Cogn Process (2005) 6: 98–116 DOI 10.1007/s10339-004-0038-7

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Marı´ a Jesu´s Funes Æ Juan Lupia´n˜ez Æ Bruce Milliken

The role of spatial attention and other processes on the magnitude and time course of cueing effects

Received: 29 July 2004 / Revised: 8 October 2004 / Accepted: 9 November 2004 / Published online: 6 January 2005 Ó Marta Olivetti Belardinelli and Springer-Verlag 2005

Abstract We are quite often exposed to multiple objects present in the visual scene, thus attentional selection is necessary in order to selectively respond to the relevant information. Objects can be selected on the basis of the location they occupy by orienting attention in space. In this paper, we review the evidence showing that attention can be oriented in space either endogenously, on the basis of central cues, predictive of the relevant location, or exogenously, automatically triggered by the salient properties of visual stimuli (peripheral cues). Several dissociations observed between orienting on the basis of the two types of cues have led to the conclusion that they do not represent just two modes of triggering the orienting of the very same attentional mechanism, but rather they modulate processing differently. We present a theoretical framework according to which endogenous predictive cues facilitate target processing by orienting attention, thus amplifying processing at the attended location. In contrast, apart from attentional orienting, peripherally presented discrepant cues might trigger additional cue-target event-integration and event-segregation processes, which modulate processing in a different way, thus leading to cueing effects that are exclusively triggered by peripheral cues.

M. J. Funes (&) Æ J. Lupia´n˜ez (&) Departamento de Psicologı´ a Experimental y Fisiologı´ a del Comportamiento Facultad de Psicologı´ a, Universidad de Granada, Campus de Cartuja, 18071 Granada, Spain E-mail: [email protected] E-mail: [email protected] Tel.: +34-958-240663 Fax: +34-958-246239 B. Milliken McMaster University, L8S 4L9 Hamilton, Ontario, Canada

Keywords Spatial attention Æ Spatial orienting Æ Exogenous and endogenous attention Æ Spatial cueing Æ Integration Æ Attentional modulation Æ IOR

Introduction Multiple objects often appear in our visual field, distributed across space but all present at the same moment in time. However, not all of these objects are equally relevant for behavior. In fact, most of the visual information available at any given moment is superfluous to performance. In addition, our effectors are restricted by the number of actions they can perform at a time, being unable to be responsive simultaneously to all visual stimulation. To deal with this problem, the visual information-processing system has evolved to select relevant information for processing that is not afforded to other irrelevant visual information. How is this selection achieved? Most theories agree that an attentional system has been developed in the visual information processing with the function of selecting the most relevant information according to our goals and intentions, or in response to any other stimulation that might be relevant for survival. Most accounts of selective attention have proposed the dimension of space to be a critical medium in which attention operates, selecting certain regions in space while ignoring others. According to this view, any object occurring inside the focus of attention is processed to high levels in the system, such as that associated with conscious awareness, while objects outside the focus of attention are rejected before this higher level of processing occurs. Although some accounts propose that attention selects relevant objects, rather than relevant locations, the results from a large number of attentional studies show that the location of an object has a special role in selection, in that it is very often the first object dimension selected. What guides attention? Under some conditions, the relevant object location can be signaled by instructions,

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or by meaningful spatial cues, that indicate the most probable target location. In this case, the attentional mechanism may be voluntarily guided by such information toward the predicted relevant object location. This mode of attentional orienting is known as ‘‘voluntary or endogenous attentional orienting’’, and is characterized by being initiated actively by the person in a top-down manner. In contrast, a location in space may be the target of attention not because it is voluntarily attended, but because of the intrinsic properties of the object that occupies that location. Salient properties of visual stimuli, such as their unique color, form, or their abrupt onset in the scene, seem to orient our attention toward them in a fast and reflexive manner, even when these stimuli are completely irrelevant for our task. This mode of orienting is thought to have an important ecological role in most species, allowing animals to be sensitive to novelty and discrepancies in the scene that could mark a predator to be avoided, or a prey to be approached (Goschke 2003; Hoffmann 1993). Recent research has referred to this phenomenon as ‘‘attentional capture’’ or ‘‘reflexive attentional orienting’’ (see Ruz and Lupia´n˜ez 2002; Yantis 2000, for recent reviews). In addition to these hardwired and perhaps innate properties of visual objects that seem to capture attention automatically, attention may be reflexively allocated toward other properties of visual stimuli that have become relevant for more transient and contextual reasons defined by the goals and motivations of the task at hand. For example, in a given visual search situation, it may be relevant to find a blue object among other green or red objects. After a few minutes of practice with the task, the attentional system might be perfectly reconfigured or biased to orient rapidly toward the location occupied by any blue object appearing in the scene. This control over orienting is described as being governed by ‘‘attentional control settings’’ (ACS), a form of endogenous control that is developed by instructions or by the current demands of the task at hand. However, once the attentional set for a given property is in place, the appearance of any object sharing this property seems to orient attention to its location in a fast and automatic manner, as if these properties were in fact hardwired attractors (Arnott et al. 1999; Folk and Remington 1998, 1999; Folk et al. 1992, 1994). In most everyday situations, our gaze is oriented together with, and in the same direction as our attention, a phenomenon known as ‘‘overt orienting’’. However, the orienting of attention can be dissociated from the orienting of the gaze, as our ability to orient our attention without orienting receptors (e.g., independently of gaze direction), which is known as ‘‘covert orienting’’ has been widely demonstrated. Although many studies have found important overlap on the anatomical mechanisms related to these two forms of orienting, there is also evidence of considerable independence between them (for recent reviews see Klein 2004; Klein and Shore 2000). In order to study covert attention, most studies introduce different strategies to control eye movements,

such as instructing participant to keep the sight fixed on a central point, while relevant targets appear at separate locations. In addition, most cover orienting studies introduce procedures in order to keep track and discard those trials where eye movement was made. In the present paper, we review previous works on cover orienting, in particular. Also, in some recent studies carried out in our laboratory, we used different procedures to study cover orienting. Apart from the issue of whether attention can be oriented covertly or overtly, and whether it can be either oriented in a purely exogenous manner, or endogenously driven by expectancies and goals, some other important questions have guided research in the field. Although not an exhaustive list, much of the research effort in the attention literature has been devoted to the following issues: (1) whether attentional selection takes place early or late in the visual-cortical processing stream, (2) the time course and trajectory of the orienting of attention in space, (3) the exact consequences for perception and behavior produced by spatial attention, and (4) whether endogenous and exogenous modes of attentional orienting are in fact two modes of orienting a unitary attentional mechanism, or rather tap qualitatively different processes by which target processing is affected. Each of these questions will be reviewed in the following sections. In the last sections, we propose a novel account that introduces additional processes separate from attentional orienting, and exclusively triggered by peripheral onset cues. We will refer to these non-attentional processes as cue-target ‘‘event integration’’ or ‘‘event segregation’’ phenomena. We will discuss how these processes could easily account for some of the results found in the literature, such as the different consequences on target processing following peripheral and central cues, which cannot be completely explained exclusively in terms of spatial attentional orienting processes.

Advance spatial information guides our attention: evidence of selection by the voluntary orienting of attention In some situations, we may know in advance where in space relevant information is likely to appear. As discussed previously, researchers have proposed that attention may be voluntarily oriented toward the most probable location of information that is relevant to our goals. One of the most used paradigms for the study of voluntary attentional orienting has been the costs and benefits paradigm introduced by Posner and colleagues (Posner 1980; Posner et al. 1978). Typically, subjects must detect or discriminate a stimulus appearing either at a peripheral location previously indicated by a spatial cue (a ‘‘valid condition’’) or at a different location (an ‘‘invalid condition’’). The spatial cue usually consists of an arrow pointing toward one of the possible target locations, indicating the most probable location of the

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upcoming target. Other variants of this cueing paradigm involve the use of symbolic cues such as colors or forms, which are associated to a specific spatial location through instructions (e.g., participants may be told that a centrally presented red circle predicts a target on the right side of space). Another form of spatial cueing involves presenting an object in the same location occupied later by the target. For this type of cueing procedure to produce voluntary orienting, the location of the peripheral cue would have to be made predictive of the location of the target, with the target appearing at the same location as the cue in most of the trials. Finally, an even simpler way to study the effect of advance spatial information on further target processing is the so-called sustained attention paradigm, which is often used in event-related potential (ERPs), single cell recording, and neuroimaging studies. This paradigm consists of instructing the individual to orient toward one location in space consistently across a series of experimental trials. The response or the brain activity produced by a target when it appears at that attended location is then compared to the response produced by a target when it appears at a non-attended location. All of these forms of cueing the target location with advance spatial information produce a robust behavioral result consisting of a reduction in reaction time (RT), and often an enhancement in accuracy (AC), for cued trials as compared to oppositely cued trials, a result known as a facilitation effect (Posner et al. 1978, 1980). The facilitation effect found with this kind of paradigm has clear electrophysiological correlates. In particular, the amplitude of the P1 (80–130 ms) and N1 (150–200 ms) ERP components over the posterior scalp is larger in the wave evoked by stimuli appearing at attended locations than in the wave evoked by stimuli appearing at unattended locations (Anllo-Vento 1995; Di Russo et al. 2003; Eimer 1993; Harter et al. 1982; Hillyard and Mangun 1987; Mangun 1995; Mangun and Hillyard 1987). These components have been localized by dipole source modeling and co-registration with neuroimaging blood flow activity maps to specific zones of extrastriate visual cortex (Clark and Hillyard 1996; Heinze et al. 1994; Mangun et al. 2001; Martinez et al. 1999). Similarly, single cell recordings from monkeys have shown an enhanced neural activity elicited by attended stimuli in retinotopic areas of extrastriate visual cortex such as V2, V3a, and V4, as well as in higher areas in the dorsal and ventral streams (Maunsell and McAdams 2000; Reynolds and Desimone 2001). The most accepted explanation for such facilitation effects is that visual attention is voluntarily oriented toward the cued location and is maintained at that location until the onset of the target. Such orientation may produce an enhancement in early sensory processing of targets appearing at the attended location, as compared to targets appearing at other locations. For targets appearing at non-attended locations, additional processes of disengagement of attention from the cued location and reorienting of attention toward the target

location would be required (Posner and Cohen 1984). This proposed early sensory modulation of attention has been recently corroborated by psychophysical studies in both humans and monkeys (Carrasco et al. 2000; Ciaramitaro et al. 2001; Lu and Dosher 1998). Thus, although neuroimaging and neurophysiological studies have found important evidence indicating that attention modulates the perceptual processing of stimuli, it is not clear whether it can also affect later stages of processing such as response selection. Psychophysical studies have also helped us to determine whether attention facilitates performance by introducing changes in processes related to decision-making (by biasing beta), as well as by producing an increase in perceptual sensitivity (by biasing d’). All such studies have found strong evidence favoring the perceptual sensitivity account, suggesting that spatial cues produce increases in perceptual sensitivity while leaving intact other factors related to response bias (Ciaramitaro et al. 2001; Downing 1988; Hawkins et al. 1990). There is some evidence showing that an increase in sensitivity may occur even in the absence of external noise (Dosher and Lu 2000; Lu and Dosher 2000). In relation to the brain sources of this early attentional modulation, Corbetta and Shulman (2002) have recently reviewed several fMRI studies, where advance spatial information was provided. They have delimited a group of brain areas that showed sustained activation during the time interval between the spatial cue and the target. These areas included the dorsal posterior parietal cortex along the intraparietal sulcus (IPs) and areas in the frontal cortex, near the human homologue of the frontal eye field (FEF). The authors call it the dorsal frontoparietal network.

Onsets and discrepancies as attentional ‘‘attractors’’: evidence of spatial selection by reflexive orienting of attention There are certain salient properties of visual stimuli, such as abrupt onsets or discrepancies with the rest of a visual array that seem to attract our attention in a fast and obligatory manner. One way to study this type of attentional orienting phenomenon is to measure the facilitative or interfering effects produced by these properties on visual search. In this kind of task, participants are asked to detect and identify a target stimulus amidst an array of distractors. What is usually found is that, when the salient stimulus happens to be the target, a facilitation effect (i.e., faster RT and reduced proportion of errors) is observed. On the other hand, if the salient stimulus is a distractor occurring in a location different from the target, its presence interferes with the detection of the target (see Ruz and Lupia´n˜ez 2002, for a recent review). This paradigm has been very useful in helping us to understand the properties of visual stimuli that most effectively capture our attention. Many studies have

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shown that singletons (i.e., stimuli that contain a physical property, such as color, that differs from the rest of the stimuli in the visual array) attract attention in an automatic manner (Theeuwes 1991, 1992, 1994). However, dynamic singletons, such as abrupt onset stimuli appearing at new locations, seem to have the greatest potential to produce attentional capture effects in a pure bottom-up manner (Jonides and Yantis 1988; Oonk and Abrams 1998; Pollmann et al. 2003; Yantis 1998; Yantis and Egeth 1999; Yantis and Hillstrom 1994; Yantis and Jonides 1984, 1996; see Ruz and Lupia´n˜ez 2002, for a recent review on the controversy about this issue). Even when they are irrelevant and orienting to them harms performance (e.g., abrupt onsets never occur at a target location), it seems that abrupt onsets capture attention. In this sense, abrupt onset stimuli appearing in new locations may constitute hardwired attractors of attention, probably because of their relevance for survival. A second paradigm used to measure attentional capture produced by onsets is the cost and benefits paradigm. The paradigm is similar to that described above used to study endogenous attention. However, in this case, peripheral cues that are not informative about the target location are used. In a given trial, an abrupt onset cue is presented at one of the peripheral spatial locations where the target is to appear. After a short cuetarget interval, the target appears with equal probability either at the location previously occupied by the spatial cue (valid trials) or at the opposite location (or in any other location if there are more than two locations; invalid trials). Depending on the task, participants must detect, localize or discriminate the target, as quickly and accurately as possible. The result typically observed is a facilitation effect; that is, a reduction in reaction time (RT) and/or an increase in accuracy (AC) for targets

Fig. 1 Time course of exogenous cueing effects. At short cue-target SOAs a facilitation effect is observed, shown as faster RT for cued (valid trials) than for non-cued locations (invalid trials). In contrast, at longer cue-target SOAs, the IOR effect is observed, consisting of slower RT for cued (valid trials) than for non-cued locations (invalid trials). Note that the transition from facilitation to IOR occurs in both tasks, but IOR appears later in the discrimination task. Data adapted from Lupia´n˜ez et al. (1997)

that appear at the cued location relative to targets that appear at non-cued locations. The introduction of longer cue-target intervals in these cueing paradigms led researchers to discover that the robust facilitation effect observed with peripheral non-informative cues is transient. Indeed, when the target onset follows the cue onset by several hundred milliseconds or more (i.e., at longer cue-target stimulus onset asynchronies, or SOAs) the opposite result is observed; responses are slower for cued than for non-cued target locations. This later negative cueing effect was first reported by Posner and Cohen (1984), and later termed inhibition of return (IOR) by Posner et al. (1985). Although the effect was initially considered to be present only in simple RT detection tasks, later it was obtained in detection and choice RT discrimination tasks, as can be observed in Fig. 1 (Lupia´n˜ez et al. 1997) (see Klein 2000 and Lupia´n˜ez et al. 1999, for reviews). It is not clear whether it is the response or the movement of attention to the cued locations that is inhibited in the IOR effect. According to the motor account of IOR, the effect would reflect a motor bias against responding to the cued location (Klein and Taylor 1994), which has been called ‘‘inhibitory tagging’’ by other authors (Klein 1988; Vivas and Fuentes 2001). In contrast, according to the attentional account of IOR, the effect is due to a reduced perceptual sensitivity at the cued location, which can be observed as a decrease in accuracy or d’ (Handy et al. 1999; Lupia´n˜ez et al. 2001). Finally, others have argued that there might be two components in the IOR effect, one attentional-perceptual and the other motor (oculomotor, Hunt and Kingstone 2003; Kingstone and Pratt 1999), both contributing to the overall measured IOR effect. This overall pattern of cueing effects, that is, facilitation followed by IOR after peripheral non-informative cues has also been observed in ERP studies that showed enhanced amplitudes of the P1 and/or N1 wave components at posterior sites for cued as compared to oppositely cued trials, but only at short SOAs. Interestingly, at long SOAs, the P1 evoked by cued targets tended to be smaller in amplitude than that evoked by oppositely cued targets, a pattern reminiscent of IOR (Doallo et al. 2004; Hopfinger et al. 2000; Hopfinger and

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Mangun 1998, 2001). In a recent study by Prime and Ward (2004), both the P1 and N1 ERP components were reduced for cued targets as compared to oppositely cued targets. Other correlates of facilitation following abrupt onsets come from single cell recordings in monkeys. In these studies, it was shown that neurons of the FEF and LIP areas respond robustly when a salient stimulus (such as an oddball or visual transient) enters its receptive field (Colby et al. 1996; Thompson et al. 1997). The framework of Posner and colleagues is often used to explain the cueing effects produced by abrupt onsets and discrepant objects. According to this framework, visual attention may be first oriented toward the cued location in an obligatory and automatic manner. If the target does not appear shortly after the cue, then attention is reoriented toward a central location because of the uninformative nature of the spatial cue. Finally, attention may then be inhibited from returning toward the cued location, with the idea that attention should be biased against re-sampling old locations thus favoring sampling of new locations, which would explain the IOR effect (Posner and Cohen 1984; Posner et al. 1985). It has been argued that this later inhibitory property of attention would produce more efficient searches of potentially relevant stimuli in our environment at new and still unexplored locations (Klein 1988, 2000; Klein and MacInnes 1999; Muller and von Muhlenen 2000; Takeda and Yagi 2000; Tipper et al. 1991, 1996).

‘‘What’’ is facilitated by the orienting of attention? As shown in the literature discussed so far, much research has been done in order to identify the kind of information that guides our attention, the stage of processing at which the attentional mechanism operates, or the time course and trajectory followed by attention. However, surprisingly little is known about the exact manner by which attention operates once deployed toward the object location in order to facilitate its processing. One important issue that needs to be further addressed is whether the orienting of attention enhances equally all visual properties of the attended stimulus, such as its color, form, and location information, or rather selectively amplifies a unique target dimension, such as the one that is relevant to performance. A general enhancement of all target properties would be in agreement with the ‘‘zoom lens’’ or the ‘‘spotlight’’ hypothesis which is, in fact, the most accepted explanation. However, it could also be that the orienting of attention enhances exclusively the target dimension that is relevant for the task at hand, such as the target color in a color discrimination task, or its shape in a form discrimination task, while leaving intact or even inhibiting the processing of other stimulus properties irrelevant for the task. A third possibility is that attention may favor the processing of the target spatial information, and leaving intact the rest of the target dimensions.

As discussed before, space has been proposed to be a particularly important medium for attentional orienting. In this context, it seems reasonable to argue that attention may facilitate the target processing by altering the processing of its spatial information. One strategy used to distinguish between these possibilities uses spatial cues to indicate the location of a potentially conflicting stimulus; that is, one in which two of its dimensions can be either congruent or incongruent to each other. This is the situation that occurs in procedures such as the Simon or Stroop task. Thus, if the interference produced by a distracting dimension that is irrelevant for the task (i.e., the color word in a color naming Stroop task) becomes larger when the target stimulus (the target and distracting dimensions) appears at the cued location, it may indicate that the dimension that carries the distracting information has been amplified by the orienting of attention. Alternatively, if the amount of interference becomes smaller on cued trials, then this may indicate that the orienting of attention has reduced the influence of the dimension that carries the irrelevant information, either by amplifying processing of the relevant dimension, or by inhibiting processing of the irrelevant dimension. Finally, if equivalent amounts of interference are produced by the irrelevant dimension on cued and uncued trials, this may indicate that the orienting of attention is indeed amplifying equally all dimensions of the target stimulus. This last possibility corresponds to the results observed by Shalev and Algom (2000). In their study, peripheral cues were presented, but they predicted the target location in 80% of the trials. The targets were color words printed in various colors, and participants were to respond either by naming the color or by reading the word. The authors found significant facilitation effects produced by peripheral cues, as well as significant color-word congruency effects. However, the congruency effects were equivalent for targets appearing at cued and oppositely cued locations. These results seem to indicate that the orienting of attention toward the target location amplified equally all target dimensions, in a zoom lens-like manner. The role of attentional orienting on the processing of target spatial information However, as we addressed earlier, it could be that spatial attention may favor the processing of the target spatial information while leaving intact the rest of the target dimensions. To see whether spatial cueing manipulations amplify the processing of spatial dimensions, we could investigate whether spatial cues do amplify the magnitude of spatial congruency effects, such as Simon or Spatial Stroop effects. In these kinds of tasks, a spatial irrelevant dimension such as the stimulus location can be congruent or incongruent with a relevant dimension (a non-spatial dimension such as color, in Simon, or another spatial dimension such as direction, in Spatial

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Stroop). Thus, if spatial cues facilitate target processing by amplifying its spatial information, we should expect larger Simon or Spatial Stroop effects for cued compared with uncued trials, due to the amplification of the distracting location information. Stoffer and Yakin (1994) used a Simon task in which target locations were precued by either a peripheral (an abrupt change of luminance) or a central spatial cue (an arrow pointing left/right), both 100% predictive of target location. Two cue-target SOAs were used, 100 and 1,000 ms, and the 100% valid cueing condition was compared to a neutral condition in which a non-informative cue was presented at fixation. They found that the Simon effect was reduced for the validly cued condition, both with central and peripheral cues, and at both SOAs, as compared to the neutral condition, for which a large Simon effect was found. However, this modulation of the Simon effect by spatial cueing has not been found in other studies using similar cueing conditions. For example, Verfaellie et al. (1988) and Proctor et al. (1992) carried out similar studies to compare the effect of precueing the target location and the effect of precueing the response location on the Simon effect in a color-discrimination task. They used both peripheral and central cues that were 80% predictive of either the target or the response location. Both studies found that precueing the response location produced an enhancement of the Simon effect, while precueing the target location did not influence the Simon effect. Similar null effects of spatial precueing on Simon effects have been found by Hommel (1993b, experiments 4–6) and Zimba and Brito (1995). In Zimba and Brito’s study, peripheral cues predicted the target location on 80% of the trials, and the interaction between spatial cueing and Simon effects was tested at a large number of cue-target SOAs, ranging from 50 to 2,400 ms within and across studies. They found that cueing effects varied as a function of SOA and that the Simon effect was not apparent at the shortest 50 ms SOA. However, neither the cueing by Simon interaction nor the cueing by SOA by Simon interaction was significant, as similar Simon effects were found for cued and uncued trials at all SOAs. In Hommel’s (1993b) study, non-informative peripheral spatial cues were presented, and different levels of SOAs ranging form 50 to 400 ms were manipulated across experiments. Again, no cueing by Simon interaction was observed. Finally, an omnibus analysis has recently been carried out that included six different cueing studies to evaluate whether facilitation and inhibition of Return effects interact with the Simon effect (Ivanoff et al. 2002). The main aim of the analysis was to rule out the possibility that the lack of previous interactions was due to a lack of statistical power. All studies included in that omnibus analysis were originally conducted with the purpose of studying the time course of cueing effects, and consisted of color or identity discrimination tasks requiring left/right responses. The stimuli appeared at two possible target locations that were precued by non-

informative peripheral cues at different levels of SOA ranging from 100 to 1,000 ms. The analysis showed that at an SOA of 100 ms, where the facilitation and Simon effects were both highly significant, the Simon effect was completely independent of cueing. However at longer SOAs, when IOR was observed, a cueing by Simon interaction was found, which consisted of an enhancement of the Simon effect at the cued location. Considering all this evidence together, it seems clear that spatial cues have little or no effect on the magnitude of Simon effects, at least when the cueing manipulation leads to facilitation effects. This result is consistent with the idea that the orienting of attention toward a location amplifies equally all dimensions of the target stimulus. However, there are two studies in which the results appear to contradict the conclusion that spatial cueing effects and spatial congruency effects do not interact (Danziger et al. 2001; Funes and Lupianez 2003). These studies differed from those described above in that they examined the effect of spatial cueing on the Spatial Stroop effect rather than the Simon effect. Participants were to discriminate the direction of an arrow pointing either left or right, which was presented either to the left or right of fixation, by hitting the corresponding left or right response. Thus, their procedure combined stimulus–stimulus (S–S) correspondence between the stimulus location and the stimulus direction, together with stimulus–response (S–R) correspondence between the stimulus location and the response location (a type 7 dimensional overlap, according to the Kornblum taxonomy 1992; Kornblum et al. 1990; Kornblum and Lee 1995). In Funes and Lupia´n˜ez’s (2003) study, the task required a left/right key press to the left/right direction of a peripherally presented arrow, appearing either at a congruent location, and requiring a spatially congruent response (i.e., a left-pointing arrow appearing on the left side of the screen and requiring a left key-press response), or at an incongruent location, and requiring a spatially incongruent response (i.e., a left-pointing arrow appearing on the right side of the screen requiring a left key-press response). In addition, peripheral non-informative cues signaled the target location on a third of the trials, and the location opposite the target on another third of the trials. No cue was presented in the remaining third of the trials. Interestingly, apart from large spatial congruency and spatial cueing effects, we found that spatial cues modulated the spatial congruency effect. In particular, the magnitude of spatial congruency was significantly smaller on cued trials (27 ms) compared to no-cue trials (44 ms) and oppositely cued ones (58 ms) (see Danziger et al. 2001, for a similar paradigm and results). Note that the difference between the Spatial Stroop tasks used in Funes and Lupia´n˜ez (2003) and Danziger et al. (2001) and the Simon tasks described above is the presence of an additional S–S source of correspondence in Spatial Stroop. Therefore, it can be concluded that spatial cues reduced the congruency effect only when the

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spatial congruency referred to the correspondence between stimulus dimensions (‘‘stimulus–stimulus’’, S–S congruency) but not when it referred to the correspondence between stimulus and response dimensions (stimulus–response, S–R congruency). The perceptual locus of the cueing reduction on spatial interference has been corroborated by another study from our lab, in which we introduced an experimental design that allowed us to compare, within the same task, the effect of spatial cues on the stimulus– stimulus (S–S) or Spatial Stroop effect with that on the stimulus–response (S–R) or Simon effect. To do so, we instructed participants to make left/right key presses to directional arrows pointing either up or down, which could appear equally often at one of the four locations: left, right, top, or bottom. With this procedure we were able to measure Spatial Stroop and Simon effects, independent of each other. In the horizontal axis, S–R Simon interference was measured (the direction of the arrow target was orthogonal to the interfering dimensions), whereas in the vertical axis S–S Spatial Stroop interference was measured (the responding hand location was orthogonal to the interfering dimensions). The results were straightforward: spatial cues reduced the S– S or Spatial Stroop effect, measured when the arrow target appeared in the vertical axis, but did not influence the S–R Simon effect, measured when the arrow appeared in the horizontal axis (Lupia´n˜ez and Funes 2004). Altogether, these results clarify the role of spatial cues on spatial congruency effects. In particular, they corroborate the hypothesis of a stimulus–stimulus locus for the influence of cueing on Spatial Stroop effects, rather than the stimulus–response locus that appears to be the source of the Simon effect. Note that most frameworks for understanding the orienting of attention would not predict the modulation of spatial cues on the magnitude of Spatial Stroop that we have observed. Thus, if the attentional mechanism were acting as a ‘‘zoom’’ that amplifies all visual properties of the cued target (in our case, both the irrelevant location and relevant direction of the target arrow), then we should have found similar Spatial Stroop effects when the arrow appeared at the cued and oppositely cued locations. Alternatively, if attention were acting as a ‘‘spotlight’’ illuminating the processing of the target location dimension (in our case, the irrelevant location of the target arrow), then we should expect larger rather than smaller Spatial Stroop effects on cued trials. Two other hypotheses could, in principle, account for this reduction of Spatial Stroop on cued trials. According to the attentional shift account (Rubichi et al. 1997; Stoffer 1991; Stoffer and Umilta 1997) a spatial code is only created with a shift of attention. Therefore, if attention has been moved toward the cued location prior to the target appearance, no attentional shift toward the target location would be necessary when the target is presented. Therefore, no spatial code would be created for the target, and consequently a null Simon or Spatial Stroop should be observed on cued locations.

Alternatively, Danziger and colleagues have recently proposed a referential coding account according to which spatial cues may constitute mere objects of reference for the creation of target spatial co-ordinates. More specifically, in a cueing paradigm, the target spatial location may be coded left-right relative to two simultaneous objects of reference, the central fixation point object and the lateralised cue object. On oppositely cued trials, the target location would be coded relative to both the left-right cued location and the central location; while on cued trials, the target location would be leftright coded only relative to the central point, because it would be coded as ‘‘same’’, relative to the cue. This explanation would also predict a reduction of Simon and Spatial Stroop effect for cued trials compared to oppositely cued trials. However, the absence of this reduction on Simon-like tasks (Hommel 1993b; Ivanoff et al. 2002; Lupia´n˜ez and Funes 2004; Proctor et al. 1992; Verfaellie et al. 1988; Zimba and Brito 1995), and more generally the dissociation found between the cueing modulation on Simon versus Spatial Stroop, are not directly predicted by either of these two accounts. On the contrary, both the attentional shift account and the referential coding account would predict the same reduction on cued trials for Spatial Stroop and Simon effects. Consequently, these theoretical accounts cannot explain why S–R spatial congruency played no role in modulating the influence of spatial cueing on the Spatial Stroop effect. In ‘‘Cue-target event-integration and event-segregation phenomena contributing to cueing effects’’, we present a theoretical account of peripheral cueing effects that could explain the pattern of data.

Do peripheral abrupt onset cues and central symbolic cues orient the same attentional mechanism? Evidence for a dissociation The paradigm of costs and benefits has been one of the most influential in the study of the orienting of attention. Many variations of the original paradigm have been used to study the properties of the spatial ‘‘spotlight’’, such as its trajectory through space or the time course for its deployment. The robustness of the facilitation effect following both central, informative cues, and peripheral, noninformative cues, is consistent with the view that there are two modes by which spatial attention can be oriented; an endogenous mode that is responsive to internally developed spatial expectancies, and a peripheral mode that is responsive to salient properties of external stimuli. A number of studies have pointed to the differences between these two modes of orienting. Two critical differences concern the magnitude of cueing effects and their temporal course. Early studies demonstrated that the facilitation effect was smaller and appeared at shorter cue-target intervals with peripheral than with central cues, and IOR effects were observed

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only following peripheral cues at long cue-target intervals (Jonides 1976, 1981; Mu¨ller and Rabbitt 1989; Posner 1980; Posner and Cohen 1984). Although these results could be interpreted in the sense that peripheral and central cues tap two separate attentional mechanisms, most of the empirical differences in performance for peripheral and central cues are quantitative in nature (size and time course), which could be interpreted to mean that they are two modes of ‘‘transporting a unitary attentional spotlight’’ (see Klein and Shore 2000, for a review). In fact, the predominant views of attentional orienting consider exogenous and endogenous orienting as two ways of orienting the very same mechanism. However, dissociations between endogenous and exogenous orienting that are qualitative in nature have been obtained more recently. These results seem at odds with the unitary attentional operator argument. Two kinds of studies can be distinguished. First, clear dissociations have been found between the qualitative consequences of peripheral and central cues for target processing. These studies suggest that, although similar amounts of facilitation can be produced by these two kinds of cues, they seem to produce their facilitation effects by affecting different stages of target processing. Second, electrophysiological and neuroimaging studies have revealed different patterns of activation, when peripheral non-informative and central predictive cues are presented. These two sets of evidence will be reviewed in the following sections. Peripheral and central cues produce different consequences on target processing Rather than studying the properties of the orienting mechanism itself, some studies have focused on behavioral effects produced by spatial cues after the putative attentional ‘‘spotlight’’ has been deployed to the target location. This line of research has revealed important dissociations between the effects produced by peripheral and central spatial cues that are difficult to reconcile with a unitary attentional operator perspective. Briand and Klein (1987) (see also Briand 1998) examined the role of attention in feature integration processes (Treisman and Gelade 1980) involved in putting together separate features presented proximal in space and belonging to the same object. To do that, they combined a cueing paradigm with either a feature or a conjunction visual search task. The specific objective was to examine whether attention benefits conjunction searches, as Treisman’s feature integration theory (Treisman and Gelade 1980) argues that attention is needed for feature integration. In Briand’s (1998) study, the feature search task required participants to decide whether or not the letter ‘‘O’’ (or a blue letter) had been presented in one of two locations. A distractor stimulus that appeared in the other location consisted of one of two other possible letters, printed in one of two other possible colors. In the conjunction search task, however,

participants had to report whether or not a ‘‘blue O’’ was presented, with the distractor being either a different letter in blue, or the letter O presented in a different color. In addition, they compared whether these tasks would be affected differently by attentional orienting elicited by central endogenous and peripheral cues. The authors found larger cueing effects for the conjunction than for the feature search task when peripheral cues were used. However, similar cueing effects were found for conjunction and feature searches when central cues were used. The authors proposed that peripheral and central cues may affect different stages of processing, with peripheral cues affecting early stages such as feature integration, and central cues affecting earlier or later stages such as feature extraction or response selection (Klein 1994). A second interesting dissociation comes from the work of Lu and Dosher (2000) with psychophysical measures. They compared spatial cueing effects with peripheral informative cues and central informative cues in a task requiring the identification of one of four pseudocharacters embedded in various amounts of external noise. A first result was that, when spatial cues were presented in advance, both types of cues reduced the threshold for target detection in the presence of high external noise, relative to a situation with simultaneous cueing. However, only peripheral precues reduced the threshold in the presence of low or no external noise. These data might be interpreted to mean that peripherally presented exogenous cues and centrally presented endogenous cues have different roles in attentional modulation. The role of central cues might be to facilitate spatial selection per se, by distinguishing relevant from irrelevant stimulation; whereas the role of peripheral cues might be more related to the creation of new object representations, a function that may be more general, and produces benefits on target processing in a broader variety of visual processing contexts, such as with and without distracting stimulation. A third line of research that provides evidence for a qualitative dissociation between central and peripheral cues comes from the work initiated by Macquistan (1997) and continued by Goldsmith and Yeari (2003). This line of research aimed at specifying the cueing conditions that favor object-based over space-based attention. Using the double-rectangle spatial-cueing paradigm (Egly et al. 1994), they found that peripheral non-informative cues surrounding the end part of one of the two rectangle placeholders consistently produced object-based facilitation; that is, participants responded faster to invalid targets that appeared inside the cued rectangle than to invalid targets that appeared inside a different rectangle, despite their equivalent distance from the cued location. This object advantage was not observed, however, with informative, central cues. Macquistan’s explanation for the finding of an object-based effect following peripheral cues, but not following central cues, was that peripheral cues may produce general priming on object files by affecting either the setting up

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of the object file, the accumulation of information within the object file, or the access of information from that file. Central cues, however, would affect earlier or later stages of target processing (Macquistan 1997). In order to understand these dissociations, it is important to note that the traditional explanation of attentional effects is based on the general idea of ‘‘signal amplification’’, by which, processing of information is amplified at the location where attention is oriented to, as compared to processing of information from other locations, to which attention is not oriented. Note, however, that this explanation does not fit well with the results reported in ‘‘The role of attentional orienting on the processing of target spatial information’’ of a reduced Spatial Stroop effect at the location cued by a peripheral salient onset cue. As already stated in this section, if exogenous orienting enhanced spatial processing at the cued location (signal amplification), the opposite result should be obtained (i.e., increased Spatial Stroop at the cued location). Alternatively, if exogenous orienting enhanced not only spatial processing but also processing of all target dimensions, no interaction should be found. Therefore, the systematic finding of decreased Spatial Stroop at the cued location is difficult to be explained by the signal amplification hypothesis of attentional orienting. However, it might be that this effect is specific to the presence of peripheral cues, while being appropriate to the amplification hypothesis for explaining endogenous orienting. In this case, a further dissociation between endogenous and exogenous attention should be observed, leading to the conclusion that the two Fig. 2 Spatial Stroop effect (difference between the congruent and incongruent conditions) as a function of exogenous versus endogenous orienting of attention. First, note the benefits of orienting attention either exogenously or endogenously (faster RT for cued/valid than for uncued/invalid trials). Second, and more importantly, note that exogenous orienting led to a reduction in Spatial Stroop, whereas endogenous orienting led to the opposite effect, i.e., an increase in the Spatial Stroop effect. Data from Funes (2004, experiment 2.2). Data are taken from the SOA at which the orienting effect was greatest (100 ms for the exogenous condition and 850 ms for the endogenous condition)

orienting systems exert their effects on processing by different means. In order to test this hypothesis, we directly compared the effects of endogenous and exogenous orienting on the Spatial Stroop effect. A central symbolic cue (a color indicating the most likely target location) was used as central cue, whereas the usual unpredictive onset (increase in luminance) at one of the target locations was used as a peripheral cue. Four levels of SOA were presented (100, 350, 600 and 850 ms) for two reasons. Following the results usually found in other discrimination tasks with peripheral non-informative cues, we expected to find a large facilitation effect at 100 and 350 ms SOA, followed by IOR for the two larger levels of SOA. For the central endogenous cue group, we expected to find a small facilitation effect at the very short SOA, which would be increasing with increasing levels of SOA. And this was exactly what we found. Regarding the cueing modulation of Spatial Stroop, if endogenous orienting modulates spatial processing by the very same attentional mechanism, differing only on the time required to be oriented, once attention is oriented (at the appropriate SOA) we should observe the very same consequences on the Spatial Stroop effect, that is, it should be smaller at the location where attention is oriented (cued trials). Results, however, completely diverged from this prediction. As can be seen in Fig. 2, exogenous and endogenous orienting had opposite consequences on the Spatial Stroop effect; whereas exogenous cues reduced the effect, thus replicating our previous results, endogenous orienting led to a significant increase in the Spatial Stroop at the valid location. These two contradicting effects were even present when comparing those SOA conditions, where facilitation effects were maximal, that is at the 100-ms SOA following peripheral uninformative cues, and at the 850-ms SOA following central cues. Thus, the opposite modulatory effects cannot be explained in terms of differences in the amount or strength of attention allocated to the cued location. The fact that endogenous spatial attention modulated the Spatial Stroop effect in the direction opposite to that

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of exogenous spatial attention has important theoretical consequences. It provides a strong argument against the unitary attentional hypothesis, suggesting that peripheral and central cues facilitate target processing by different means, and add to the growing number of quantitative and qualitative empirical dissociations concerning the effects of these two kinds of cues. In order to explain the reported dissociation, it is necessary to propose that, at least partially, endogenous and exogenous attention are independent mechanisms, affecting processing in different ways. One feature that is common to all these studies is the fact that all dissociations found between peripheral and central cueing effects were present regardless of whether the peripheral cue was informative or uninformative about the target location. This feature of the results appears to favor the hypothesis that the dissociations stem from the different spatial relations between the cue (central vs. peripheral) and the target (always peripheral), while they seem to be unrelated to other factors such as the informativeness of the cue. Peripheral and central cues produce different consequences in the brain In addition to the behavioral evidence described above, electrophysiological and neuroimaging studies have revealed brain activity dissociations between peripheral and central cueing conditions. Direct comparisons of brain activity following peripheral and endogenous central cues have been conducted by Rosen et al. (1999) using functional magnetic resonance imaging (fMRI). Although they reported overlapping areas activated by both kinds of cues (bilateral parietal and dorsal premotor regions, including the frontal eye field, see also Peelen et al. 2004, for a similar finding), they also found some areas specifically activated by each type of cue. Thus, the right dorsolateral prefrontal cortex (BA 46) was activated exclusively in the central endogenous cue condition, while the left ventrolateral nucleus of the thalamus was active in the peripheral cue condition. A recent event-related fMRI study compared the differential cortical activation following voluntary and reflexive overt orienting (Mort et al. 2003). Contrary to the finding of Rosen and colleagues, these two forms of overt orienting led to large differences in cortical activations. Voluntary saccades toward the direction indicated by a central arrow cue produced greater activation within the frontal eye field (FEF) and the intraparietal sulci. On the other hand, reflexive saccades toward peripherally presented abrupt onsets produced greater activation of the angular gyrus of the inferior parietal cortex, particularly in the right hemisphere. Although some of these cortical dissociations may reflect differences that are exclusively related to voluntary and reflexive overt orienting (i.e., overt shifting of the eyes), they may represent a dissociation between voluntary and

reflexive processing that is common to covert and overt orienting. Brain activity dissociations have also been found following peripheral and central cues with the use of ERP measures. Eimer (2000) investigated differences in the time course of cueing effects by comparing the ERPs elicited by letter targets following either peripheral or central cues. In this case, both the peripheral and central cues were informative about the target location. He found that certain evoked potentials appeared 100 ms earlier for peripherally cued targets than centrally cued targets with a short CTI (cue-target interval; 200 ms). With a longer CTI (700 ms), a similar enhancement of components for valid trials was found following both kinds of cues. He interpreted the results as reflecting that peripheral cues capture attention in a fast and automatic manner relative to central cues, which may require additional time to initiate an attentional shift. However, it should be noted that this finding could also reflect additional time required to decode the central cues, rather than a difference between two forms of orienting. Fu et al. (2001) have recently pointed out to some important qualitative differences between ERPs elicited by targets following peripheral informative cues, and those found in other studies that used central cues or sustained attention tasks. Whereas tasks that used central cueing or sustained attention procedures have led to contralateral P1/N1 enhancements for attended stimuli that are similar in latency and scalp distribution across studies (e.g. Harter et al. 1982; Mangun and Hillyard 1991; Mangun et al. 1993, 1997), the pattern of results found in their study that used peripheral informative cues (75% informative) was quite different. First, although they did find a P1 contralateral enhancement for cued trials, they found an N1 enhancement at ipsilateral occipital sites. Second, they found that the contralateral enhancement of P1 was accompanied by a delay in peak latency, which was completely absent in other studies that used either central cueing or sustained attention procedures. Third, they found that both the amplitude and latency of the P2 component was affected by cue validity, an effect that is completely absent in studies that use central cueing or sustained attention procedures. Yamaguchi et al. (1994) made direct comparisons between the ERPs evoked by peripheral and central cues. They found differences not only in the onset latency, but also in the scalp distribution and duration of the negative potentials at contralateral sites that are difficult to explain in terms of mere differences in time course. Rather than different time courses of orienting of a unique attentional operator, these results may reflect qualitatively different forms of orienting. Finally, studies of brain-lesioned patients have also suggested the existence of separate neural systems for orienting after peripheral and central cues. Reflexive attentional orienting toward new objects in a scene seems to be severely impaired in some brain damaged patients, such as those suffering from a deficit known as

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neglect. These patients, typically with large right parietal lesions, fail to consciously perceive or respond to objects appearing in the contralesional (usually left) visual field, together with an abnormal attraction toward right-sided objects. This orienting deficit occurs despite an absence of sensory loss (Rafal 1998). A related deficit also associated to right parietal lesions is unilateral extinction, which consists of a perceptual impairment to perceive contralesional stimuli but only when a simultaneous stimulus is also present in the ipsilesional field. These deficits have often been interpreted as attentional deficits in the automatic orienting to events on the contralesional side. The spatial cueing paradigm has been used to systematically measure the deficits of these patients, and to compare their performance with that of persons without brain damage. Apart from the general difficulty in detecting contralesional targets, these patients usually show the largest reaction times (RT) when the target appears on the contralesional side following a cue at the ipsilesional side. This effect is often described as an extinction-like pattern, and is interpreted as a deficit in disengaging attention from a right-sided event when attention has to be re-engaged on a left-sided object (Posner et al. 1984) (reviews in Bartolomeo and Chokron 2002; Losier and Klein 2001). However, the extinction-like patterns found with neglect patients in spatial cueing tasks is found only with peripheral cues and not with central cues (La`davas et al. 1994; see Losier and Klein 2001, for a review). Recent neuroimaging studies (reviewed in Corbetta and Shulman 2002) have suggested that the brain contains two partially segregated systems for visual orienting; a dorsal network (intra-parietal sulcus and frontal eye field), bilaterally represented and concerned with endogenous orienting, and a more ventral network (temporo-parietal junction and inferior frontal gyrus) subserving exogenous orienting. Importantly, the ventral network is lateralized to the right hemisphere, and co-localizes with the brain regions most often damaged in unilateral neglect.

Peripheral abrupt onset cues trigger additional processes apart from the orienting of attention Taking into consideration all the evidence reported in the last section, it is difficult to maintain the existence of a single attentional mechanism, which can be oriented either endogenously, according to the individual’s intention, or exogenously, automatically triggered by external salient events. The fact that peripheral exogenous and central endogenous cues modulate processing differently, even in opposite ways, as it is the case of the reviewed evidence of cueing modulation on the Spatial Stroop effect, strongly support the view that these two kind of cues might trigger different processes. Once the unitary attentional view is disregarded, there are several possibilities to explain independence between the cueing effects. First, it might be that the two

types of cues tap two attentional mechanisms that are completely different in nature and work quite independently of each other. However, in order to support this hypothesis, we should have to explain how coherent behavior is accomplished in everyday life, where external events and internal goals compete for the control of action and thought. In addition, contrary to this hypothesis is the existence of some commonalties among the processing contexts triggered by both types of cues. In fact, we believe that once these attentional control systems are initiated by the occurrence of either peripheral or central cues, they may trigger a similar orienting process, in order to select the spatial relevant location. The fMRI study of Rosen et al. (1999) described in ‘‘Peripheral and central cues produce different consequences in the brain’’, where similar brain areas were recruited with both type of cues, may constitute a good evidence for common processes. Such an orienting process seems to occur faster and with a more transient life following peripheral non informative cues than when central informative cues are presented. The main differences between these two modes of orienting may come from the fact that the central cue is informative and need to be decoded given its symbolic nature. These differences may explain the different time course of their effects at behavioral (e.g. Mu¨ller and Rabbitt 1989) and electrophysiological levels (Eimer 2000), for which there may not be strong reasons to believe that they constitute separate orienting processes. An alternative framework could be that the main difference between these two modes of orienting attention stems from the fact that peripheral cues constitute real objects or events. Due to the spatial correspondence between the cue and the target events, which only occurs in the case of peripheral cues, additional processes might be triggered by this type of cues, apart from the orienting of attention. These spatial and temporal relations between the cue and the target events may represent a ‘‘special’’ situation that is exclusive to peripheral cues, and may introduce interactions between the processing of the cue and the target that are not possible in the case of central informative cues. These additional processes triggered by peripheral abrupt onset cues may contribute to cueing effects such as facilitation, jointly with the orienting of attention. However, in order to test further whether peripheral abrupt onset cues lead to additional processes apart from the orienting of attention, it is necessary to find a way to dissociate exogenous cueing effects of peripheral abrupt onset cues that are independent of attentional orienting. Recent research carried out in our laboratory have been done in this context, directly aiming at studying the relationship between endogenous and exogenous orienting, and targeting additional effects of peripheral cues which might be independent of spatial orienting (in the sense of the attentional spotlight metaphor).

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Peripheral cues lead to cueing effects independent from the orienting of attention We (Lupia´n˜ez et al. 2004) have recently devised an experimental procedure to test the above claim. A cost and benefits paradigm was used, in which the general logic of predictive cues (endogenous attention) was combined with the logic of abrupt onset cues to measure exogenous cueing. In one block of trials the cue predicted the target to appear at the same location as the cue on 80% of the trials, and participants were instructed to orient attention toward the cued location. In another block of trials, the cue predicted the target to appear at the location opposite the cue on 80% of the trials, and participants were instructed to orient attention to the opposite cued location. By testing both of these block conditions, a target that appeared at the same location as the cue could appear at the expected location (where attention would be oriented to) or at the unexpected location (where attention would not be oriented to). Similarly, when a target appeared at the location opposite the cue it could in fact appear at the location where attention was oriented (expected location) or at the opposite location (unexpected location). In this manner, we were able to test the above predictions regarding the interaction between exogenous cueing effects and (endogenous) orienting of attention. As can be seen in Fig. 3, the results we obtained were clear-cut. Regarding endogenous orienting of attention, as predicted, responses were faster at the expected location, where participants were instructed to orient attention to (note in Fig. 3 that RT is shorter at the

Fig. 3 Time course of exogenous cueing effects as a function of whether the target appeared at the endogenously expected location (where attention would be oriented), or at the endogenously unexpected location (where attention would be not oriented). Note that responses are faster when attention was oriented on the target location (left panel). However, comparable exogenous cueing effects (facilitation followed by IOR) were obtained independently of whether attention was oriented on the target location or not. Data adapted from Lupia´n˜ez et al. (2004)

location where attention is oriented—left side of the figure, especially at long SOAs). On the other hand, regarding exogenous attention, the traditional time course of cueing effects was obtained, with facilitation at the short SOA and IOR at the longest one. However, the most interesting result was that exactly the same pattern of exogenous cueing effects was observed independently of endogenous orienting of attention. Therefore, our results confirm that, unless under certain circumstances, endogenous and exogenous cueing effects can be completely independent of each other. Although these results might be seen as counterintuitive, similar results have been now obtained with a variety of experimental procedures. Our methods used exclusively peripheral cues, and modulated participants’ endogenous expectancies by varying the level of cue predictiveness. In a later study, we manipulated the two block conditions (predictive and counterpredictive cues) within the same block of trials (Chica and Lupia´n˜ez 2004), and obtained comparable results. Berlucchi et al. (2000) used peripheral cues to measure exogenous attention, and instructions (without additional cue predictiveness) to manipulate endogenous attention. Finally, Berger and Henik (2000; see also Rafal and Henik 1994) used predictive central cues (arrows presented at fixation) to manipulate endogenous attention and peripheral cues to measure exogenous cueing effects. Importantly, comparable results were obtained in all of these studies in the sense that exogenous cueing effects were independent on endogenous orienting of attention. The similarity of the results in the face of important changes in method suggests that the independence of cueing effects due to the cue-target spatio-temporal correspondence from the cueing effects due to spatial expectancies (triggering the orienting of attention) is a robust phenomenon. Opposite effects (facilitation/IOR) of exogenous cueing depending on target demands Further evidence for the existence of cueing effects following peripheral non-informative cues, independently

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of the orienting of attention comes from an experimental series in which we manipulated the nature of the target, within blocks of trials, and obtained opposite cueing effects for different targets. This target manipulation is important because it leaves intact all processes taking place before the target appearance. Being the kind of target manipulated within blocks of trials, the same orienting of attention has to occur for all targets. Therefore, any difference observed between the cueing effects measured for the different targets cannot be explained on the basis of differences on attentional orienting. In one experiment (Lupia´n˜ez et al., submitted, experiment 3), the frequency of the target was manipulated, so that a given target letter occurred much more often than the two other target letters (e.g., the letter ‘‘X’’ appearing on 75% of the trials, compared to the letters ‘‘O’’ and ‘‘U’’ each appearing on 12.5% of the trials). Participants were instructed to make discriminative key-press responses depending on the identity of the letter. The frequency manipulation was introduced to foster the expectancy for one of the targets, the most frequent one. Furthermore, we gave explicit information about the frequency manipulation and we encouraged participants to give priority and be ready to respond specially fast to the most frequent target. As can be observed in Fig. 4, the nature of the cueing effect depended on the type of target that appeared on a given trial. More specifically, we observed qualitatively different cueing effects as a function of the target type. An IOR effect was observed for those trials in which the most frequent letter was presented. In contrast, a facilitation effect was obtained for those trials in which one of the infrequent targets was presented. As stated above, this pattern of results requires an explanation different from orienting processes evoked by the peripheral onset cue because all targets were equated in this regard (all trials were identical until the target appeared). Therefore, the appropriate explanation must consider processes evoked by the target at the moment it appears, which would interact with the processes evoked by the cue in determining cueing effects.

Fig. 4 Cueing effects for the frequent target and the infrequent target observed by (Lupia´n˜ez et al., submitted). Note that IOR was observed for the frequent target, whereas the opposite effect (facilitation) was observed for the infrequent targets. See text for details

Taken together, we can conclude that, apart from orienting attention exogenously, peripheral cues might have additional effects on target stimulus processing independent from the orienting of attention. Thus, the fact that (1) exogenous cueing effects can be completely independent on endogenous orienting of attention, (2) exogenous cues can produce opposite effects (facilitation or IOR) depending on whether target detection or spatial selection processes are tapped by target processing, and (3) peripheral cues produce different effects than central cues on other well-known effects such as Spatial Stroop, has led us to propose the existence of additional post orienting processes triggered exclusively by the presence of peripheral cues that have independent consequences on target processing. In the following section we will describe our event integration hypothesis of peripheral cueing effects.

Cue-target event-integration and event-segregation phenomena contributing to cueing effects Cue-target event integration phenomena Our event integration proposal (Lupia´n˜ez and Milliken 1999; Lupia´n˜ez et al. 2001) borrows from Kahneman et al. (1992) the idea that current perceptual information is integrated with memory representations of prior experience in order to interpret the current stimuli. According to the authors, the onset of a visual object triggers the retrieval of memory representations of similar prior events (object or event file representation). In an exogenous cueing paradigm, cue and target can be considered as perceptual events. Under conditions of enough spatio-temporal match between successive events (i.e., cued trials at short SOAs), the event representation initiated by the onset of the cue may benefit the processing of the target by allowing target processing to be initiated earlier. That is, identifying the target as a continuation of the cue event provides a short-cut to processing of partial target information, such as its spatial information. If cue and target belong to the same

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event, then processing of the spatial location of the cue can be used or borrowed to localize the target, rather than having to compute the spatial location of the target independent of the cue. This effect of advance spatial target information owing to previous onset of a peripheral cue may constitute a benefit to target processing. The existence of spatial cue-target proximity may be crucial for event integration to occur. In this sense, it may occur reliably on cued trials following peripheral cues, where the spatial continuity is maximal. However, perceptual integration following central cues or uncued trials following peripheral cues, would be unlikely to occur, given the spatial discontinuity between the cue and the target in these conditions, and a new object representation must be initiated. By assuming that creating a new object representation takes longer than updating an existing one, facilitation effects produced by exogenous peripheral cues can be easily explained. In addition to spatial proximity, temporal proximity between the cue and the target is also likely to favor the occurrence of cue-target integration (Kahneman et al. 1992). This will be important in coming to terms with the findings of Maruff et al. (1999) who showed the relevance of temporal relations between the cue and the target for obtaining positive or negative cueing effects in detection tasks (facilitation was easily found under conditions of cue-target temporal overlap). Note that, according to this event integration hypothesis, peripheral cueing would facilitate target processing by ‘‘speeding it up’’, a form of target modulation that differs significantly from the one produced by the orienting of attention, that is, by producing its perceptual amplification, as defended by the spotlight and zoom lens theories of attention. This dissociation between the ‘‘attentional anticipation’’ produced by peripheral cues and the ‘‘attentional amplification’’ produced by orienting of attention elegantly may account for some of the results previously described in this review. In this regard, the findings obtained by Macquistan (1997) and Goldsmith and Yeari (2003), where object-based cueing effects were observed following peripheral but not central cues, may occur because peripheral cues trigger the creation of a new object representation, whereas central cues may trigger the creation of an ‘‘empty’’ spatial representation about the target location (see Macquistan 1997, for a similar explanation). Similarly, the dissociation reported by Briand and Klein (1987; Briand 1998; see ‘‘Do peripheral abrupt onset cues and central symbolic cues orient the same attentional mechanism? Evidence for a dissociation’’), according to which only peripheral cues seem to have different effects on conjunction and feature searches, could also be related to our proposal of peripheral cues triggering the creation of an object representation. Thus, the finding of larger cueing effects for conjunction searches, where the creation of an object representation is presumably required (e.g., Treisman and

Gelade 1980), may indicate that a peripheral cue presented at that location initiates the encoding of an object representation in advance, leading to an advantage in performance compared to opposite cued trials. Thirdly, the opposite modulation of central symbolic and peripheral cues over the Spatial Stroop effect described above can also be accounted for by our proposal. Thus, under conditions of peripheral cueing, event integration processes may take place on cued trials at short SOAs. Such phenomena may allow the target’s irrelevant visual location information to be processed faster, by borrowing the spatial processing already initiated by the cue, as compared to the processing of the target relevant direction information, which would be initiated later. The separation in time of these two perceptual codes might result in a reduction of the spatial congruency effect (Hommel 1993a). We believe that this event integration process may occur exclusively when the spatial codes triggered by the cue and the target are both perceptual in nature, thus explaining why the reduction on interference by cueing only occurred for the S–S congruency effects (see ‘‘What’’ is facilitated by the orienting of attention?). In contrast, due to the lack of spatial correspondence following central symbolic cues, the cue representation cannot be integrated with the target representation. In this sense, centrally presented endogenous cues may lead to the orienting of attention toward the cued location and, consequently, to the amplification of the target location information. As a result, the amplified location information would compete more strongly with the target direction information, thus leading to an increased Spatial Stroop effect. A strong piece of evidence favoring the view that it is the cue-target perceptual correspondence that determines the cueing effects different from orienting comes from one important fact. In all the above studies, the dissociations observed between exogenous and endogenous cues could not be explained in terms of differences in the predictiveness between the two types of cues, given the constancy in the behavioral dissociations independently of whether peripheral cues were predictive or completely unpredictive about the target location (Briand 1998; Briand and Klein 1987; Goldsmith and Yeari 2003; Macquistan 1997). Furthermore, in a recent Cueing-Spatial Stroop experiment in our lab (Funes 2004, experiment 2.1), we included a peripheral cueing condition where spatial cues were predictive about the target location. Note that such a condition was similar to the exogenous orienting condition described above in the sense that spatial cues were peripherally presented. On the other hand, such a condition was similar to the endogenous orienting condition given that it provided advance information about the most probable target location. In agreement with our hypothesis, we found the very same reduction of Spatial Stroop as the one obtained following peripheral non-informative cues.

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Cue-target event segregation phenomena The spatio-temporal continuity between the cue and the target seems to be a sufficient condition for the event integration processes to occur. However, when the time interval between the cue and the target becomes longer, event integration processes may be less favored, and instead, a new and separate event representation is more likely to be created with the onset of the second event. The processes involved in encoding a target as a separate event from a preceding stimulus represent the complement of those involved in event integration; here we call them event segregation processes. As we described before, the detection of a target event is often favored when that target occurs at a ‘new’ location in space, perhaps prompting its encoding as a new object or event (Jonides and Yantis 1988; Pollmann et al. 2003; Yantis 1998; Yantis and Hillstrom 1994). However, given the discussion above, it follows that this advantage in processing targets at new locations in space ought to depend on how favorable the cues are in general for processing the target as a new event. Thus, if a target event is presented at a new location, but close in time to an old event, this set of cues would not be optimal for the rapid encoding of the target as a new event. In contrast, if a target event is presented at a new location and its onset is temporally distant from an old event, then this set of cues would more fully foster the rapid encoding of the target as a new event. According to Milliken and colleagues (Milliken 2002; Milliken et al. 1998, 2000), it is in this sense that the inherent tendency of the perceptual system to categorize stimuli as old (and requiring updating) or new (and requiring the encoding of a new event) may contribute to the time course of spatial cueing effects, with IOR usually found only at long SOAs. By this view, IOR can be understood as location alternation advantage, which occurs as a natural outcome of the perceptual system categorizing events as known/old or unknown/new, rather than the product of inhibitory processes that prevent attentional reorienting of the cued location. In addition to producing a detection advantage for new targets appearing at new locations, event integration/segregation processes may produce a cost or disadvantage for new targets appearing at old locations (i.e., location repetition costs). Thus, according to Hommel (1998) and Hommel and Colzato 2004, when two consecutive stimuli repeat or match in some of their dimensions (e.g., color and/or shape) but mismatch in another dimension (e.g., spatial location), large costs are observed to respond to the second stimulus compared to when the two stimuli completely match or mismatch in all dimensions (complete match condition and mismatch conditions, respectively). The authors explained these costs for the ‘‘partial match condition’’ in terms of the second stimulus reactivating or retrieving the representation of the first stimulus on the basis of the matched dimension. However, given the mismatch within the other dimensions, the former representation cannot be

appropriately reactivated. Finally, additional effort may be required to undo the misleading representation and to initiate a completely new object representation for the second event. Such a process may consume more time and resources than the complete match case, where the representation created for the first stimulus can be perfectly re-instantiated for the second event, or a situation of complete mismatch, where the lack of similarity between the two events may lead to the fast creation of a new representation for the second event. Similar explanations based on inappropriate transfer or retrieval processes have also been applied to explain negative priming effects and negative response repetition effects, where the response to a target is largely impaired when the very same response was previously required for a prime stimulus differing in some dimension with the probe (Kane et al. 1997; Lowe 1985; Marczinski et al. 2003; Milliken et al. 1998). We believe that this cost for partial mismatches could also contribute to IOR effects. According to this view, IOR would be due not only to a location alternation benefit for opposite cued trials (as described above), but also to a location repetition cost for the target on cued trials. That is, on the one hand, a long cue-target SOA may favor the processing of a new target appearing at a new location. However, in addition, the fact that the cue and the target are usually different in some dimension such as shape or luminance (e.g. a box flickering cue, followed by a letter or figure target) may make cued trials very similar to the partial match trials described by Hommel. That is, the cue and the target match in one dimension (location) but mismatch in other dimensions (e.g., shape or luminance). Such a perceptual situation, at long SOAs, may lead to inappropriate retrieval processes for cued trials, contributing to IOR. These proposed negative and positive consequences for location repetition and location alternation, respectively, are also in agreement with the results of Taylor and Donnelly (2002), where IOR effects were larger when S1 (cue) and S2 (target) were different in more than one dimension, while no IOR effect was obtained when S1 and S2 were identical. Whether event integration/segregation processes are sufficient to explain IOR effects, or whether other attentional processes that inhibit the return of attention to the cued location are responsible for IOR effects (e.g., Posner et al. 1985), remains unclear. Although most researchers in the field take for granted the original inhibition hypothesis, this hypothesis has not been extensively tested and may require further research.

Top-down modulation of cue-target event integration and segregation We assume that the integration of information across time is a ‘‘by default’’ characteristic of the visual system, thus explaining perceptual continuity in spite of visual changes that do not disrupt spatio-temporal

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correspondence. At shorts cue-target SOAs, the temporal correspondence would favor event integration processes, thus leading to facilitation effects (faster RT at the cued location); whereas, at long SOAs, the lack of temporal correspondence would favor event segregation, thus leading to IOR effects. However, rather than being strictly automatic, these processes are flexible in the sense that whether an old event representation is updated (event integration) or a new event representation is created (event segregation) might depend both on spatio-temporal correspondence and on the attentional set that participants adopt in response to task demands. For example, detection tasks require special sensitivity to the onset of the target. Therefore, when performing a detection task, a set to segment cue and target representations in time, an ‘‘event-segregation’’ set, should rather be adopted, as the integration of two stimuli into a single event representation would make quite difficult the detection of the second of two stimuli (target). In contrast, discrimination tasks require a richer target representation. Therefore, a set to rely on the default tendency of the system to integrate events within a single representation, ‘‘event-integration’’ set, would be appropriate. The differential adoption of the ‘‘eventsegregation’’ versus the ‘‘event-integration’’ sets would easily explain the different time course of cueing effects observed in detection and discrimination tasks (Lupia´n˜ez et al. 1997, 2001; Lupia´n˜ez and Milliken 1999; see Fig. 1). Note, however, that the adoption of a different set for integrating versus segregating events across time cannot explain the results reported by Lupia´n˜ez et al. (submitted; see Fig. 4), where opposite cueing effects (facilitation vs. IOR) were observed for different types of targets. Given that the type of target was manipulated within blocks of trials, the same attentional set must be operating at the moment the target appears, with independence of the type of target that is presented. In order to account for these results, we (Lupia´n˜ez et al., submitted) have argued that a cue can trigger multiple processes (likely in multiple brain areas), each one having either a positive or negative influence on different stages of target processing. As argued above, processes elicited by an exogenous cue might hinder processes required to detect a following target at that location (detection cost). At the uncued location the onset of the target is spatially distinct and therefore would signal the appearance of a new object; a process that would not take place at the cued location, because the onset of the target is not spatially distinct from that of the cue. As a consequence, target detection would be faster at the uncued (new) location than at the cued (old) location. In parallel, the exogenous cue could facilitate spatial selection of the cued object representation, which might be required to discriminate the identity of the target (spatial selection benefit). By manipulating the nature of the target, the relative contribution to performance of these processes (‘‘detection cost’’ versus ‘‘spatial selection benefit’’) is varied, leading to a net

cueing effect on RT to the target that can be either negative or positive.

Conclusions In this chapter, we have reviewed the literature on attentional spatial selection. The evidence indicates that attention may be able to select objects, but the property by which such an object is first selected seems to be its spatial location. However, more research is needed to understand the exact consequences of attention on the processing of spatial information. We have reviewed two forms of attentional orienting through space, an endogenous mode, triggered by informative cues providing advance information about target location, and an exogenous mode, triggered by the salient properties of visual stimuli. These two modes of orienting appear to vary in the time taken to initiate attentional orienting, and in terms of processes related to reorienting away from the cued location when the target is not presented immediately after the cue. However, in principle, the orienting process that is itself triggered by either peripheral non-informative or central informative cues could be one and the same; a unique attentional operator, or spotlight. However, we have described behavioral, electrophysiological, and neurofunctional dissociations found between peripheral and central informative cues, which clearly speak against the view that the same attentional operator is merely driven differently by the two kinds of orienting cues. Instead, the general pattern of results invites an explanatory framework in which there are intrinsically different processes underlying facilitation effects that follow peripheral and central informative cues. One possibility is that the two types of cues tap two attentional mechanisms that are completely different in nature. Alternatively, some authors have proposed that it is not necessary to go quite that far, given that the two kinds of cues could orient the same attentional mechanism, but with additional stages of processing that are unique either to peripheral or central informative cues (Klein and Shore 2000). The finding of some commonalties both in the consequences on target processing, and in the modulation of common activations in the brain, seems to favor the latter interpretation, which is the view that we favor in this review. We have proposed that the main difference between these two modes of orienting attention stems from the fact that peripheral cues trigger additional processes apart from the orienting of attention, due to the spatial correspondence between the cue and the target, which is only present following peripheral cues. These spatial and temporal relations between cue and target represent a ‘‘special’’ situation that is exclusive to the use of peripheral cues and that introduce interactions between the processing of the cue and the target that are not possible in the case of central informative cues. These interactions lead to event-integration and event-segre-

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gation processes that result in positive (facilitation) and negative (IOR) effects of peripheral cueing, in addition to the exogenous orienting of attention. At the same time, event-integration processes might modulate processing in a way different from attentional orienting, thus leading to modulations on Visual Search, Spatial Stroop or Object-Based processing, that are specific to exogenous cueing by peripheral onset events. Acknowledgements This research was financially supported by the Spanish Ministerio de Ciencia y Tecnologı´ a with a research project grant (BSO2002-04308-C02-02) to the second author. We would like to thank the two anonymous reviewers for helpful comments on a previous version of the manuscript. Please direct correspondence to Marı´ a Jesu´s Funes (e-mail: [email protected]) or to Juan Lupia´n˜ez (e-mail: [email protected]), both at the Departamento de Psicologı´ a Experimental y Fisiologı´ a del Comportamiento, Facultad de Psicologı´ a, Universidad de Granada, Campus Universitario de Cartuja s/n, 18071-Granada, Spain. http://www.ugr.es/neurocog/

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