Exp Brain Res (2015) 233:2619–2626 DOI 10.1007/s00221-015-4331-8
RESEARCH ARTICLE
The configural properties of task stimuli do influence vigilance performance Neil R. de Joux1 · Kyle Wilson1 · Paul N. Russell1 · William S. Helton1
Received: 13 January 2015 / Accepted: 15 May 2015 / Published online: 31 May 2015 © Springer-Verlag Berlin Heidelberg 2015
Abstract Sixty-one participants performed a sustained attention task in which they were required to respond to a critical signal requiring feature discrimination. Three separate groups performed the task with different global display configurations. The local feature elements (directional arrow shapes) were displayed on either a circle, a circle broken apart or a reconnected figure. For two of the groups, the entire display consisted of a clear global shape (circle and reconnected), and for one of the groups, the display had no discernible global element (broken circle) despite the critical signal being the same for all the groups. Analyses of hit rate and A′ scores indicated that the broken circle group had impaired performance compared to the global figure groups. A configural superiority effect was found in which performance was improved by having a global shape property to the entire display. These results provide a behavioural base for further research utilizing measures of cerebral activation, as cerebral activity during vigilance tasks may be dependent on both task difficulty and hierarchical aspects of the display. The configurable or hierarchical aspects of vigilance displays may be critical in understanding sustained attention performance and its hemispheric lateralization. Keywords Configural superiority effect · Displays · Sustained attention · Vigilance
Introduction People regularly need to monitor their environments during extended temporal searches for rare or infrequently * Neil R. de Joux
[email protected] 1
Department of Psychology, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
occurring stimuli. Psychologists refer to this process as vigilance or sustained attention. A consistent finding in the scientific literature is that vigilance or sustained attention appears to be right hemisphere cerebrally lateralized. Blood flow and metabolic activity are elevated in the right as compared to the left hemisphere during sustained attention tasks, a finding which has been determined by employing a variety of imaging techniques, including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), transcranial Doppler sonography (TCD) and functional near-infrared spectroscopy (fNIRS) (Berman and Weinberger 1990; Buchsbaum et al. 1990; Cohen et al. 1988; Helton et al. 2007; Hitchcock et al. 2003; Lewin et al. 1996; Parasuraman et al. 1998; Shaw et al. 2009; Stroobant and Vingerhoets 2000; Warm et al. 2009; for a more concise perspective, however, see Helton et al. 2010). Additionally, research with commissurotomized (splitbrain) patients has demonstrated improved performance when signals are presented to the right as opposed to the left hemisphere (Diamond 1979a, b; although see Ellenberg and Sperry 1979). Recent research has also shown associations between reductions in right hemisphere cerebral blood flow with declines in vigilance task performance (Shaw et al. 2013). Nevertheless, the right lateralization of vigilance may require some caveats. For example, the cerebral lateralization resulting from the vigilance task appears to be a function of task characteristics other than prolonged temporal search. Helton et al. (2010) suggested that the laterality profile may be a function of task difficulty, with more difficult vigilance tasks eliciting more bilateral cerebral activity. This finding matches the literature on tasks other than vigilance that shows increased bilateral activity with increased task challenge (Gur et al. 2000; Ferrari et al. 2014). In addition to overall task difficulty, it is also a possibility that
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compositional or hierarchical features of the stimuli or the stimuli’s background may be influential. Visual objects, for example, are ordered in a hierarchical fashion, in which larger objects are composed of a number of smaller features or shapes. Those smaller features themselves may be composed from even smaller elements, and so on. The smaller features of an overall object are referred to as local shapes or features, while the overall object itself can be referred to as a global shape or feature. Local–global feature discrimination, and cognitive processing of such features, has been thoroughly investigated from a perceptual standpoint (Navon 1977; Kimchi and Palmer 1982; Kimchi 1988, 1992; Lamb and Robertson 1990; Pomerantz 1983). The discrimination of local and global visual objects in sustained attention tasks, however, has received relatively little investigation. Research that has been performed in this area has found that the processing of local and global visual objects produces differing effects on both objective measures of vigilance performance and measures of cerebral activity (De Joux et al. 2013; Helton et al. 2009). Vigilance tasks in which a local feature or shape discrimination is required appeared to elicit more bilateral activity than tasks requiring a global shape or feature discrimination. The differences between local and global feature vigilance tasks may occur due to each type of feature discrimination requiring the utilization of separate cognitive resources across hemispheres. This raises the issue of whether feature discrimination in sustained attention tasks is governed by a unitary resource, or whether multiple resource pools contribute towards performance. Given that local and global feature discrimination result in differing responses and cerebral activation trends, these findings may be more in support of multiple resource theory (MRT; Wickens 1980, 1984, 2008; Wickens and Hollands 2000). Studies investigating local–global discrimination using various brain imaging techniques indicate a right hemisphere bias for global discrimination and left hemisphere bias for local discrimination (Lux et al. 2004; Yamaguchi et al. 2000; Van Kleeck 1989). Evidence from a number of studies also suggests that the left and right hemispheres could be considered
Fig. 1 Examples of the visual stimuli
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to contain their own cognitive resource pools (Herdman and Friedman 1985; Friedman et al. 1982; Friedman and Polson 1981). From an MRT perspective, the differences between local and global discrimination during sustained attention tasks found in previous research may be due to local discrimination recruiting a greater overall amount of cognitive resources that are in turn able to supply vigilance performance, given increased bilateral activation during such tasks. This may also account for differences found between local–global perception research and local–global vigilance research. De Joux et al. (2013; in press) found that global feature discrimination displays linear trends (a traditional vigilance decrement), while local feature discrimination displays a quadratic trend in performance, with accuracy and reaction times initially showing a decrement before improving in the final periods of the vigil. From a unitary resource perspective, it could be argued that global feature discrimination is more taxing, therefore creating a more immediate decline in performance. However, this explanation may not quite match the relating literature on local–global discrimination which finds that global features are more readily distinguishable, as well as subjectively easier to attend to, compared to local features (Navon 1977; Miller 1981; Martin 1979; Paquet and Merikle 1984). Although vigilance or sustained attention may often be right-lateralized, this may somewhat reflect object feature hierarchy processes as well, in which many vigilance tasks employ more global-based feature discriminations. This raises the issue of how configural elements of stimuli used in vigilance tasks may influence task performance and cerebral activity. Funke et al. (2010) in a recent study, for example, employed a task requiring participants to monitor four arrows (simulating aircraft) which were orientated in the same direction, with the rare target stimuli represented by one of the arrows being orientated in the opposite direction to the other arrows. In this experiment, the four arrows were placed on a ring shape (see first image in Fig. 1, for example). Funke and colleagues using TCD and fNIRS failed to detect the usual right lateralization patterns associated with vigilance tasks. This has also been noted in other more recent studies using this particular task (Nelson et al.
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2014; Funke et al. 2012). In the context of local–global feature processing, these four arrows may be considered as local features of the global circle shape, a point raised by Funke and his associates. One explanation of the deviant laterality findings with this task is that it evokes local feature processing, therefore recruiting greater left hemisphere or bilateral activity. Indeed, the compositional properties of a vigilance task and how the brain processes the features may be an issue which has been previously overlooked. Pomerantz and associates (1977, 1986, 1989) have previously noted the configural superiority effect, in which stimuli which form a gestalt or whole are processed more efficiently than stimuli which fail to form a gestalt or whole (see also Bennett and Flach 2011). The present study was therefore designed to explore the importance of configural or hierarchical elements of vigilance task stimuli on vigilance performance. The configural nature of the Funke and associates’ task may be revealed by manipulating the global shape while maintaining the local target features which determine a target across conditions and examining the impacts that these alterations in global shape (or configuration) have directly on task performance. Three global shapes were used in the current experiment: a circle, a reversed broken circle and a reconnected shape (see Fig. 1, for example). The circle global shape was chosen as it provided a similar configuration to the Funke et al. (2010) experiment. In terms of local and global configuration, this display may be considered as local features (arrows) on a global shape (circle). The reversed broken circle shape consists of the same overall level of information; however, by splitting the circle, the overall global shape itself no longer forms a gestalt or configurative whole. The broken circle condition may be considered as local features (arrows) on separate objects. The reconnected shape is the reversed broken circle shape which has been reconnected in order to again form a full global shape or configurative whole. This shape may be considered similar to the circle shape (local features on a global shape), but with more spatial extension. Funke et al. (2010) found a decrease in the percentage of hits over time: the vigilance decrement. As their experiment used stimuli which were very similar to the circle condition used in the current research, we expected that there would be a decline in the mean proportion of hits over time for the circle condition. By extension, it was also expected that the two remaining groups would experience this decrease in hit proportion over time. The broken circle shape, however, should exhibit a lower level of performance than the circle and reconnected shapes, as it lacks configurative properties. The broken circle shape should force the search for a deviation among four completely separate features or objects, not local features of
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a global shape, and therefore, should not be as likely to recruit the same neural processing resources as the intact shapes should. Proctor et al. (2004) also noted that signals are more accurately detected when target objects were presented on a meaningful background. It is suggested that the underlying mechanism for this is that different visual processing systems are utilized in the processing of background and foreground visual information (Julesz 1978). While we expect that the circle and reconnected conditions will display higher accuracy due to both shapes forming a full gestalt, we expect that the circle shape will have higher accuracy compared to the reconnected condition, as it is a more common, or more “meaningful”, background object. The current research only examined performance effects, as the potentially important role of the configurative properties or feature hierarchy on vigilance performance needs to be established before incurring the costs of follow-up brain imaging research. If, as we suspect, configurative properties influence vigilance performance, then this may spur further research and theorizing on the underlying brain mechanisms for these effects.
Methods Participants Sixty-one participants (21 men, 40 women) from the University of Canterbury completed the study. All participants had normal or corrected-to-normal vision. Ages ranged from 18 to 49 (M = 21.48, SD = 4.28). Materials The visual stimuli consisted of four black arrows on a white shape, which was centred on a solid red circle. The black arrows act as the local component of the overall object, while the white shapes are considered the global component. The screen position and size (75 mm × 80 mm) of the black arrows were uniform across all conditions, while the white global shape was manipulated. Three manipulations of the white global shape were presented: enclosed circle (circle); disconnected “broken” circle (broken); and reconnected “broken” circle (reconnected; see Fig. 1, for example). There were also clockwise and anti-clockwise versions of the local arrow shapes, which served as counterbalancing measures. The width of the white line was kept the same across all conditions (120 mm), while the overall size of the global objects differed slightly (circle = 10 cm × 10 cm; broken = 9.5 cm × 9.5 cm; reconnected = 15 cm × 15 cm).
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The experiment was performed by participants in groups of 5–10 people in a laboratory setting where each participant was assigned to a separate cubicle workstation. Each participant was randomly assigned into one of the three groups available, and into either the clockwise or anticlockwise version of that group. Participants were shown a brief instructional screen and then completed a 30-s practice period of the task. The task required participants to monitor brief displays of the stimuli and respond whenever one of the four black arrows was orientated in an opposite direction to the other three black arrows. The opposite arrow could occur on any of the four positions shown in Fig. 1. Responses were to be made using the central space bar on a computer keyboard. The red central circle was displayed at all times, while the global shape was displayed for 500 ms, with a 1000-ms interval between displays. It was during this 1500-ms period that participant responses were recorded. There were 120 trials per period, and each period went for 4 min. Participants completed five periods in total. There was no rest break between periods as is typical for vigilance tasks. The overall time including all periods and practice trials was 20.5 min. Distracter and target stimuli were presented in random order with a target display probability of 6.6 % and a neutral display probability of 93.4 %. This probability was consistent between the practice trials and main trials. Immediately following the experiment, participants were debriefed before leaving.
Results For each individual for each period of watch, the proportion of correct detections (hits), the proportion of false alarms and the signal detection theory metric A′ were calculated. A′ is a metric used in signal detection theory to measure perceptual sensitivity (Stanislaw and Todorov 1999). Participants who were unable to detect any critical targets during the short practice phase were excluded from analysis. This resulted in six participants being excluded. Of those remaining, there were no significant differences between groups at the practice stage. A 3 (shape: circle, broken and reconnected) by 5 (periods of watch) mixed analysis of variance was performed for each metric, with preplanned orthogonal polynomial contrasts or trend analyses (see Keppel and Zedeck 2001; Ross et al. 2014; Ruxton and Beauchamp 2008). While repeated-measures ANOVA is more commonly used in vigilance research, orthogonal contrasts are a more powerful statistical test (Rosenthal and Rosnow 1985; Rosnow and Rosenthal 1996; Rosenthal et al. 2000). Specifically, such tests avoid problems related to the assumption of sphericity and are direct tests of trend
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differences (changes over periods of watch) between conditions. For the pre-planned orthogonal contrasts, we limited the contrasts to the linear and quadratic trends. In the case of hit proportions, there was a significant main effect for shape, F(2, 58) = 3.72, p = .030, η2p = .114. For the periods effect, there was a significant quadratic effect, F(1, 58) = 18.06, p < .001, η2p = .237, and for the shape by period interaction, there was a significant linear effect, F(2, 58) = 3.24, p = .046, η2p = .100. Mean proportion of hits are presented in Fig. 2. It is evident here that there is a higher hit proportion rate in both the circle and reconnected conditions than the broken condition. Moreover, the circle and reconnected conditions have similar patterns of change over periods of watch. This was confirmed with a separate analysis of just the circle and reconnected conditions in which there were no statistically significant group interactions, although there were overall significant linear and quadratic effects for periods of watch, p < .02. Analysing the broken condition separately revealed that for periods of watch, there was no significant linear effect, p = .186, nor quadratic effect, p = .053. In the case of false-alarm proportions, there was no significant main effect for shape, F(2, 58) = .25, p = .776, η2p = .009. For the periods effect, there were significant linear, F(1, 58) = 42.61, p < .001, η2p = .424, and quadratic effects, F(1, 58) = 8.71, p = .005, η2p = .131, but neither a significant shape by period linear, F(2, 58) = .50, p = .607, η2p = .017, nor quadratic interaction effect, F(2, 58) = 2.76, p = .071, η2p = .087. Mean proportion of false alarms are presented in Fig. 3. All conditions showed a decrease in proportion of false alarms made over time, with little difference between conditions. It should be noted, however, that the overall false-alarm rate was low for this task. In the case of mean A′ scores, there was a significant main effect for shape, F(2, 58) = 3.32, p = .043, η2p = .103. .80
Broken Circle
.70
Hit Proportion Rate
Procedure
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Joined
.60 .50 .40 .30
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Period Fig. 2 Mean proportions of hits over five periods of watch
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.070
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False Alarm Rate
.060
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.050 .040 .030 .020 .010 .000
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Period Fig. 3 Mean proportion of false alarms over five periods of watch
.920 .900 .880
A'
.860 .840 .820
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.800 .780
Joined 1
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Period Fig. 4 Mean A′ scores over five periods of watch
For the periods effect, there was a significant quadratic effect, F(1, 58) = 9.24, p = .004, η2p = .137, and for the shape by period interaction, there was a significant linear effect, F(2, 58) = 5.26, p = .008, η2p = .153. Mean A′ scores are presented in Fig. 4. Again, it is evident that A′ scores are higher in the circle and reconnected conditions compared to the broken condition. Similar to hit proportion rates, the circle and reconnected conditions display similar patterns over time. The separate analysis of circle and reconnected conditions revealed no statistically significant group interactions. There was a significant quadratic effect for periods of watch, p < .001. The separate analysis of the broken condition revealed a significant linear effect for periods of watch, p = .004.
Discussion As hypothesized, the broken condition displayed an impaired level of performance compared to the circle and reconnected conditions. While false alarms displayed no significant differences between the three groups, hits were found to be at much lower levels in the broken group compared to the circle and reconnected groups. This was also found with mean A′ scores, where although the broken condition displayed an upward trend in accuracy over time it still remained at lower levels of accuracy comparatively. An orthogonal contrast found a statistically significant linear trend for the period by shape interaction. This trend was found in both hit proportions and A′ scores. These results support the claim that overall shape configuration may influence how configural elements of a shape are processed. The period by shape interaction observed in hit rate proportions indicates significant differences between conditions over time, with visual inspection suggesting a vigilance decrement found in the circle and reconnected conditions, and not in the broken condition. It was expected that hit rates would follow a similar pattern to that observed by Funke et al. (2010), in that there would be a decrease in hit rates over time in the circle condition. A linear trend was found in both the circle and reconnected conditions, in which hit proportions decreased over time. A vigilance decrement was found in these two conditions. The broken condition does not display a similar trend. Hit proportions start at a much lower level in the broken condition and remain at that low level over the five periods. It is possible that this is a function of overall task difficulty in this condition. Task difficulty has been found to have effects on both blood flow lateralization and task performance. Higher levels of task difficulty are associated with more bilateral activation (Helton et al. 2010), which in turn may have mitigating effects on vigilance decrement trends. Alternatively, the task may have been difficult enough to have essentially bottomed out in regard to performance. While performance could have in theory even gone lower, most vigilance tasks eventually see performance declining to a stable asymptote (the decrement is a decelerating downward trend for accuracy metrics, for example, the quadratic trends found in the present study). Vigilance theorists have suggested that this performance asymptote may be the point at which the resource requirements of the task for the performance level achieved are matched by the ability of the nervous system to replenish the required resources. It may be that the broken display reached that asymptote very quickly due to its inherent difficulty. The significant linear trend for period by shape found in mean A′ scores again indicates that shape configuration affects performance. All three conditions show an
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initial decrease in performance before improving over the remaining periods. The circle and reconnected conditions do not rise above their initial A′ scores, and overall display impaired levels of performance over time. The broken condition improves overall from its initial score, indicating an improvement in performance over time. Despite this improvement, the broken condition still performs worse than the remaining two conditions throughout the five periods. These findings are due to a lack of change in hit rates over periods, but a decrease in false alarms over periods for the broken circle group. Again, the performance improvement observed in this condition could be in part attributed to a hypothetical increase in bilateral activation with higher task difficulty. As suggested above, increased bilateral activity may increase the overall resource capacity for participants in this group, resulting in a lack of a clear decrement pattern. Indeed, this may have resulted in the A′ increase noted for this group. Increased cerebral activation has been found to predict improvements in performance in perceptual learning experiments (Ong et al. 2013), and other researchers have noted concerns regarding the impact perceptual learning may have on vigilance tasks (Head and Helton 2015; Szalma et al. 2004). It was expected that the circle condition would have the highest level of accuracy, followed by the reconnected condition, and the broken condition performing the worst. The results revealed that the broken condition was indeed the worst performing of the three conditions, while the circle and reconnected conditions exhibited similar levels and trends of accuracy. The comparatively poorer performance in the broken condition is attributed to the split or breaks between the separate components of the shape, which in turn may cause the object to be processed as four separate local objects, as opposed to a singular overall global object like the circle and reconnected conditions. Of particular interest is the finding of the reconnected condition performing at a similar level to the circle condition. The expectation that the circle condition would display improved performance when compared to the reconnected condition was due to the increased spatial extension that the reconnected group displayed (broadening possibly the focus of attention), as well as the circle condition having a more common or “meaningful” shape (Proctor et al. 2004). Additionally, Julesz (1978) suggests that different processing mechanisms are used in the processing of background and foreground information. The reconnected shape was also extremely similar to the reversed broken circle shape in regard to the positioning of the breaks in the global shape. While the current findings are not necessarily what was hypothesized, they do somewhat fit with the aforementioned research, given that these two conditions clearly display higher accuracy compared to the broken condition. Perhaps though, instead of background and foreground
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processing mechanisms being utilized, there is another mechanism that is utilized when the background shape forms a full gestalt. The lack of a significant difference between the circle and reconnected conditions does support Pomerantz and associates’ (1977, 1986, 1989) configural superiority effect, in which objects which form a whole are easier to process than those which do not. As a result of this configural superiority effect, the broken circle task without a clear global aspect was more difficult for participants to perform compared to the global aspect tasks (circle and reconnected conditions). Based on these interpretations, two extensions of the current research should be undertaken. Firstly, the current paradigm should be used in conjunction with the utilization of measures of hemispheric activation (brain imaging), which will examine whether the behavioural patterns found here correspond with patterns of cerebral activation. The current investigations’ findings suggest that there are differences in activation during the task. A measure such as fNIRS (de Joux et al. 2013; Ong et al. 2013; Helton et al. 2007; Hitchcock et al. 2003; Parasuraman et al. 1998) which has been utilized in similar vigilance studies in the past would be appropriate for such an investigation. Secondly, the effects of task difficulty should be more fully explored. This may be possible through the inclusion of a transition between conditions while using the current paradigm. If task difficulty is increasing bilateral activation, a transition from or to a more difficult condition should in theory yield a number of performance changes. A transition effect between local and global processing has been examined using simple Navon objects (de Joux et al., submitted for publication) with findings suggesting performance changes between local and global processing; however, this task was of a relatively low difficulty. The use of a transition in a much more complex object, however, has not been examined. In addition to this, further investigation into the relationship between local–global configuration and task difficulty should be explored. It is unclear from the current investigation whether local tasks are intrinsically more difficult tasks than global tasks, which may also be a factor in the results. Previous research suggests that local and global feature discrimination do not significantly differ in subjective difficulty (de Joux et al. 2013), again; however, the stimuli in the current paradigm appear to be more complex, which may yield different findings when compared to investigations using relatively simple stimuli. Further selfreport measures of perceived workload and effort should be included along with objective measurements to explore these relationships. The research aim was to investigate whether global shape configuration has an impact on the performance of a vigilance task, with the intent of establishing whether any behavioural differences found warrant further investigation
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utilizing measures of cerebral activation. The results found in the current study suggest that global shape configuration does impact vigilance task performance. The configural superiority effect observed here means that the hierarchical or configurable properties of vigilance tasks may be important aspects of understanding vigilance performance and, plausibly, its cerebral activation patterns. The results found also suggest that overall task difficulty may be influencing patterns of response and that this is in line with previous research involving measures of cerebral activation. The results of the current research seem to suggest that the disparity between conditions is due to differences in local– global object processing, differences in task difficulty, as well as the processing demands of sustained attention tasks. These results warrant further investigation using measures of cerebral activation. Some of the atypical cerebral activity studies of vigilance tasks, in which right lateralization is not found, may be due to these tasks using more complex stimuli than those standard in more common laboratory studies of vigilance (which usually use very simple stimuli). More research on the hierarchical properties of vigilance displays and their impact on both performance and cerebral activity are warranted.
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