Biological Cybemet

Biok Cybernetics38, 75~83 (1980)

9 by Springer-Verlag 19~

Extraction of Objects from Structured Backgrounds in the Cat Superior Colliculus. Part II* G. Fr/Smel Arbeitsgruppe III (Biophysik),Institut fiir Zoologie,Johannes-Gutenberg-Universiti4tMainz, FRG

Abstract. Specific changes occur in the cells of the upper layers of the cat's superior colliculus when a two dimensional noise (background) is superimposed onto a deterministic signal (spot of light). Some of the measurements can be interpreted as meaning that some cells only react to certain relative movements of object (spot) and background (noise). The movement of the visual background is interpreted as environmental movement occurring due to the animal's own movement. The results of the measurements provide all the necessary presuppositions for a distinction between the animal's own velocity and that of the object (Part I). The experimental results can be interpreted with a model. The essential factor for the interpretation is the direction specific behavior of the cells which is bound up with an asymmetrical spatial coupling of the neurons with each other. The decisive advantage of asymmetrical systems for the pattern recognition of moving objects is that they can work without distortion and spatial displacement over large ranges of velocity (Part II). 1. Simulation of the Experimental Results in a Model System

The superior colliculus of the cat is part of a system, which controls coordinated eye movements necessary for spatial orientation. For this purpose, a "localization" of moving stimuli must take place, The parameters required for this task are : the size of the object (to a certain degree also its form), its position relative to the animal, the velocity of the object as well as its direction of movements with reference to the animal. These parameters have to be determined accurately for the sake of establishing an exact reaction (e.g. movement). A possible realization is a "situation-specific" tilter, which extracts a certain combination of parame* This research was supported by DFG Grant Se 251/7. Prof. Dr.-Ing. W. v. Seelenwas in charge of the project

ters and transmits them for further evaluation or for direct localization. Due to temporal asymmetry in the time domain (causality), symmetrical spatial coupling (e.g. as in the retina) shows distortions in the spatial domain, so that pattern recognition is rendered difficult. Moreover, due to the temporal properties of the coupling, the determination of where a moving object is located is subject to a systematic spatial error, which is dependent on velocity. It will be shown, how these drawbacks of symmetrical (directionunspecific) spatial coupling can be circumvented by asymmetrical spatial coupling necessary for producing direction specificity. Directionspecific responses can be generated by largely asymmetrical spatial coupling, combined with the causality dependent inherent asymmetry of the time-dependent parameters. Spatial and temporal asymmetry can become effective in the same direction, thereby increasing the direction specificity, or they can, within a certain range of velocity, produce opposite effects, thus decreasing the specificity. In their latter combination they are applicable as equalizing networks to the detection of moving patterns. The systems being largely linear (cf. Part I, Sect. 3), the veiocity-dependent increase in amplitude observed arises from frequencydependent phase shifts, which means that emphasizing parts of the pattern is generally asscociated with their distortion (Fr6mel, 1977). Asymmetrical spatial couplingccan be caused by: 1. 2. 3. 4.

reciprocal shifts of He(r, s) and H~(r, s), coupling which is only unilaterally effective, different coupling widths in opposite directions, continuously changing coupling widths

B = B(r, s).

From the neuroanatomical point of view, spatial asymmetries can be interpreted, for instance, as an dendritic growth : 0340-1200/80/0038/0075/$ 01.80

76

selves, spatially weighted in accordance with their distance from one another (1), show in addition a temporal behaviour, which in its simplest form, can be described by a first-order low-pass falter. As a result one obtains the complex spatio-temporal spectrum

klFI sl,,t,,I

3~(H2(r, t))=

iiiiiiiii!iii::i

2mlB1 (1 + u2B~)(1 +jcoT1)

iiiiii::". ~"

2mzB 2

:~11'....:~,-

(3)

(1 + u2B~)(1 +jcoT2) '

J;-;;3.~-r, t Fig. 1. Qualitative curve of the space time frequencycharacteristic plotted over the u - m-level.The curvesof [,~(H(r,t))[are shownonly for u=0 and co=0. The effectivefrequency characteristic for a certain velocityv can be read off from the respectivevelocityvector

a) Growth of excitatory or inhibitory dendrites in the same direction (Sect. 1.2) or in opposite directions (Sect. 1.3). b) Symmetrical growth of the excitatory and inhibitory dendrites, the centers of the dendritic trees being shifted reciprocally (Sect. 1.4). c) A "dendritic tree" has a different size in its various directions. d) The above-mentioned situation arises, when the stimulus passes through receptive fields of different sizes (inhomogenlty). The simulations according to items 3. and 4. do not help to interpret the experimental data and will, therefore, not be considered any further.

1.1. Formation of Distortions in Moving Patterns For the sake of a simplified mathematical description, a different coupling function curve that in Eq. (I.1) will be chosen. The qualitative differences regarding the system are negligible. Spatially symmetrical receptive fields show an entirely real behaviour in the frequency domain. For a one-dimensional spatial coupling of the form

H 2(r) = ml e- I,I/B,_ m2 e- Irl/B2

(1)

co representing the temporai frequency and T1,2 the time constants of excitation and inhibition. These systems have a frequency-dependent phase angle, which brings about the distortion of the patterns. The two-dimensional spatio-temporal frequency response (3) can be mapped as a mountain mass above the u-co-plane. When moving a pattern at a constant velocity v, the effective transmission function lies above the straight line co = u- v

(Fig. 1) because of the well-known relation

.,~:(y(r + vt)) = Fr(y(r)). 6((o - uv).

2mlB1

y(r + vt) ~ 2a. H2(r).c I

1 + u2B 2"

(6)

on the condition that the stimulus size is sufficiently small and the velocity sufficiently high

a
(7)

respectively

v~A.BJT i

(8)

if the error due to the coupling function is determined by

~-

/ ~ ) ~ (Hz(r)-y(r+vt))2dr 7~-~ - -

J-V

2maBz 2

(5)

If the velocity v chosen is sufficiently low, the straight line (4) approaches the u-axis. Thus, by slowly moving patterns it is possible to approximate the temporally independent spatial frequency response, provided the size of the pattern (in general, the bar width 2a of an optical stimulus) is sufficiently small. The temporally nearly independent output of the system is given by

one obtains the Fourier transform ~-(H(r)) - 1u +2 ~

(4)

J~ H2(r)2dr

]

< A.

(9)

r=-~

(2)

r denoting the spatial coordinate, B1, 2 the coupling width of excitation or inhibition, u the spatial frequency and mr, 2 an congtant. At first sight, the disappearance of the phase angle seems advantageous, especially since this allows a simpler mathematical approach. The coupling of the neurons among them-

The result of an equivalent measurement is shown in Fig. 2. Despite a complex (temporal) behaviour of the systems, an adequate distortion-free transformation can be made, if the patterns are moved at a sufficiently low velocity v only. The measuring directions, as described above, can turn out to be contradictory, when applying this

77

method to neurophysiological experiments. On the one hand, according to (7), the stimulus is required to be sufficiently small; on the other hand, for reasons of available energy (6), an output can in this casetimes no longer be observed. On the one hand, the velocity v is to be as low as possible (8), on the other, as is well-known, the neuronal systems under investigation no longer respond in such cases. Consequently, as far as the error (9) is concerned, an optimal stimulus size and an optimal velocity must be chosen for every cell. When in doubt, the velocity v rather than the stimulus size 2a ought to be increased to obtain a measurable output, since the velocity has a less significant impact on the error accruing from the measurement Due to the inherent asymmetry of the time domain (causality), phase shifts of spatial frequencies occur causing a deformation of the moving patterns. If the velocity is low enough (8), these phase shifts can be neglected. The results obtained for a symmetrical, on-type receptive field at different velocities v are shown in Fig. 3. Better responses (regarding distortions and resolving power) are produced by receptive fields with an on-off characteristic and given certain combinations of parameters. Both types are, however, directionally unspecific. In addition, when the velocity is increases, the maximum of the output value lags behind that of the input value, Le. an error occurs when determining the position of moving patterns (Fig. 4). In order to meet the requirements of Sect. 1, it is necessary to compensate for the inherent temporal asymmetry by a spatial asymmetry. Different approaches exist to produce such a spatial asymmetry, by means of which a compensation can be achieved under different conditions.

!~1-1

I

spot

rs

m~ n value

-800

0

I *800

= r/turn

0

-800

Fig. 2. P S T H for stimulation with a very slowly moving bar of fight. Taking the mean value of the histogram as the zero line the result is a field size of 385 m m which approximately agrees with that ascertained with stimulus by hand. The inhibitory parts can be clearly seen on either side of the m a x i m u m s

1 o r,o Yt)(

.'"

i v=lO0

--ON

I

" i

v .-

."

Ftg. 3. Transformation of a bar of light (length 2 a = 2 0 ) moving to the left at a velocity of v = 10 and v = 100 respectively. The on-off system has a considerably higher spatial resolution capacity than the on system at v = 100. The amplitudes are standardized

1.2. Equalization of Movement by Unilateral Spatial Coupling The temporal asymmetry of an asymmetrical spatial coupling brings about distortions of the spatially dependent output values. By asymmetries in the spatial frequency domain such distortions can, at least in part, be counter-balanced. For systems described by

G(r, t):

y(r.vt) -v=5

iT,

m, ) T6 e-Irl/",-t/r, . (1 - o-(r))

(10)

(Fig. 5) or their Fourier transform &~-(H3(r, t))=

-1~0 '

m6B6

i

I

J riB1

msB5 (1 -juB)(1 +jo) Ts) - (1 -juU6)(1 +jcoT6)

J

(10

Fig. 4. Change in the form of the output caused by velocity. Shift of the output with respect to the stimulus deplacement for the same system as in Fig. 3. The distortions and the deplacement increase proportionally to the velocity v

78

which are used for the generation of a direction specificity, a velocity-dependent equalization can be accomplished. Given the velocity vm=B/T for each individual system and given a moving coordinate system, one obtains as output value

H3(r) i H4{r)

(/t3(r, t/)= \(l+u2B~) "-o,. -10

H3(r)

on--~l , ' " ' ' ~ I

""

10 r/B 5

Ha(r)

Fig. 5. One-sided asymmetrical spatial coupling [H3(r), excitation and inhibition] for eqnaliring movement

(l+u2B~))

the relations in the time independent case. The Fourier transform is a purely real value; distortions can no longer occur. The exponential function is due to the movement of the patterns. It has no influence on the form of the output. Following adjustment of the velocity, the phase of the system disappears, so that no matter what moving input is used, it will .be processed without temporal distortion (Fig. 6). The deformation of the input by the spatial filter itself is preserved. Consequently in spite of the movement of the optical stimuli, the distortions caused by that movement will disappear. Given an onsystem, the condition K = L and K > 1, L > 1 or

v~= Bl/T~

(13)

y(r+vt)

v=257~I I

I

I

|,

i

~

-10

I

~' ~

must be valid for all individual systems. The distortions increase with increasing upward or downward deviation from the adjusted velocity. An additional off-system (Fig. 5), similar to that described in I, Sect. 2.1 by

v=25 100

0 rib 7

Fig. 6. Output of the system in Fig. 5 at the adapted velocity v. = 25 and at v = 100. The maximum are standardized. The curve of H2(r) is obtained directly at v = v~ = 25, other velocities lead to distortions which are considerably lower than those in Fig. 4

=pike= . r I 300.

2OO"

100

H,,(r,t)=

e Irl/~7-t/r7 m8 e-Irl/Bs-t/Tel" if(r) / r8

brings about an equalization of the system, which, within a considerable range, is independent of velocity (10). The output of the off-system disappears at the adjusted velocity of the on-system. For v < v a one obtains predominantly positive output values, for v > v= the sign is reversed, so that the superposition can produce a non-overlapping asymmetrical on-off system equalized largely independent of velocity. It follows that ~a~-(v(r+ vt))l. . . . ~ 3~-(H2(r)). exp (juvt)

o

16o

~

~o

--,"

v/Or1

,~------ O

Fig. 7. Velocity dependent responses of two cells in the superior colliculus of the cat. Completely direction specific cell ( - - ) , partially direction specific cell (. . . . . ). The larger response evoking velocity range is the result of the larger field diameter

(14)

(15)

Le., temporal and spatial asymmetries are compensated in such a way that a phase-free or an undistorted value is produced, which is constant within a large range of velocities. Only at

v>6BJT~

(16)

does it drop to half its maximum value. Moreover, it does not show any shift (Fig. 6).

79 The above considerations are valid only for stimuli moved in the direction of negative spatial coordinates, they are not generally applicable to stimuli moved in the direction of positive spatial coordinates. Here, the usual distortions occur, apart from a few exceptions. The advantage of equalization by asymmetrical coupling is linked to the condition that, at the same time, direction specificity becomes necessary (Figs. 7 and 8). A velocity-dependent increase in amplitude on the part of the output signals is, of necessity, associated with a deformation of the pattern processed. Consequently, the preferred direction of movement (defined as the direction, in which a moving stimulus produces the higher output value) is equivalent to that, in which the input is deformed dependent on velocity. If the stimulus is moved in the opposite direction, it will, to be sure, be transformed without distortion, but, on the other hand, it will produce a smaller output value. Therefore, the mostly asymmetrical or directionspecific collicular fields would lend themselves to an equalization of movement. Due to the necessary condition K > 1 and L > 1, the configuration described can also produce a relatively velocity independent processing in the pattern recognizing areas of the cat visual cortex. Thus, pattern recognition does not absolutely require a stationary (apart from the tremor) retinal image; even while displacements during pursuit eye movements occur, pattern recognition can prove to be satisfactory within ranges of velocity adjusted to the system. The variety of potential velocities in biological systems is masked by the variation of the connecting parameters.

1.3. Equalization of Movement by Bilaterally Asymmetrical Coupling

Hs(r, t) = m9 e-Irl/B9-t/rg. if(r)

T9

i%

- a(r))

(17)

(lS)

~(H~(r)) characterizes the Fourier transform of a symmetrical spatial coupling, similar to (2) ~(Hs(r))_ 2m9B9 1 + u2B z"

0

10

100 v/Bi.s-1

50

iii

Fig. 8. Velocity dependent response of an on system. The degree of direction specificity agrees with the neurophysiological findings dependent on the velocity v

IH5(r)

r/ B190tr

-10

Following backward transformation in the temporal domain, a bar of light of the width 2a produces the output from

~(y(r + vt)) = ~a~r(H~(r))~,~(x(r))

(Fig. 9), one obtains for the spatial domain with B9=B~o and T9= Tio

juB 9 ~.~(Hs(r, t)) = F(H~(r)) 1 +jogT9"

y(v)

Flg. 9. Two-sided asymmetrical spatial coupling Hs(r) for equalizing movement

ff the coupling function is given by

ml~ e-Irl/m~176

l

(19)

-

juB 9 exp (]uvt). 1 +juvT 9 -

(20)

If the stimulus is moved at a high velocity in the direction of negative spatial coordinates, (20) can be expressed more simply as ~r(Y( r + ?)t))[v ~,1

J~(H~(r))..~ (x(r)) ~ - exp (]uvt) v19

(21)

80 ly(r. vt)

lilT(r) ]~v=

1000

v= 25,100,1000/ ~/~--v=100 [[1[

__J I 'C

lO

I ~

riB9

Fig. 10. Outputs of the system in Fig. 9 for various velocities.The result resembles that in Fig. 6 except that the distortions only disappear at higher velocities.The maximums are standardized

9176 1I

1

"1

l

-10

i

10

r/Bi2

H6(r,s) ~ ~ o~

/

A

r~.

0

Fig. 11. Asymmetricalspatial couplingH6(r , s) for producingpattern recognition largely flee from distortion and direction specific behaviour without displacementof the output

Fig. 12. Shift of the center of the intfibiting with respect to the stimulating spatial coupling and view from the top of the receptive field. B~2=l , K=2, ro2=2

(Fig. 10). As output value of the asymmetrical system, one thus obtains the input value filtered by a symmetrical coupling and weighted by 1Iv. If, however, the input is moved in the direction of positive spatial coordinates, i.e. in the opposite direction, one obtains, due to

A similar behaviour is shown by a coupling of the form

a~(v(r + vt) =

~ ( H ( r , t ) - ~ ( x ( r ) ) a ( c o g- uv)

(22)

(23)

If a threshold exists at the output of the system, one obtains an output value only when there is movement in one direction. In such a case, the system is absolutely direction-specific. Moreover, it maps the input without deformation (except for the filter processing of the symmetrical spatial filter) and without delay, i.e. it does not lag behind the input. Given moving stimuli that are sufficiently slow (Sect9 1.1) the curve of the asymmetrical spatial couplhag (17) is approximated.

(24)

L1 (Jensen, 1978) (Fig. 11) or with F(H~(r)) according to

(19) Jir(H6(r , t)) = ~ Z ( H (r]) juB l l . . . . . (1 q-jcoT11)m11"

an output value with a reversed sign. y(r =- vt) = - y(r + vt).

H6(r, t) = ml 1. r. e-E,Im. -~/rll

(25)

A comparison with a coupling according to (18) shows a filter with a narrower bandwidth and a steeper flank, but with the same maximum value. Qualitatively, there are no differences.

1.4. Equalization of Movement Shifting the Centres of the System Another possibility of generating spatially asymmetrical coupling consists in a displacement of the centres of

81 excitatory and inhibitory coupling. With a coupling of the form

HT(r, t)= m12

e-lr+roll/Bla-t/TJ2

/?'/13 (26)

9e-Ir+,o21/B,3-t/r~3

(Fig. 12) and K = 1 one obtains

~:r(Y{r+vt))~(x(r))~'~(Hs(r))l(l +j~VTlz

( r0x

+ 1 + ~ - r 1z

1~ 3

1+j~vT13 ) '

If, in addition, L = 1, roE>0 and ro~ = -ro2 as well as a high velocity v are selected, the complex term is eliminated and the relation acquires the form

~,(y(r + vt)) ~ .~(H~(r)).~r(x(r)) ~ - ej~t Vllz

(28)

similar to the systems described in Sects. 1.2 and 1.3. As in the preceding expressions, the exponential function characterizes only the movement of the pattern or that of the output value.

1.5. Determination of Relative Velocities If different patterns pass through a receptive field, the output values are superimposed in accordance with the properties of the system. Due to the largely linear behaviour of the systems, except at their threshold (cf. I, Sect. 3.1), linearity will, for the present, be assumed. Given a coupling function according to (17) [or (24)], one obtains for two different patterns x 1 and Xz, which move at high velocities v 1 and v z in the same directions

f f r(Y(r + Vt)) ~ ~ " ~:(Hs(r))

Fig. 13. Input couplings of the superior colliculusfor simulation of the findings in I, Figs. 8-10. Hi(r, s, t) represents the direct retinal input with symmetrical receptive fields, Hs(r,s,t) the direction specific spatial coupling which inhibits the collicular cells

If the directions of movement are the same, the terms of both patterns add up in the output value. If the directions are opposed to each other, the difference of the inverse velocities weighted with the spatial filter of the patterns is established (relative velocity). If the temporal frequency response is a high-pass filter (je)), compensation can be achieved by a spatial low-pass filter (1/ju). In this case, according to (29)-(31), the output value is multiplied by v instead of by 1/v. The same effect is achieved, with good approximation, by a temporal band-pass. In this case, one obtains for small values of v (1/{ l + juvT} ~_1-juvT) a response proportional to the velocity and for high values of v a drop in the response, depending on 1/v. This relationship is in agreement with neurophysiological findings. One obtains accordingly at low velocities of vl, v2 for a temporal low-pass filter.

ze

(%v,-}-0~2/)2)

y(r -k-vt) ~ B9 -(rl + vlt)/B9 (29) Considering the situation, in which both patterns moved at vl and v2 are superimposed in the centre of the receptive field (experimental situation: rl+v~t =rz+Vzt), one obtains

y(r + vt) ~ -B9 - e -(,1 +vl0/B9[~1 + ~2] T9 [vl ~22 "

(30)

Where cq and ~z represent the input values x~ and x 2 weighted with the spatial filter H,(r). If pattern x: is moved in the direction of positive spatial coordinates (i.e. into the direction opposed to that of pattern xl) , one obtains, in analogy to the above with

y(r, vt) ~ T99

(32)

an evaluation of the relative velocity, which is proportional to the velocities used. In this way, the experimental results presented in I, Figs. 8, 9, and 12 can be interpreted by means of the schematic coupling diagram in Fig. 13. Retinal coupling is spatially symmetrical. Therefore, the stimuli of an area with a small surface in comparison with the field center are transformed in a directionally unspecific manner. The generally direction-specific cells of the visual cortex have, in the presence of a threshold, a mostly inhibitory effect on collicular cells (Hoffmann, 1973). Particularly to small stimuli, they thus produce direction-specific responses in the colliculus, which should be opposed to those given by corticotectal cells. This problem has, so far, not been clarified.

82

180"

90 ~

//

270"

O' 360"

Fig. 14. Differentdirection characteristics of a cell when measured with noise moving in a circle (cs I, Sect. 3.1) and a spot moving rectilinearily. Unlike I, Fig 4, where only a very small output resulted when the noise was used, a change in the direction characteristic occurs here in addition. This cell shows different direction characteristicsfor small stimuli and for large surfaceones Most of the receptive fields in the superior colliculus possess a surround, which is located asymmetrically with regard to its centre (Dreher and Hoffmann, 1973). Therefore, this surround shows a directionspecific response to moving stimuli of large-surface area (Wickelgren and Sterling, 1969). Responses to small-surface stimuli are not modified by the surround. By using large-surface patterns a decoupling of the subsystems field centre and surround (or of the input from object and background) can be avoided. Accordingly, inhibition of the activity of the superior colliculus in response to a moving object is produced by the visual cortex, while inhibition as a response to a moving background is produced by the inhibitory surround. Due to the different mechanisms for their generations the direction-specificities of the visual cortex and of the inhibitory surround are of a different nature. Therefore, the measurement of directionspecificities produces different results, when using moving spots and circularly moving noise processes (Fig. 14). Measurement of the relative velocity and of the direction-specific suppression of the background is thus achieved by the non-linear coupling (threshold) of two asymmetries, which, at a given time, possess a different filter behaviour. In Part I, Fig. 9, an almost exclusively directionspecific response of the efferent cortical cells can thus be

observed; as a result, an almost exclusively directionspecific cell is also obtained in the superior colliculus. If a spot and a large-surface stimulus are moved simultaneously, the asymmetrical surround in the superior colliculus is also activated and suppresses the direction-specificity of the cell. The different types of direction specificities necessary for detection of the different types of movements performed by object and background are obscured by the variety of biological systems.

2. C o n c l u s i o n

The effect of a structured background projected simultaneously with a spot of light on the activity of single cells in the colliculus of the cat was studied. The background was stationary or moved with the spot; it could have different space and time spectrums. The form and the degree of direction specificity of the output essentially depend on the background in this case. In the experiments the background: 1. exercized no influence, 2. superposed its effect linearly on that of the spot, 3. evoked a dominant (positive) response, so that a spot moving simultaneously with the background could not be "recog-niTed", 4. had only an inhibitory effect (even when the background moved a little), 5. caused the relative velocity of object and background to be evaluated or respectively caused an output only when there was relative movement between the object and the background. When the background alone is moved (without simultaneous movement of the object) some cells of the classes 4 and 5 can react like class 1 cells. Thus a combined stimulation is necessary for a complete system analysis. The individual findings are not distinctly separable but rather the transitions between them are gradual. The results can be interpreted in terms of system theory as well as in terms of the conditions in the biotope. In the first case the output is determined by the system's fdter characteristics which can have a velocity dependent, direction specific inhibitory effect. The results can be simulated with a model which evaluates the difference in velocity between moving patterns. In addition these models have the advantage of compensating the distortions caused by the velocity of the transformed patterns in symmetrical neuronal systems as well as of enabling the position to be determined more exactly. In the second case the findings in the biotope can be interpreted as certain systems performing different partial functions

83 o f o r i e n t a t i o n in a m o v i n g e n v i r o n m e n t . I n this case t h e b a c k g r o u n d is i n t e r p r e t e d as e n v i r o n m e n t d u e to its s p e c t r a l features w h i c h m o v e b e c a u s e o f t h e a n i m a l s own movement. T h e results i n d i c a t e d fulfill all p r e r e q u i s i t e s for a d i s t i n c t i o n b e t w e e n self m o v e m e n t a n d m o v e m e n t o f a n object.

Acknowledgements. I would like to express my thanks to Prof. Dr.Ing. W. v. Seelen for many valuable discussions and to Prof. Dr. K.-P. Holtmama for his helpful comments.

Hoffmann, K.-P. : Conduction velocity in pathways from retina to superior colliculus in the cat: a correlation with receptive field properties. J. NeurophysioL 36, 409-424 (1973) Jensen, J. : Untersuchung zur Formerkennung im visuellen System der Katze (Clare Bishop Area) mit statistischen Analysemethoden. Bericht zum Forschungsprojekt Se 251/9 im Rahmen des Schwerpunktprogramms ,,Neuronale Mechanismen des Verhaltens" (1978) Wickelgren, B.G., Sterling, P. : Influence of visual cortex on receptive fields in the superior colliculus of the cat. J. Neurophysiol. 32, 16-23 (1969)

References

Received: January 18, 1980

Dreher, B., Hoffmann, K.-P. : Properties of excitatory and inhibitory regions in the receptive field of single units in the cat's superior colliculus. Exp. Brain Res. 16, 333-353 (1973) Fr6mel, G. : Neuronal network characteristics in the cat superior colliculus. Biol. Cybernetics 28, 1~26 (1977)

Dr. Ing. G. Fr6mel Armsener Strage 35 D-2800 Bremen 44 Federal Republic of Germany