Thus, the color space in monkeys and fish consists of a hypersphere in four- ... the identification of the perceptive brightness space in monkeys, using training to.
Neuroscience and Behavioral Physiology. Vol. 28, No. 3, 1998
B R I G H T N E S S PERCEPTIVE SPACE IN M O N K E Y S (RHESUS MACAQUES)
D. V. Evtikhin, A. V. Latanov, and E. N. Sokolov
UDC 612.843.3+612.846+612.821.6
Operant differentiation was used to study black~white stimulus intensity discrimination (over the range 1-37 cd/rn2; CIE-31 coordinates X = O.340, Y = O.304) in two monkeys (rhesus macaques). Confusion matrices were constructed from the probabilities that animals would make operant responses when required to select from pairs (a conditioned stimulus and one of 9 differentiation stimuli) of stimuli in 10 series of experiments with conditioned stimuli of 10 intensities. Correlation matrices for vectors consisting of stimuli in the confusion matrices were assessed by factor analysis for identification of their intrinsic vectors. A perceptive space of two-dimensional structure was obtained for the brightness of black~white stimuli, and the main characteristics of this were similar to those of the analogous perceptive spaces for humans, fish (carp), and rabbits. The coordinate axes of this space can be interpreted in terms of excitation of two intensity-encoding channels: light and dark.
A number of reports have demonstrated that perceptive spaces for stimuli discriminated in terms of color tone and brightness can be constructed using direct subjective assessments of interstimulus differences in humans [3, 7]. A perceptive color space of animals, like the brightness space, can be constructed using the probability of obtaining a conditioned reflex response during training to differentiation, an analog of the stimulus categorization method used in humans (the color name method) [7]. Color spaces have been constructed in monkeys (rhesus macaques) [4] and fish (carp) [5] using the probability of conditioned reflex responses during training to color differentiation. The experimental approach was based on alternation of the reinforced color stimulus with a number of differentiation stimuli presented sequentially [4] or simultaneously in pairs, using a selection method [5]. When random errors (responses to nonreinforced stimuli and lack of responses to signal stimuli) are made, the color which is reinforced in a given series of experiments is characterized by a vector derived from the probabilities of responses to all the stimuli used. Different colors were reinforced sequentially to generate a probability matrix for conditioned operant responses (a confusion matrix), and this was used to calculate matrices of the correlations between vectors representing the colors. Factor analysis identified four factors making up the color space in animals (these are the coordinate axes of the color space) were interpreted by the authors as neuronal channels encoding color differences: opponent red-green ( R + G - ) and yellow-blue ( Y + B - ) axes, along with a bright and dark axes (B- and D-neurons respectively, according to the Yung classification [11]). Thus, the color space in monkeys and fish consists of a hypersphere in fourdimensional Euclidean space. The three projection angles for different planes in this hypersphere corresponded to the psychological characteristics of color stimuli: color tone, lightness (subjective brightness), and saturation [7]. Data obtained for color differentiation in fish [5] and monkeys [4] showed that subjective brightness in these animals is represented by an excitation vector whose components correspond to the level of excitation of the bright and dark channels. Experiments on fish (carp) [2] and rabbits [6], also using differentiation, were used to investigate the discrimination of stimuli differing only in terms of brightness. The results of these studies showed that the brightness space in fish and rabbits is determined by two factors (coordinate axes), which were interpreted as brightness and darkness neuronal channels. The present report addressed the identification of the perceptive brightness space in monkeys, using training to operant differentiation of stimuli differing only in terms of brightness and with application of multidimensional analytical methods. M. V. Lomonosov Moscow State University. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova. Vol. 47, No. 1, pp. 98-108, January-February, 1997. Original article submitted September 13, 1996: revision submitted October 2, 1996. 0097-0549/98/2803-0285S20.00
~1998 Plenum Publishing Corporation
285
METHODS Studies were performed using two male macaques aged 10-12 years, kept in animal house conditions. Animals were trained to differentiate 10 achromatic stimuli in 10 series of experiments: only one of these stimuli was reinforced in each series. Experiments were performed in a chamber with a background illumination at about 50 Lx. Animals were placed in a primate chair, but were not immobilized, and two levels were placed under the hands. Stimulus presentation, recording of level movements, and provision of reinforcement were automated using a special computer program. Displacements (positional coordinates) of the levers in the "towards self/away from self" direction (in relation to the animal) were digitized at 100 Hz and recorded on a personal computer disk. Stimuli were presented using a color graphics monitor located 60 cm from the monkeys' eyes. Correct responses led to the automatic provision of 0.1 ml of milk using a peristaltic pump controlled by the computer program. Stimuli (squares of size 4 ~ were presented in pairs on a dark screen background; the distance between stimulus centers was 140 ram. A reinforced stimulus was present in each pair. The position of the reinforced stimulus within the pair (left or right) and the sequence of differentiation stimuli used were changed pseudorandomly. Experiments involved the use of 10 black/white stimuli with brightnesses of 1-37 cd/m 2, with coordinates in the International Color Specification System CIE-31 [1] of X = 0.340 and Y = 0.304 (Table 1). Stimulus specification was determined using a video colorimeter/brighmess meter. The basis for development of the conditioned reflex in the food-obtaining method used here was the combination of visual stimulus, a "grasping" movement by the monkey on the lever "towards itself," and the monkey's feeding act upon reinforcement. Training was carried out in three stages. Animals were initially trained to make operant responses - - moving either of the levers "towards self" with the hands in response to presentation of pairs of black/white stimuli. Animals then had to move the lever spatially (left or right) corresponding to the position of a stimulus of given intensity, i.e., the conditioned stimulus. The monkeys' task was gradually increased in difficulty, by increasing the number of differentiation stimuli to nine. Conditioned responses were considered correct and were reinforced if the animals moved the lever within 10 sec of stimulus presentation. The full travel of level movement was taken as 100%: the extreme "away from self" position corresponded to an amplitude of 0% and the extreme "towards self" position corresponded to 100%. The "start range" (0-20%) and the "response range" (80-100%) were also defined, i.e., the animals have to move the lever by more than 80% of its entire travel for a response to have been made. Stimuli were switched off when animals made a movement response or 10 sec after switching on, and the next pair of stimuli was presented only after the monkey had returned the lever to the "start" position. The next pair of stimuli was presented after an interval of 2-6 sec, this being changed pseudorandomly to exclude the development of a reflex response to time. During the experiments, responses to all stimuli presentations (including the conditioned and differentiation stimuli) were recorded, along with the number of intersignal responses (lever movements during interstimulus intervals), and the latent periods of responses. The latent period was the duration of the monkey's movement response between the moment of stimulus presentation and the point at which the level reached the "response level." Each experiment involved presentation of 270-360 stimulus pairs. Thus, combination of the conditioned stimulus with each of the 9 differentiation stimuli was repeated 30-40 times. After the probability of operant responses to the conditioned and differentiation stimuli had stabilized at a particular level (i.e. had plateaued), the next series of experiments, with a different conditioned stimulus, was started. Each series consisted of 5-9 experiments, depending on the level of success with which differentiation was developed. Experimental results from each series were used to analyze the dynamics of the proportion of response made by the monkeys to stimuli of different intensity, along with the dynamics of intersignal responses and "refusals." The perceptive brightness space for black/white stimuli was identified by constructing a confusion matrix based on the probabilities of operant responses at the stage at which the conditioned reflex stabilized (Tables 2, 3). In these matrices, each stimulus was represented by a vector (a column) defined by the probability of obtaining an operant response to the stimulus in each of the experiments series performed. Confusion matrices were used to calculate matrices of correlations between vectors representing different intensities, which were then subjected to factor analysis (using the STADIA statistics program suite) to identity the basic axes of the space corresponding to these vectors. 286
TABLE 1. Stimulus Characteristics Stimulus No.
Stimulus No.
Slimulus brightness, cd/m 2 I
I
37
6
2 3 4 5
27 22 16 10
7 8 9 10
Stimulus brightness, c d / m 2 6
4 2 1,5 I
TABLE 2. Confusion Matrix of Probabilities of Conditioned Responses, %, for Monkey No. 1 using Stimuli of Different Intensities Reinforced stimulus 10 I 2 3 4 5 6 7 8 9 I0
41.7 38,4 30.0 18,4 15 6.7 3.3 1.7 1,7
37.2 29,9 21.4 18.2 8.7 6.8 0.8 0.9 0.0
39.4 31.8
24.2 25.9 35,6 38,7 26.1 19.3 6,5 1,6 1.6
37,9 25.8 18.2 10.7 9,1 1.5 0.0
16.0 17.6 33,1 41,4
10.6 10.1 17.1 28,7 43,0
38,2 30,0 12.7 6.4 3.2
3.3 6.7 8.1 16.7 26.3 31,2
,.10.8 15.6 9.3 12,6
1.7 3,4 5,0 6,7 15.0 23.4 36.7 41,7 40,0
33,4 21.3 23.4
7.5 10.6 7.4 7.5 10.6 19,4 16.7 32,9 61.1
130 1.7 3.3 6.5 9.9 24.7 26.7 40.0 50,0 -
Notes. Here and in Table 3, matrix columns contain the probabilities of conditioned responses to the stimuli presented (Table 1). Rows correspond to series of experiments with particular reinforced stimuli. TABLE 3. Confusion Matrix of Probabilities of Conditioned Responses, %, for Monkey No. 2 using Stimuli of Different Intensities Stimuli presented
Reinforced stimulus
I
2 3 4 5 6 7 8 9 I0
I
2
3
41.7 31.2 18,4 16.7 10.0 8.4 5,0 0.0 0,0
42.4
29,5 39.7
42,5 35.0 15.1 21.3 6,2 4.5 1.6 3,0
39,1 28,0 16.2 13,3 1,5 4.3 1,5
,I
4
26.7 21.7 38,4 35,0 25,0 18,4 13.4 5.0 6,7
5
I I. 1 t 7.5 20.0 48.8 43.8 27.5 10.0 12.5 5.0
i,
6
8.3 9.8 21.2 27.8 .11..3 46.9 22.8 I 1.6 6.6
i
t L
7
5.0 8,4 10.0 23,4 33,4 41.7 36.7 18.4 18.4
I
8
4,5 4,5 5,9 7,4 20.9 26,9 35.9 47.1 36.8
1
9
1.6 0,0 6,6 9.6 6.6 21.3 29.2 36.1 46,8
10
0.0 4.7 4.8 9.4 12,6 25.3 23,7 33.5 38,1
RESULTS
Each set of experiments with each monkey involved 10 series of black/white stimulus differentiation tests. The conditioned stimulus in each series was one of the 10 stimuli of different intensity. As training proceeded, the total probability of obtaining a conditioned response to the reinforced stimulus, calculated for combinations with all the differentiation stimulus, increased, usually stabilizing after the fourth experiment: changes in subsequent experiments were insignificant. The proportion of responses to the differentiation stimuli decreased gradually and stabilized at a certain level, this level becoming lower as the brightness difference between the conditioned and differentiation stimuli increased (Figs. I and 2). After the probability of obtaining a response to stimuli had stabilized, a stimulus of different intensity was selected as the conditioned stimulus. In the first experiment, each new series showed sharp increases 287
80
8
g.to
7.
4,
d
Fig. 1. Dynamics of the probabilities of obtaining operant responses in monkey No. 2 in a series of experiments with differentiation of stimuli of different intensities (the conditioned stimuli was stimulus No. 4). The abscissa shows the number of the experiment within the series, and the ordinate shows the probability, %. Curves are numbered according to stimulus numbers as given in Table I. The probability of a response to the conditioned stimulus was calculated for combinations with all differentiation stimuli.
SQ
6
8
s,a
~-
4,
7
Z
I I
O Z
~ j
6 #
5 ,0
Fig. 2. Dynamics of the probabilities of obtaining operant responses in monkey No. 1 in a series of experiments with differentiation of stimuli of different intensities (the conditioned stimuli was stimulus No. 1). For further details see caption to Fig. 1. in the probability of obtaining responses to all stimuli, the highest probability of conditioned responses being with stimuli reinforced in the previous experiment and with stimuli close to it in brightness. In subsequent experiments, there were generally increases in the levels of responses to the reinforced and close stimuli, while the level of responses to other differentiation stimuli decreased sharply. The proportion of erroneous responses reached 35-50% in different experiments for differentiation stimuli with brightnesses close to that of the conditioned stimulus (Figs. 1 and 2). Learning of the brightest and darkest stimuli was taster than for stimuli of intermediate intensities. At ends of series, the proportions of conditioned responses to the brightest and darkest conditioned stimuli were 85 % (Fig. 2). In these series. the probability of obtaining responses to the differentiation stimuli most different from the conditioned stimuli decreased to 288
TABLE 4. Coordinates of Stimuli in Two-Dimensional Euclidean Space for the Two Monkeys M o n k e y No. 1 Stimulus
stimulus coordinates
Monkey No. 2
vector radius
stimulus coordinates
vector radius
X1 I 2 3 4 5 6 7 8 9 10
0.931 0.922 0,905 0.813 0.438 --0.294 -0.569 -0,938 -0,970 -0,884
XI
X.. --0,168 -0.193 0.133 0,536 0,798 0.848 0,817 0.078 -0,060 -0.211
0,946 0,942 0.914 0.979 0.910 0.898 0.995 0,941 0,972" 0.909
X~_ 0,926 0,909 0.919 0,742 0.272 -0.292 --0.644 -0.916 -0.913 -41.907
-0,107 -0.143 0.028 0.622 0.896 0.892 0.718 0,201 0.052 -0.137
0.932 0.921 0.919 0.960 0.937 0.939 0.965 0.938 0.915 0.917
Notes. For monkey No, 1, the mean vector radius was 0.940 + 0.010; the coefficient of variation was 3.5%. For monkey No. 2, the mean vector radius was 0.935 + 0.006; the coefficient of variation was 2%.
less than 10% by the second experiment, with reductions to almost 0% in subsequent experiments (Tables 2 and 3; Fig. 2). In series in which the conditioned stimuli were of intermediate intensities, the probabilities of correct responses reached 7580% (Fig. 1). Training to stimuli of intermediate brightnesses was noticeably more difficult and generally required more experiments. Thus, the series in which stimulus 4 was reinforced (Fig. 1), monkey No. 2 quite quickly discriminated dark stimuli, but continued to be confused during the next three experiments. The proportion of intersignal responses (relative to the total number of presentations) in the first two or three experiments in each series increased, and the number gradually decreased in subsequently experiments, which was in good agreement with the dynamics of the probability of obtaining responses to reinforced stimuli within series. However, complete quenching of intersignal responses did not generally occur. Additionally, the first experiment of each series showed relatively large numbers of performances in which there was no response either to the conditioned or the differentiation stimulus (so-called "refusals"). As a rule, the number of responses of this type at the beginning of each series, after a change in the conditioned stimulus, was 2-10% of all presentations; this proportion gradually decreased to zero over subsequent experiments within a series. The sensory space for black/white stimulus intensity was determined independently for each monkey by constructing confusion matrices of dimensionality 10 x 10; the columns in these matrices corresponded to the stimuli used and the rows to series with particular reinforced stimuli (Tables 2 and 3). The probabilities making up each matrix were calculated from two sequential experiments in each series, i.e., when the conditioned response had stabilized. Thus, each stimulus in a given matrix was characterized by a vector consisting of the column of response probabilities. The matrix contained probability values for the conditioned stimulus, as, in conditions of selection from a pair, the probability of a response to the stimulus has to be considered separately for combinations with each of the differentiation stimuli. These matrices were used to construct pairwise correlation matrices between individual vectors. Such correlation matrices were assessed by factor analysis for identification of the basic axes of the sensory brightness space for monkeys. Factor analysis of correlation matrices for each monkey individually yielded five factors for each, which accounted for 97.5% (monkey No. 1) and 96.3% (monkey No. 2) of the dispersity of the experimental data. In the case of monkey No. 1, the intrinsic values of the five factors identified were 6.40, 2.10, 0.60, 0.35, and 0.30 (accounting for 64.0%, 21.0%, 6.0%, 3.5%, and 3.0% of the dispersity). In the case of monkey No. 2, the intrinsic values of the five factors identified were 6.20, 2.10, 0.67, 0.38, and 0.28 (accounting for 62.0%, 21.0%, 6.7%, 3.8%, and 2.8% of the dispersity). Theoretically, the minimal dimensionality of the actual Euclidean space is determined by the number of factors identified; however, because of random errors in experiments, the analysis yields factors which in fact account for negligibly small proportions of the dispersity. For the two animals, the first two factors accounted for 85% and 83% of the dispersity of the experimental data, so we were able to limit the factors for construction of the sensory space to these two.
289
X~
O5
Fig. 3. Projection of points corresponding to stimuli of different intensity on plane XIX 2, formed by the light (Xl) and dark (X2) axes o f the sensory space of monkey No. 1. Points are numbered according to Table 1.
~
I( z
,;,a
~
/:: -~,oolo -o5 ,
o -0,5
/
I, OXl
-tO
Fig. 4. Projection of points corresponding to stimuli of different intensity on plane XLX2, formed by the light (Xl) and dark (X2) axes o f the sensory space of monkey No. 2. For further details see caption to Fig. 3. Thus, the sensory spaces for achromatic stimuli identified here were two-dimensional_ The coordinates of stimuli of different intensity within these spaces are shown in Table 4. When projected onto the plane defined by these two factors, points corresponding to the stimuli form an arc-shaped trajectory corresponding to monotonic changes in intensity (Figs. 3 and 4). Points corresponding to stimuli 1-3 and 8-10 were located at opposite ends of the X l axis o n these trajectories (Figs. 3 and 4). These correspond to the brightest and darkest stimuli. The lengths of the vector radii corresponding to the stimuli 290
varied insignificantly, as shown by the small values for the coefficients of variation of these radii, which were no more than 3.5% (Table 4). This means that points corresponding to stimuli of different intensities were located virtually along a semicircle on the two-factor plane.
DISCUSSION The results of these studies show that development of differentiation using stimuli differing only in terms of brightness was more effective in lower monkeys than when stimuli differing in both brightness and color tone were used [4]. This is indicated by the rapid stabilization of the conditioned reflex at the level of 77-85 % and by the small numbers of intersignal responses and "refusals" in all series of experiments. The insignificant numbers of intersignal responses in each series shows strengthening and stabilization of inhibitory processes which support the differentiation of nonsignal stimuli. On the other hand, the number of "refusals" characterizes the level of food-related excitation of the animal. Reductions in the numbers of interstimulus responses and the almost complete absence of "refusals" by the end of each series are criteria showing stabilization of the conditioned reflex. Identification of the sensory space for stimuli differing in terms of brightness was performed using the probability of obtaining operant relationships in two sequential experiments in each series, when the numbers of interstimulus responses and "refusals" were insignificant .and the probability of obtaining a response to stimulus presentation had reached a plateau, i.e., when the conditioned reflex had stabilized. The nature of the distribution of points corresponding to stimuli when projected onto the plane defined by the coordinate axes (extracted factors) (Table 4; Figs. 3 and 4) suggests an interpretation of the axes of this sensory brightness space in monkeys similar to the interpretation of the coordinate axes forming the analogous spaces in humans [3, 7], carp [2], and rabbits [6]. According to the brightness encoding model proposed by the authors of these reports, all the stimuli of different intensity corresponded to points lying on a semicircle in two-dimensional space. The coordinates (factor loadings) of points corresponding to the stimuli on this plane are interpreted as the levels of excitation of hypothetical light and dark neurons (corresponding to the B- and D-neurons in the Yung classification [11]) resulting from presentation of stimuli of the corresponding intensity. The brightest stimuli produce more excitation of a light neuron, while the response of a dark neuron is close to zero. As the brightness decreases, excitation of the light neuron also decreases and the response of the dark neuron increases, reaching a maximum for stimuli of intermediate intensity. The response of the light neuron decreases to zero, and then changes sign when the stimulus brightness decreases further. According to the model, the distribution of stimuli on the circumference of a circle occupying two quadrants (Figs. 3 and 4) corresponds to simultaneous contrast when the selection consists of choosing one of two stimuli [8]. The positions of the stimuli with the greatest brightness differences at the opposite ends of the bipolar axis X 1 provides a basis for terming this the brightness axis. The distribution of stimuli along the X 2 axis, over a range from 0 to 1, identifies this as the dark axis. Fomin et al. [8] considered a neuron model which explains this ratio of responses from light and dark neurons. When stimuli are presented in pairs, comparison occurs in conditions of simultaneous contrast. According to the model [8], simultaneous contrast leads to an interaction of two local analyzers -- two regions of the retina with different levels of illumination, between which there are reciprocal inhibitory relationships. Formally, this interaction is described by transformation of the corresponding excitation vectors. The resultant vector in a given analyzer is the value of the initial excitation vector in this analyzer minus the excitation vector arising in the neighboring local analyzer, multiplied by the interaction coefficient. The effect of simultaneous contrast is to change the characteristics of the hypothetical light and dark neurons: light neurons have a negative sign on illumination of neighboring regions, while the excitation level of dark cells decreases. When the stimuli have large brightness differences, the dark stimulus becomes darker on the background of the lighter stimulus than would be the case in the absence of contrast, and vice versa. When the stimuli are close in terms of brightness, the effect of simultaneous contrast decreases. In our experiments, stimuli were presented in pairs, i.e.. the monkeys "selected" the conditioned or differentiation stimulus in conditions of simultaneous contrast. Thus, accordmg to the model for interactions between local analyzers [8], the response of the hypothetical light neuron (coordinate X l) becomes negative for darker stimuli (stimuli 6-10 in Figs. 3 and 4). This model can be regarded as being based neurophysiologically on neurons with properties first described in the cat visual system by Yung [11]. B- and D-Neurons show pronounced reciprocal relationships in relation to a specified change in 291
illumination. D-Neurons are maximally excited in the dark, while B-neurons are most excited on application of light. Some lateral geniculate nucleus neurons show similar responses [10, 12], as do some ganglion cells in the monkey retina [91. According to the spherical model of brightness discrimination, the central angle between the vectors corresponding to intensities is a measure of the difference between the stimuli [7]. The sequential positions of points on the arch of a circle (Figs. 3 and 4) show that the central angle formed by the corresponding vectors can be regarded as a measure of the subjective difference between two stimuli in conditions of simultaneous contrast. The coordinates on the dark axis for points corresponding to stimuli of very different intensities (the lightest stimulus 1 and 2 and the darkest stimuli 9 and 10) have small negative values. These insignificant distortions in the space are due to unknown causes, which are reflected by factors with small intrinsic values. Thus, the two-dimensional configuration of the brightness space in monkeys is determined by two parallel channels for processing brightness information. This space can be considered within the framework of a single spherical model of color discrimination as a sub-space, reflecting only the brightness differences between stimuli, when the activity of the color channels is zero (for achromatic stimuli) or remains constant as color stimuli of different intensities are used. In summary, the results obtained here, taken along with data from studies on monkeys [4], carp [2, 5], and rabbits [6], using stimuli differing in terms of both brightness and color, indicate that matrices describing the probabilities that animals will make operant responses contain information on the structure of the sensory space for differentiation stimuli. The dimensionality of this space reveals the number of proposed sensory mechanisms involved in processing information about stimulus properties. The configuration of the spaces reported in this study shows the common nature of the organizational principles of the intensity analyzer in vertebrates, including humans.
CONCLUSIONS 1. Confusion matrices consisting of the probabilities that monkeys will make operant responses to stimuli differing only in terms of intensity contain information about the structure of the sensory brightness space were constructed. 2. The two axes of the brightness space can be interpreted as two channels of achromatic vision: a light channel and a dark channel. This study was supported by the Russian Fund for Basic Research (project code 93-04-20511).
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