OC4?-5989,79 07014813M200~0
BINOCULAR
AND THE BINOCULARITY SUMMATION OF CAT VISUAL CORTEX’ MICHAEL VON GR~~NAU
Department of Neurophysiology,
Max-Planck Institute for Psychiatry. Munich. West Germany* (Rem-iced
1 September
1978)
Abstract-Monocular and binocular contrast sensitivity functions were determined behaviourly for normal kittens and those that were raised with alternating monocular exposure (alternators). The result that. in contrast to normal kittens, alternators have no binocular summation, suggests that binocular summation depends on a normal population of binocular cortical units. Kr!, U’ortls-binocular
summation: visual cortex: cat.
INTRODUCTION
of having two eyes rather than only one is not limited to providing us with stereoscopic depth perception. Many other visual tasks are improved when viewing is binocular. This phenomenon has been called “binocular summation” (Blake and Fox. 1973). Of special interest has been the summation that occurs at threshold. Here a superior performance of two eyes as compared to one can be predicted for statistical reasons alone, referred to as “probability summation” (Pirenne, 1943). It is therefore not obvious that any obtained summation effect can be interpreted as resulting from a physiological interaction (Thorn and Boynton, 1974). Recently, however, unambiguous evidence has been presented to show that the obtained binocular summation clearly exceeds that predicted by probability summation (Wolf and Zigler, 1963; Campbell and Green, 1965; Westendorf and Fox, 1977). The existence of physiological summation at threshold can therefore, no longer be in doubt. A possible neural basis for binocular summation is suggested by the finding that most units in the visual co&es of cat and monkey are binocular, i.e. they receive inputs from both eyes (Hubel and Wiesel, 1962 1968). At least a subset of these have interactive properties that could account quantitatively for the amount of binocular summation that is found psychophysically. The existence of similar binocular cells in the human visual system has been inferred from many psychophysical experiments (e.g. _ Blakemore and Hague, 1972; Mitchell and Ware, 1974; Mitchell et al.. 1975: Wade. 1976). If these binocular cells actually represent a neural basis for binocular summation, their absence in the visual cortex would be expected to result in an absence of binocular summation in experimental situations that would normally show a strong binocular effect. Since it is possible, by proper rearing procedures, to raise animals which lack normal binocuThe advantage
i A preliminary report of the results was read at the A.R.V.O. Meeting in Sarasota. FL., 1978. 2Present address: Department of Psychology, Queen’s University, Kingston, Ontario, Canada “.R_ 19 7-F
813
larity in areas 17 and 18, but in all other respects have a normal visual cortex (Hubel and Wiesel, 1965: von Griinau and Singer, 1979), this hypothesis can be easily tested. In the present experiment, the monocular and binocular contrast sensitivity functions of kittens raised with alternating monocular exposure (alternators) and those of normally raised control kittens were measured. The results show that binocular summation does not occur in alternators, while normal controls exhibit a fairly strong superiority of binocular viewing. METHODS Two kittens from the same litter were kept in total darkness until the end of the third post-natal week. From then on they received monocular exposure to a normal laboratory environment for an average of 8 hr per day. The other eye was always covered by an opaque contact lens, and the eyes were alternated daily. The rest of the time the animals spent in total darkness. This rearing procedure allowed them to receive normal input to each eye, but they could never see with both eyes simultaneously. It has been shown previously that thii rearing procedure produces animals with mainly monocular cortical cell populations (von Griinau and Singer, 1979). The control animals grew up under normal binocular conditions. All animals were first trained by operant techniques. using a “‘jumping St&d”. to discriminate between a vertical grating and a homogeneous grey of the same average illumination. The ‘jumping stand” was similar to the one described by Mitchell et al. (1977). The stimuli were projected via a mirror onto two trap doors. The luminance of the projector and that of the second light source. which kept the total luminance.at a constant level of 5.5 cd/m’ during contrast changes. were regulated by sets of polarizing filters. The grating was projected onto the left or right trap door in a pseudo-random manner and the kittens. who were slightly food-deprived. were rewarded for correct responses with a small chunk of baby food. When they responded incorrectly, they received no reinforcement and sometimes fell through the unlocked trap door. At the time of jumping, the minimum distance between the kittens’ eyes and the stimuli was 3Ocm. After having learned to jump consistently to the grating, the kittens’ contrast thresholds for sine gratings of various spatial frequencies were determined. using a modified Comsweet staircase method. Contrast was varied in steps of 3 or 4% from a maximally possible 57% down to 1%. Test-
MICHAEL LOX GRCNAL
Alternators
Normal cats
Cat
644
Cat C4
Cat
642
Cat Cl
.I8
Spatial frequency
36
74
RESULTS AND DlSCUSSlON Contrast sensitivity functions, comparing two alternators with two normal kittens. are presented in Fig. 1.
1.5
(cycles/degree)
Fig. 1. Contrast sensitivity functions for two experimental and two control kittens, (= l/contrast threshold) IS plotted against spatial frequency on a log-log scale. left eye monocular (t3) and right eye monocular (0) cases are compared to the The kittens were trained by operant conditioning techniques to discriminate sine spatial frequencies from a homogeneous field.
ing was always started with the grating at a high contrast level and proceeded to the next lower contrast when at least three out of four trials were correct. or to the next higher contrast when at least two out of four trials were incorrect. The threshold was defined as the lowest contrast which led to a further lowering of contrast in at least three out of the last four occasions. In each session, testing was either left eye monocular (LE. @) or right eye monocular (RE. a) or binocular (LE + RE. 0). Six values of spatial frequency were ptesented within each testing condition in a pseudo-random order, and the conditions appeared according to a counterbalanced design. In this way. thresholds were determined twice for each condition and each frequency.
99
Contrast sensitivity In each graph. the binocular case (0). gratings _f various
In each graph. contrast sensitivity (= l/threshold contrast) is plotted as a function of spatial frequency on a log-log scale. The curves have been fitted by eye. Clearly, in the two alternators (Cat 642 and Cat 644), binocular vision is not superior to monocular vision. Analysis of variance shows no significant difference between the three conditions. Only one curve is therefore drawn to represent all the data. In contrast, the results for the normal control cats (Cat Cl and Cat C4) show a significant increase of contrast sensitivity for the binocular condition when compared to the monocular conditions @ < O.Ol), which themselves were not different from each other. One curve is drawn to represent the average of the monocular conditions, the other to represent the binocular case. For comparison, a human observer was tested on the same apparatus with the same stimuli and indi-
81.5
Binocular summation and the binocularity of cat visual cortex
Normal
human
observer LW
0
1.0 Spatia:’
8.3 12.114.7 3.9 5.3 frequency (cycles/degree)
Fig. 2. Contrast sensitivity functions for a human observer. The 2 curves represent the average of the left eye monocular (a) and the right eye monocular (0) conditions and the binocular (0) condition. cated verbally when she detected the grating. The results are shown in Fig. 2. Again, the binocular
thresholds are lower than the monocular thresholds. and this is statistically significant (p < 0.01). The size of the summation effects that were found for the two control kittens and the human observer are somewhat large, but lie within the range reported in the literature (e.g. Rose, 1978). As a further control, monocular and binocular contrast senstivity functions were measured for a Siamese cat. These cats are known to lack the normal binocularity of units in cortical areas 17 and 18, due to an abnormal crossing of optical fibers at the cbiasm. One would therefore not expect to find binocular summation in these animals for similar reasons as for the alternators. It is quite clear from Fig. 3 that there is absolutely no binocular summation effect for the Siamese cat. The contrast sensitivity is, overall, lower for the control kitten Cl, as compared to the alternators and the other control cat. The reason for this is probably the 1oNer age of Cat Cl, since Mitchell et al. (1976) have shown that the development of acuity in kittens reaches adult levels at an age of about 3-4 months and cat Cl was only 10 weeks old at the beginning of testing. and C4, on the other hand, . . The ,_ alternators ._ were older (b and 5 months, respectively).
In the data for the alternators. no evidence for the existence of binocular summation can be detected, not even for probability summation. The latter should be apparent especially with alternators, since here the assumption that the eyes function as two independent detectors is most likely fulfilled. From recent studies, however, it is known that the size of probability summation is much smaller than originally assumed (Blake and Fox, 1973). and may be within the variability of the present results. It has been reported (Blake et al., 1974; Packwood and Gordos 1975) that alternators and Siamese cats have interocular alignments that deviate from the slightly divergent pupillary alignment of normal cats. This misalignment by itself might have made it impossible for these animals to achieve fusion and thus binocular summation. The alternators and the Siamese cat of the present study were found to have slightly convergent pupillarv alignments. With the large vertical grating stimuli employed here, this slight misalignment did not, however, preclude fusion. In conclusion, the binocular summation obtained in the normal kittens and the human observer must be interpreted as physiological summation, based on certain interactions of the monocular inputs in the visual pathway. The lack of this summation in alternators and Siamese cats who also lack normal binocularity in their visual cortex, suggests that binocular units in area 17 and perhaps area 18 play a vital role in binocular summation. Binocular units in these areas are also thought to be involved in the mechanism of stereopsis (Barlow et al., 1967; Nikara er al.. 1968). It has been shown that stereoblind people lack interocular transfer of aftereffects (Hohmann and Creutzfeldt. 1975; Banks er al., 1975) and that alternators and Siamese cats are not able to use stereoscopic depth cues (Packwood and Gordon, 1975). These cortical binocular units may even be concerned specifically with stereoscopic depth perception, since other binocular abilities, such as interocular transfer of pattern discriminations and binocular combination of
Siamese
I
I
38
Spatial
.36
frequency
cat
I
.74 .99
l.5
(cycles/degree)
Fig. 3. Contrast sensitivity function for a Siamese cat. The curve represents the average of the left eye monocular (s). the right eye monocular (0) and the binocular (0) I... conamons.
pattern information. are not disturbed by an abnormal binocularity (Sherman, 1971: Ganz er al., 1972: von Griinau and Singer, 1979). It is therefore suggested that binocular summation and stereopsis may share a common mechanism in these binocular cortical units. In this connection, it has already been shown that people who lack stereopsis aho have no binocular summation (Lema and Blake, 1977). The present experiment demonstrates that kittens which lack the normal cortical binocularity and therefore do not see stereoscopic depth. also have no binocular summation. Furthermore. recent experiments investigating the precise spatial and temporal conditions under which binocular summation occurs. show a close correspondence between these conditions and those that give rise to stereopsis (Westendori and Fox. 1974, 1975, 1977). Since the magnitude of binocular summation is usually comparatively small. it appears then that binocular summation may only be a blproduct of the binocular interaction that results !n stereoscopic depth perception.
Hohmann A. and Creutzfeldt 0. D. (1975) Squint and the
development of binocularity in humans. Yot~re. LO&. ,254. 613-614. Hubel D. H. and Wiesel T. N. (1962) Receptlre fields, binocular interaction and functional architecture in the cat’s visual cortex. J. Ph.uiol.. Land. 160. 1G6-15-l. Hubel D. H. and Wiesel T. N. (1968) Receptive fields and functional architecture of monkey striate cortex. j. f%xsioi., Lo&. 1%. 215-243. Hubel D. H. and Wiesel T. N. (1965) Binocular interaction in striate cortex of kittens reared with artificial squint. J. Neurophysiol 28, l&l-1059. Lcma S. A. and Blake R. (1977) Binocular summation in normal and stereoblind humans. Vision Res. 17, 691-695. I~itcheli D. E.. Giffen F. and Timney B. (19771 A behavioral technique for the rapid assessment of the visual capabilities of kittens. Perception 6, 181-193. Mitchell D. E.. Giffen F.. Wilkinson, F., Anderson P. and Smith M. L. (1976) Visual resolution in young kittens. Ksion Res. 16, 363366. Mitchell D. E.. Reardon J. and Muir D. W. (1975) Interocular transfer of the motion after-effect in normal and stereoblind observers. Expl Brain Res. 22, 163133. Mitchell D. E. and Ware C. (1974) Interocular transfer of a visual aftereffect in normal and stereoblind humans. Acknowledgements-This work was supported in part by J. Phpsiol.. Lond. 236, 707-721. funds from the Deutsche Forschungsgemeinschaft, SFB SO. Nikara T., Bishop P. 0. and Pettigrew J. D. (1968) Analysis Kybernetik. I wish to thank Dr W. Singer for his generous of retinal correspondence by studying receptive fields of support and most valuable advice. binocular single units in cat striate cortex. Expl Brain Res. 6, 3X3-372. Packwood J. and Gordon B. (1975) Stereopsis in normal domestic cat. Siamese cat. and cat raised with alternating REFERENCES monocular occlusion. J. Neurophysiol. 9, 1485-1499. Pirenne M. H. (1943) Binocular and uniocular threshold Banks M. S., Aslin R. N. and Letson R. D. (1975) Critical of vision. Narure, Lond. 152, 698-699. period for the development of human binocular vision. Rose D. (1978) Monocular vs binocular contrast thresholds Science 190, 675-637. for movement and pattern. Perception 7, 195-200. Barlow H. 8.. Blakemore C. and Pettigrew 1. D. (1967) Sherman S. M. (1971) Role of visual cortex in interocular The neural mechanism of binocular depth discrimitransfer in the cat. &.rpi Neu~ol. 3% 34--t5. nation. J. Physiol, Lond. 193, 3X-342. Blake RI Crawford M. L. J. and Hirsch H. V. B. (1974) Thorn F. and Boynton R. M. (1974) Human binocular summation at absolute threshold. Vision Rrs. 14. Consequences of alternating monocular deprivation on 445-458. eye alignment and convergence in cats. fnresr. Ophthal. Wade N. J. (1976) On interocular transfer of the movement 13, 121-126. aftereffect in individuals with and without normal binBlake R. and Fox R. (1973) The psychophysical inquiry ocular vision. Percrprion 5. 113-l 18. into binocular summation. Percepr. Fs~c~ph~s. 14, Westendorf D. H. and Fox R. (1974) Binocular detection 161-185. of positive and negative Rashes. Percepr. Psychophys. 15, Blakemore C. and Hague B. (1972) Evidence for disparity 61-65. detecting neurones in the human visual system. J. f’h~Westendorf D. H. and Fox R. (1975) Binocular detection siol.. Lond. 255, 437-455. of vertical and horizontal line segments. Vision Res. 15, Campbell F. W. and Green D. G. (1965) Monocular vs 471-476. binocular visual acuitv Narure. Lond. ZQ& 191-192. Westendorf D. H. and Fox R. (1977) Binocular detection Ganz L, Hirsch H. V.-B. and Tieman S. B. (1972) The of disparate light flashes. Vision Res. $7, 697-702. nature of perceptual deficits in visually deprived cats. Wolf E. and Zigler M. 3. (1963) Effects of uniocular and Brain Res. 44, 547-568. binocular excitation of the peripheral retina with test Griinau M. W. van and Singer W, (1979) The role of binfields of various shapes on binocular summation. J. apt. ocular neurons in the cat striate cortex in combining Sot. Am. 53, 1199-1205. informationfromthe twoeyes.ExplErainRes.31.133-142.
BINOCULAR
AND THE BINOCULARITY SUMMATION OF CAT VISUAL CORTEX’ MICHAEL VON GR~~NAU
Department of Neurophysiology,
Max-Planck Institute for Psychiatry. Munich. West Germany* (Rem-iced
1 September
1978)
Abstract-Monocular and binocular contrast sensitivity functions were determined behaviourly for normal kittens and those that were raised with alternating monocular exposure (alternators). The result that. in contrast to normal kittens, alternators have no binocular summation, suggests that binocular summation depends on a normal population of binocular cortical units. Kr!, U’ortls-binocular
summation: visual cortex: cat.
INTRODUCTION
of having two eyes rather than only one is not limited to providing us with stereoscopic depth perception. Many other visual tasks are improved when viewing is binocular. This phenomenon has been called “binocular summation” (Blake and Fox. 1973). Of special interest has been the summation that occurs at threshold. Here a superior performance of two eyes as compared to one can be predicted for statistical reasons alone, referred to as “probability summation” (Pirenne, 1943). It is therefore not obvious that any obtained summation effect can be interpreted as resulting from a physiological interaction (Thorn and Boynton, 1974). Recently, however, unambiguous evidence has been presented to show that the obtained binocular summation clearly exceeds that predicted by probability summation (Wolf and Zigler, 1963; Campbell and Green, 1965; Westendorf and Fox, 1977). The existence of physiological summation at threshold can therefore, no longer be in doubt. A possible neural basis for binocular summation is suggested by the finding that most units in the visual co&es of cat and monkey are binocular, i.e. they receive inputs from both eyes (Hubel and Wiesel, 1962 1968). At least a subset of these have interactive properties that could account quantitatively for the amount of binocular summation that is found psychophysically. The existence of similar binocular cells in the human visual system has been inferred from many psychophysical experiments (e.g. _ Blakemore and Hague, 1972; Mitchell and Ware, 1974; Mitchell et al.. 1975: Wade. 1976). If these binocular cells actually represent a neural basis for binocular summation, their absence in the visual cortex would be expected to result in an absence of binocular summation in experimental situations that would normally show a strong binocular effect. Since it is possible, by proper rearing procedures, to raise animals which lack normal binocuThe advantage
i A preliminary report of the results was read at the A.R.V.O. Meeting in Sarasota. FL., 1978. 2Present address: Department of Psychology, Queen’s University, Kingston, Ontario, Canada “.R_ 19 7-F
813
larity in areas 17 and 18, but in all other respects have a normal visual cortex (Hubel and Wiesel, 1965: von Griinau and Singer, 1979), this hypothesis can be easily tested. In the present experiment, the monocular and binocular contrast sensitivity functions of kittens raised with alternating monocular exposure (alternators) and those of normally raised control kittens were measured. The results show that binocular summation does not occur in alternators, while normal controls exhibit a fairly strong superiority of binocular viewing. METHODS Two kittens from the same litter were kept in total darkness until the end of the third post-natal week. From then on they received monocular exposure to a normal laboratory environment for an average of 8 hr per day. The other eye was always covered by an opaque contact lens, and the eyes were alternated daily. The rest of the time the animals spent in total darkness. This rearing procedure allowed them to receive normal input to each eye, but they could never see with both eyes simultaneously. It has been shown previously that thii rearing procedure produces animals with mainly monocular cortical cell populations (von Griinau and Singer, 1979). The control animals grew up under normal binocular conditions. All animals were first trained by operant techniques. using a “‘jumping St&d”. to discriminate between a vertical grating and a homogeneous grey of the same average illumination. The ‘jumping stand” was similar to the one described by Mitchell et al. (1977). The stimuli were projected via a mirror onto two trap doors. The luminance of the projector and that of the second light source. which kept the total luminance.at a constant level of 5.5 cd/m’ during contrast changes. were regulated by sets of polarizing filters. The grating was projected onto the left or right trap door in a pseudo-random manner and the kittens. who were slightly food-deprived. were rewarded for correct responses with a small chunk of baby food. When they responded incorrectly, they received no reinforcement and sometimes fell through the unlocked trap door. At the time of jumping, the minimum distance between the kittens’ eyes and the stimuli was 3Ocm. After having learned to jump consistently to the grating, the kittens’ contrast thresholds for sine gratings of various spatial frequencies were determined. using a modified Comsweet staircase method. Contrast was varied in steps of 3 or 4% from a maximally possible 57% down to 1%. Test-
MICHAEL LOX GRCNAL
Alternators
Normal cats
Cat
644
Cat C4
Cat
642
Cat Cl
.I8
Spatial frequency
36
74
RESULTS AND DlSCUSSlON Contrast sensitivity functions, comparing two alternators with two normal kittens. are presented in Fig. 1.
1.5
(cycles/degree)
Fig. 1. Contrast sensitivity functions for two experimental and two control kittens, (= l/contrast threshold) IS plotted against spatial frequency on a log-log scale. left eye monocular (t3) and right eye monocular (0) cases are compared to the The kittens were trained by operant conditioning techniques to discriminate sine spatial frequencies from a homogeneous field.
ing was always started with the grating at a high contrast level and proceeded to the next lower contrast when at least three out of four trials were correct. or to the next higher contrast when at least two out of four trials were incorrect. The threshold was defined as the lowest contrast which led to a further lowering of contrast in at least three out of the last four occasions. In each session, testing was either left eye monocular (LE. @) or right eye monocular (RE. a) or binocular (LE + RE. 0). Six values of spatial frequency were ptesented within each testing condition in a pseudo-random order, and the conditions appeared according to a counterbalanced design. In this way. thresholds were determined twice for each condition and each frequency.
99
Contrast sensitivity In each graph. the binocular case (0). gratings _f various
In each graph. contrast sensitivity (= l/threshold contrast) is plotted as a function of spatial frequency on a log-log scale. The curves have been fitted by eye. Clearly, in the two alternators (Cat 642 and Cat 644), binocular vision is not superior to monocular vision. Analysis of variance shows no significant difference between the three conditions. Only one curve is therefore drawn to represent all the data. In contrast, the results for the normal control cats (Cat Cl and Cat C4) show a significant increase of contrast sensitivity for the binocular condition when compared to the monocular conditions @ < O.Ol), which themselves were not different from each other. One curve is drawn to represent the average of the monocular conditions, the other to represent the binocular case. For comparison, a human observer was tested on the same apparatus with the same stimuli and indi-
81.5
Binocular summation and the binocularity of cat visual cortex
Normal
human
observer LW
0
1.0 Spatia:’
8.3 12.114.7 3.9 5.3 frequency (cycles/degree)
Fig. 2. Contrast sensitivity functions for a human observer. The 2 curves represent the average of the left eye monocular (a) and the right eye monocular (0) conditions and the binocular (0) condition. cated verbally when she detected the grating. The results are shown in Fig. 2. Again, the binocular
thresholds are lower than the monocular thresholds. and this is statistically significant (p < 0.01). The size of the summation effects that were found for the two control kittens and the human observer are somewhat large, but lie within the range reported in the literature (e.g. Rose, 1978). As a further control, monocular and binocular contrast senstivity functions were measured for a Siamese cat. These cats are known to lack the normal binocularity of units in cortical areas 17 and 18, due to an abnormal crossing of optical fibers at the cbiasm. One would therefore not expect to find binocular summation in these animals for similar reasons as for the alternators. It is quite clear from Fig. 3 that there is absolutely no binocular summation effect for the Siamese cat. The contrast sensitivity is, overall, lower for the control kitten Cl, as compared to the alternators and the other control cat. The reason for this is probably the 1oNer age of Cat Cl, since Mitchell et al. (1976) have shown that the development of acuity in kittens reaches adult levels at an age of about 3-4 months and cat Cl was only 10 weeks old at the beginning of testing. and C4, on the other hand, . . The ,_ alternators ._ were older (b and 5 months, respectively).
In the data for the alternators. no evidence for the existence of binocular summation can be detected, not even for probability summation. The latter should be apparent especially with alternators, since here the assumption that the eyes function as two independent detectors is most likely fulfilled. From recent studies, however, it is known that the size of probability summation is much smaller than originally assumed (Blake and Fox, 1973). and may be within the variability of the present results. It has been reported (Blake et al., 1974; Packwood and Gordos 1975) that alternators and Siamese cats have interocular alignments that deviate from the slightly divergent pupillary alignment of normal cats. This misalignment by itself might have made it impossible for these animals to achieve fusion and thus binocular summation. The alternators and the Siamese cat of the present study were found to have slightly convergent pupillarv alignments. With the large vertical grating stimuli employed here, this slight misalignment did not, however, preclude fusion. In conclusion, the binocular summation obtained in the normal kittens and the human observer must be interpreted as physiological summation, based on certain interactions of the monocular inputs in the visual pathway. The lack of this summation in alternators and Siamese cats who also lack normal binocularity in their visual cortex, suggests that binocular units in area 17 and perhaps area 18 play a vital role in binocular summation. Binocular units in these areas are also thought to be involved in the mechanism of stereopsis (Barlow et al., 1967; Nikara er al.. 1968). It has been shown that stereoblind people lack interocular transfer of aftereffects (Hohmann and Creutzfeldt. 1975; Banks er al., 1975) and that alternators and Siamese cats are not able to use stereoscopic depth cues (Packwood and Gordon, 1975). These cortical binocular units may even be concerned specifically with stereoscopic depth perception, since other binocular abilities, such as interocular transfer of pattern discriminations and binocular combination of
Siamese
I
I
38
Spatial
.36
frequency
cat
I
.74 .99
l.5
(cycles/degree)
Fig. 3. Contrast sensitivity function for a Siamese cat. The curve represents the average of the left eye monocular (s). the right eye monocular (0) and the binocular (0) I... conamons.
pattern information. are not disturbed by an abnormal binocularity (Sherman, 1971: Ganz er al., 1972: von Griinau and Singer, 1979). It is therefore suggested that binocular summation and stereopsis may share a common mechanism in these binocular cortical units. In this connection, it has already been shown that people who lack stereopsis aho have no binocular summation (Lema and Blake, 1977). The present experiment demonstrates that kittens which lack the normal cortical binocularity and therefore do not see stereoscopic depth. also have no binocular summation. Furthermore. recent experiments investigating the precise spatial and temporal conditions under which binocular summation occurs. show a close correspondence between these conditions and those that give rise to stereopsis (Westendori and Fox. 1974, 1975, 1977). Since the magnitude of binocular summation is usually comparatively small. it appears then that binocular summation may only be a blproduct of the binocular interaction that results !n stereoscopic depth perception.
Hohmann A. and Creutzfeldt 0. D. (1975) Squint and the
development of binocularity in humans. Yot~re. LO&. ,254. 613-614. Hubel D. H. and Wiesel T. N. (1962) Receptlre fields, binocular interaction and functional architecture in the cat’s visual cortex. J. Ph.uiol.. Land. 160. 1G6-15-l. Hubel D. H. and Wiesel T. N. (1968) Receptive fields and functional architecture of monkey striate cortex. j. f%xsioi., Lo&. 1%. 215-243. Hubel D. H. and Wiesel T. N. (1965) Binocular interaction in striate cortex of kittens reared with artificial squint. J. Neurophysiol 28, l&l-1059. Lcma S. A. and Blake R. (1977) Binocular summation in normal and stereoblind humans. Vision Res. 17, 691-695. I~itcheli D. E.. Giffen F. and Timney B. (19771 A behavioral technique for the rapid assessment of the visual capabilities of kittens. Perception 6, 181-193. Mitchell D. E.. Giffen F.. Wilkinson, F., Anderson P. and Smith M. L. (1976) Visual resolution in young kittens. Ksion Res. 16, 363366. Mitchell D. E.. Reardon J. and Muir D. W. (1975) Interocular transfer of the motion after-effect in normal and stereoblind observers. Expl Brain Res. 22, 163133. Mitchell D. E. and Ware C. (1974) Interocular transfer of a visual aftereffect in normal and stereoblind humans. Acknowledgements-This work was supported in part by J. Phpsiol.. Lond. 236, 707-721. funds from the Deutsche Forschungsgemeinschaft, SFB SO. Nikara T., Bishop P. 0. and Pettigrew J. D. (1968) Analysis Kybernetik. I wish to thank Dr W. Singer for his generous of retinal correspondence by studying receptive fields of support and most valuable advice. binocular single units in cat striate cortex. Expl Brain Res. 6, 3X3-372. Packwood J. and Gordon B. (1975) Stereopsis in normal domestic cat. Siamese cat. and cat raised with alternating REFERENCES monocular occlusion. J. Neurophysiol. 9, 1485-1499. Pirenne M. H. (1943) Binocular and uniocular threshold Banks M. S., Aslin R. N. and Letson R. D. (1975) Critical of vision. Narure, Lond. 152, 698-699. period for the development of human binocular vision. Rose D. (1978) Monocular vs binocular contrast thresholds Science 190, 675-637. for movement and pattern. Perception 7, 195-200. Barlow H. 8.. Blakemore C. and Pettigrew 1. D. (1967) Sherman S. M. (1971) Role of visual cortex in interocular The neural mechanism of binocular depth discrimitransfer in the cat. &.rpi Neu~ol. 3% 34--t5. nation. J. Physiol, Lond. 193, 3X-342. Blake RI Crawford M. L. J. and Hirsch H. V. B. (1974) Thorn F. and Boynton R. M. (1974) Human binocular summation at absolute threshold. Vision Rrs. 14. Consequences of alternating monocular deprivation on 445-458. eye alignment and convergence in cats. fnresr. Ophthal. Wade N. J. (1976) On interocular transfer of the movement 13, 121-126. aftereffect in individuals with and without normal binBlake R. and Fox R. (1973) The psychophysical inquiry ocular vision. Percrprion 5. 113-l 18. into binocular summation. Percepr. Fs~c~ph~s. 14, Westendorf D. H. and Fox R. (1974) Binocular detection 161-185. of positive and negative Rashes. Percepr. Psychophys. 15, Blakemore C. and Hague B. (1972) Evidence for disparity 61-65. detecting neurones in the human visual system. J. f’h~Westendorf D. H. and Fox R. (1975) Binocular detection siol.. Lond. 255, 437-455. of vertical and horizontal line segments. Vision Res. 15, Campbell F. W. and Green D. G. (1965) Monocular vs 471-476. binocular visual acuitv Narure. Lond. ZQ& 191-192. Westendorf D. H. and Fox R. (1977) Binocular detection Ganz L, Hirsch H. V.-B. and Tieman S. B. (1972) The of disparate light flashes. Vision Res. $7, 697-702. nature of perceptual deficits in visually deprived cats. Wolf E. and Zigler M. 3. (1963) Effects of uniocular and Brain Res. 44, 547-568. binocular excitation of the peripheral retina with test Griinau M. W. van and Singer W, (1979) The role of binfields of various shapes on binocular summation. J. apt. ocular neurons in the cat striate cortex in combining Sot. Am. 53, 1199-1205. informationfromthe twoeyes.ExplErainRes.31.133-142.
