Asymmetry of motility of the eyes and change of binocular properties of cortical cells in adult cats

Immobilization of one eye in the adult cat following section of the oculomotor cranial nerves causes a decrease of the proportion of binocularly activated cells in the striate cortex. This change in the binocular properties of cortical cells also takes place if the animal is deprived of vision from the day on which the nerves are cut to the day of the electrophysiological recording. No similar change in the proportion of binocular cortical cells is observed in cats where both eyes have been immobilized.

Neurogenesis in the visual system of the rat. An autoradiographic investigation

Rats of the BD III strain were injected with a single dose of 3H-thymidine on either the twelfth, fourteenth, sixteenth, eighteenth or twentieth day of gestation (ED 12. . . . .ED 20) or on the postnatal day one, three, or seven. Animals were killed at age 22 to 24 days. DNA synthesis, as an indicator of cell division, was studied in matrix precursors of nerve and glial cells in the visula centers, including the lateral geniculate body (LGB), the superior colliculus (SC) and the visual cortex (VC). It was found that proliferation of matrix precursors of nerve cells destined for all the regions studied was in progress on ED 12. In the subcortical regions (LGB, SC) this process was substantially more advanced than in the VC. The first neuroblasts appeared in the SC (ED 12) and only later (ED 14) in the LGB and VC. In comparison with the LGB, VC neuroblasts were quite rare on ED 14 and were present only in layer VI. They appeared more frequently in this region only after injection of isotope on ED 16. Matrix cell proliferation and nerve cell formation ceased in the LGB between ED 16 and ED 18. The number of labeled cells arising after injection of the isotope on ED 16 indicates that neurogenesis ceased somewhat earlier in the dorsal nucleus of the LGB than in the ventral. In the SC the last neurons arose between ED 18 and ED 20, and in the VC, with the possible exception of a few granular neurons (which may continue division into the first few days postnatally), proliferation continued until the end of gestation. The origin of neuroblasts initially followed a caudo-rostral gradient. Later, the times of neurogenesis in the regions studied overlapped significantly. This is clear, for example. on ED 16, when neurogenesis in the mesencephalic SC continued for about two days longer than in the more postral LGB, and coincided with that in the VC, especially in the deep layers. The end of neurogenesis in the LGB, especially in the ventral nucleus, coincided with the time of neurogenesis in the deep cortical layers. In the VC, and partly also in the SC, an inside-out pattern of proliferation and neuron formation was confirmed. The times of proliferation of precursor cells, with the exception of the very end of neurogenesis, substantially overlapped within both these regions. The degree of this overlapping, described in terms of Labeling Index values, decreased towards the end of the neurogenetic period. Division of neuroglial cell precursors, started as early as on ED 14 in/for subcortical centers (LGB, SC), but not until ED 18 in/for the VC. A few labeled endothelial-like cells were observed in all regions studied after isotope injection on ED 12.

Representation of the visual field on the medial wall of occipital-parietal cortex in the owl monkey

The medial visual area is located on the medical wall of occipital-parietal cortex. A much larger proportion of this area is devoted to the representation of the more peripheral parts of the visual field than in any other cortical area or subcortical visual structure than has been mapped previously in any species of primate.

Visual receptive fields in the lateral suprasylvian area (Clare-Bishop area) of the cat

Single units were recorded from the visual area of the lateral suprasylvian gyrus (LSSA or Clare-Bishop area) in 20 unanesthetized cats. Most LSSA units were poorly responsive to stationary visual stimuli, but they responded vigorously to moving visual stimuli. Their receptive fields appeared to be constituted of a large activating region (discharge area) often surrounded by inhibitory flanks. Relating unit behavior to changes of stimulus length, the LSSA neurons could be subdivided into 5 categories. The first category (22 out of 95 units tested, 23.16%) consisted of units showing summation inside the discharge area. Expanding the stimulus outside the discharge area did not affect the response. The second category (7.37%) was formed by units which showed summation inside the discharge area and inhibition when the stimulus was extended outside the discharge area. The third category (21.05%) consisted of units largely insensitive to the stimulus length inside the discharge area, but surrounded by inhibitory flanks. The fourth category (41.05%) consisted of units which showed inhibition of the response when the stimulus, well inside the discharge area, became longer than a certain optimal lenght. They were surrounded by inhibitory flanks. The fifth category (7.37%) was formed by units insensitive to variations of the stimulus length inside as well as outside the discharge area. Almost all units, independent of their category, were directionally specific, that is their response could be decreased 50% or more by varying the direction of movement away from that which gave the maximal response (preferred direction). Typically the response was halved when the stimulus was moved +/- 50 degrees from the preferred direction. Among the directionally specific units, 71% showed the minimal response 180 degrees away from the preferred direction (direction specificity curve type 1), 20% had the minimal response 90 degrees from the preferred direction (direction specificity curve type 2); the remaining could not be classified in this respect. Of LSSA units, 87% (all those of type 1 and many of those of type 2) were directionally selective, that is their response to movement in the preferred direction was at least double that in the opposite direction. The LSSA units usually preferred stimuli moving at rather high speeds. The optimal speed for 71% of units was 20 degrees/sec or greater. Almost all units responded over a wide range of speeds, many of them from 5-10 degrees/sec to over 100 degrees/sec. Most neurons had a low spontaneous activity and some of them remained completely silent for seconds.

An examination of the participation of neurons in readout from memory

Adult cats were implanted with a movable microelectrode and were trained to perform for food reward in response to diffuse light flicker at two different frequencies. After substantial overtraining, the patterns of cell response (poststimulus histogram) were obtained during generalization trials, using an intermediate frequency stimulus. An average of 29% of the cells examined in lateral geniculate nucleus and visual cortex traverses showed statistically significant differences in the late component of the neuronal response when different responses to the same generalization stimulus were compared.

Evoked potential-unit relationships in behaving cats

Adult cats were implanted with a movable microelectrode, and trained to respond differentially to two different frequencies of light flicker. Unit responses and evoked potentials were recorded along trajectories in the visual cortex and lateral geniculate nucleus. Although strong correlations are shown to exist between certain components of the evoked potential and peaks in the poststimulus histogram, it is demonstrated that it is impossible to specify a causal, predictive relationship.

Responses of simple and non-oriented cells to stimuli of variable duration in areas 17 and 18 of cat visual cortex

Single unitresponses to photic stimuli of variable duration were studied in the cat visual areas 17 and 18. The closed-chamber technique was used to record extracellularly impulsive electrical activity in locally anaesthetized, Flaxedil-paralyzed and artificially ventilated cats. Stimulus duration ranges between 0.020 and 10 sec, the background luminance and the intensity of stimuli being constant. We found a first group of cells which fire independently of the stimulus duration; a second group which fire in close relation to the stimulus duration in a rather wide range of values, and a third group which fire monotonously as long as the stimulus goes on. The three groups of cells are supposed to be three functionally different types of cortical cells.

Central mechanisms of vision: parallel processing

Despite repeated attempts by several laboratories to discover increasingly complex "feature detector" neurons in higher visual centers of mammals, there has been little convincing evidence for the existence of such neurons. What had been reported instead is that many neurons in higher centers show less rather than more specificity when compared with cells in areas 17, 18 and 19. Studies in mammalian retina have revealed multiple processing systems apparently operating in parallel at the level of the ganglion cells. Striate cortex receives at least two different kinds of visual input from the lateral geniculate, and sends at least two different parallel outputs to other brain regions. Within striate cortex there is some segregation of different functional cell types in separate layers. The accumulated evidence suggests the existence of parallel visual processing mechanisms beginning in mammalian retina and extending through striate cortex to higher cortical centers. The notion of separate processing systems for the detection of different features in the visual world recalls earlier work by Lettvin, Maturana, McCulloch and Pitts concerning visual processing in the frog retina and optic tectum.

A quantitative study of the projection area of the central and the paracentral visual field in area 17 of the cat. I. The precision of the topography

1. In the course of long oblique penetrations through the postlateral gyrus a variation in the position of the receptive fields (RF-scatter) of single cells recorded extracellularly is observed. This is superimposed on the continuous topological representation of the retina. Spezifying the RF-positions by the azimuthal and elevation coordinates of their geometrical centers, the standard deviation (SD) of the mean RF-positions of cells recorded in 200 mum long horizontal sections of cortex is calculated and the total radial scatter of RF-positions (Sanderson, 1971) as defined: (see article) is determined. The radial scatter is found to have its smallest value (1 degree visual angle (v.a.)) in the projection area of the functional center of the area centralis increasing to 3-4 degrees v.a. at 10 degrees eccentricity. 2. The mean RF-diameter as defined: (see article) is centrally 0.7 degrees v.a. increasing to 2.6 degrees v.a. at 10 degrees eccentricity. The ratio of the largest RF-diameter to the smallest RF-diameter is between 7-9 and remains almost constant over the central 10 degrees of the projection area. The magnification factor (M) as defined: mm Cortex/degree v.a. is centrally 2.3, decreasing paracentrally to 0.6. 3. The cells in area 17 whose RFs have the same direction in the visual field constitute the spatial subunit of the retinocortical projection. The diameter of the spatial subunit is calculated as: (see article). The spatial subunit functionally represents, therefore, that part of the visual field whose location and area is calculated by averaging over the RFs of the individuals of its cell population. It is found that the cells belonging to a spatial subunit are distributed within a cortical cylinder of 2.6-2.8 mm in diameter, the peak of the distribution coinciding with the central axis of the cylinder. 4. Within the projection area of the central 10 degrees of the retina in area 17 the spatial subunits have the same diameter. This suggests that each retinal ganglion cell is functionally connected with an equal number of cells in area 17 irrespective of its position within the retina and that, therefore, the retinocortical projection is organized on the basis of a stereotyped schema if a basic spatial relationship is concerned.

Deficits in binocular depth perception in cats after alternating monocular deprivation

Allowing very young kittens to see with only one eye at a time greatly reduces the proportion of binocular cortical cells. Compared to normal cats these specially reared animals suffer deficits in binocular depth perception while retaining normal activity in the two eyes. Evidently, binocular cells play a crucial role in stereopsis.

The origin of efferent pathways from the primary visual cortex, area 17, of the macaque monkey as shown by retrograde transport of horseradish peroxidase

The retrograde transport of horseradish peroxidase has been used to identify efferent cells in area 17 of the macaque. Cells projecting to the lateral geniculate nucleus are small to medium sized pyramidal neurons with somata in lamina 6 and the adjacent white matter. The projection to the parvocellular division arises preferentially from the upper half of lamina 6, while that to the magnocellular division arises preferentially from the lower part of the lamina. The projection to both superior colliculus and inferior pulvinar arises from all sizes of pyramidal neurons lying in lamina 58 (Lund and Boothe, '75); at least pyramidal neurons of lamina 5B send collateral axon branches to both destinations. Injections with extensive spread of horseradish peroxidase show that many cells of lamina 4B and the large pyramidal neurons of upper lamina 6 also project extrinsically but their terminal sites have not been identified. Other studies have indicated that cells of laminae 2 and 3 project to areas 18 and 19. Therefore every lamina of the visual cortex, with the exception of those receiving a direct thalamic input, contains cells projecting extrinsically. Further, each lamina projects to a different destination and from Golgi studies can be shown to contain cells with specific patterns of dendritic branching which relate to the distribution of thalamic afferents and to the patterns of intracortical connections. These findings emphasise the significance of the horizontal organisation of the cortex with relation to the flow of information through it and contrast with the current concept of columnar organisation shown in physiological studies.

The intrinsic, association and commissural connections of area 17 on the visual cortex

An experimental neurohistological study has been made of the intrinsic connections of the cortex of area 17 of the monkey, of the commissural connections of the visual cortex of the cat and monkey and of the association fibres passing into area 17 of the cat. In light microscopic studies the axonal degeneration method of Nauta has been used, and the site and mode of termination of the degenerating fibres has also been determined with the electron microscope...

[Demonstration of direct projections between the ponto-mesencephalic tegmentum and the visual cortex of the cat using a peroxidase technique]

Using a retrograde tracer technique with horseradish peroxidase, we have identified the afferent connections to the visual cortex from the dorsolateral pontine tegmentum (locus coeruleus, nucleus parabrachialis medialis and nucleus parabrachialis lateralis) as well as from the dorsal raphe nucleus.

Receptive fields of single cells in the visual cortex of the hooded rat

The receptive fields of 107 single cells in area 17 of the hooded rat were examined. About half the cells responded to stationary as well as moving stimuli and about half only to movement. A variety of receptive field types were observed. Some of the cells responding to stationary stimuli had circular receptive fields, some with and some without annuli, some had elongated receptive fields, some had irregular receptive fields. Of the cells that responded only to movement, some were orientation or direction specific and some were not. Only two cells were found that responded to stimulation of the ipsilateral eye. Columnar organization of the cortex was not observed.

[On the pigmentation pattern of the human area striata (author's transl)]

Pigment-architecturally, within the area striata of man, IVcbeta differs considerably from bright adjacent layers because of the presence of deeply staining lipofuscin granules.

Responses of striate cortical cells to moving edges of different curvatures

The responses of twenty cells to stimuli of varying curvature were measured in cat's striate cortex. All the investigated cells were sensitive to the orientation of lines and not hypercomplex. Fourteen cells showed a systematic change of response with curvature. The optimal curvatures of the cells were distributed over the whole range investigated. Six cells were insensitive to curvature. The responses from all the typical simple cells (8) varied with curvature, whereas all the complex cells (5) were insensitive to curvature changes. The curvature tuning curves were broad and the variability to individual stimuli was high, independent on whether the cell responded best to straight or to curved edges. The findings do not support the view that individual cells of area 17 could detect curvature.

Complex tight junctions of epithelial and of endothelial cells in early foetal brain

?The morphology of epithelial and of endothelial intercellular junctions in human foetal (9-15 weeks gestation) and sheep foetal (50, 60 and 125 days gestation, term 147 days) brain has been studied using the freeze-fracture technique and thin section transmission electronmicroscopy. Freeze-fracture replicas of the choroid plexus of both early human and sheep foetuses showed that the choroidal ependymal cells are linked at the ventricular surface by tight junctions. Freeze-fracture replicas of foetal cortical endothelial cell junctions showed that they are still more complex than those of choroidal epithelial cells, in all specimens so far examined. In some 60 day sheep foetuses the dye Alcian blue, which binds to plasma albumin and which iselectrondense when treated with osmium tetroxide, was injected intravenously a few minutes prior to fixation. The dye penetrated from blood into brain extracellular space and c.s.f. but apparently not by an intercellular route. The dye was found in a tubular system (endoplasmic reticulum) in both choroidal epithelial and cortical endothelial cells. The possibility that protein penetrates into the foetal brain and c.s.f. by a transcellular route is discussed. The possible significance of these findings in relation to previous ideas and studies of the development of blood-brain barrier mechanisms is also considered.

A study of inhibitory antagonism in cat visual cortex

Since there seems to be good evidence that GABA may act as an inhibitory neurotransmitter in the mammalian cortex, we tested the effects of an antagonist of GABA, namely the alkaloid bicuculine, on the response properties of visual cortex neurons, using a computer-controlled stimulus presentation system to assess quantitatively the changes in receptive field organization after the drug. Complex cells were most affected, increasing both evoked and spontaneous activity and losing some of their specificities for stimulus parameters such as orientation and direction. Hyper-complex cells lost their inhibitory flanks, responding equally well to long and short bars after the drug. Simple cells were the least affected, usually becoming somewhat depressed after the drug. Preliminary tests with another inhibitory amino acid antagonist, strychnine, showed that it excited simple cells, indicating that possibly more than one inhibitory transmitter is at work in the cortex. The results are discussed with relation to the synaptic anatomy of the cortex, and it is concluded that a class of stellate cells, using GABA, is a likely candidate for the transmitter of some intracortical inhibition.

[Classification of cortical neurons according to the character of their background impulse activity]

The method of basic components and cluster analysis was used to classify 75 units in the visual cortex of alert rabbits, proceeding from the empiric form of distribution of interspike intervals (DII), the mean frequency of impulsation and the relative number of intervals up to 500 msec. They were classified into nine groups containing from two to twelve units. Besides the cues, used as a basis for classification, the groups of units also differed in the structure of burst activity, the correlation of adjacent intervals and the ratio between the short (up to 20 to 60 msec) and long intervals. The latter served to make suggestions on the genetic affinity of bimodal DII to those with a bend on the waning after the maximum, and on the nature of formation of high frequency bursts and intervals exceeding 20 to 60 msec.

Eye movement-related inhibition of primate visual neurons

The influence of saccadic eye movements (EM) upon spontaneous neuronal activity was studied in the lateral geniculate nucleus (LGN) and striate visual cortex (VC) of encéphale isolé monkeys. EM were spontaneous and occurred in total darkness to eliminate the effects of retinal image displacement. The activity of LGN cells was not altered in association with EM. In contrast, 76% of cells studied in VC displayed a period of inhibition related to spontaneous EM in total darkness. EM-related inhibition of VC neurons was directionally specific; for each cell there was one quadrant of EM direction for which inhibition was most prominent. The majority of VC neurons showed inhibition in relation to EM directed into only one quadrant of the visual field. Reliable detection of EM-related inhibition required the formation of average histograms of neuronal firing time-locked to EM. For individual EM (even of optimum direction), a consistent degree of inhibition was not seen. The time course of EM-related inhibition of VC neurons is consistent with that reported for saccadic suppression. These results support the concept of a central mechanism (corollary discharge) acting at the cortical level being of significance in saccadic suppression.

The morphometry of the branching pattern in dendrites of the visual cortex pyramidal cells

An analysis has been made of the three-dimensional branching structure for the basal and apical dendrites of cortical neurons in an adult rabbit. The real branching angles of basal dendrites and apical oblique branches are in the same range, but differ from those of the apical main shaft. Therefore, several different parts of the apical dendrite have to be distinguished on anatomical grounds, coincident with the presynaptic areas distinguished in the literature. The bifurcations of basal dendrites are essentially symmetrical. The mode of outgrowth, however, is non-symmetrical. Redirection of dendrites will, therefore, occur. This redirection is often not complete, so that a large variability of branching angles results. The possible significance of the observed symmetry is discussed.

Receptive fields of single cells and topography in mouse visual cortex

The visual cortex was studied in the mouse (C57 Black/6J strain) be recording from single units, and a topographic map of the visual field was constructed. Forty-five percent of the neurons in striate cortex responded best to oriented line stimuli moving over their receptive fields; they were classified as simple (17%), complex (25%) and hypercomplex (3%). Of all preferred orientations horizontal was most common. Fifty-five percent of recpetive fields were circularly symmetric: these were on-center (25%), off-center (7%) and homogeneous on-off in type (23%). Optimal stimulus velocities were much higher than those reported in the cat, mostly varying between 20 degrees and 300 degrees/sec. The field of vision common to the two eyes projected to more than one-third of the striate cortex. Although the contralateral eye provided the dominating influence on cells in this binocular area, more than two-thirds of cells could also be driven through the ipsilateral eye. The topography of area 17 was similar to that found in other mammals: the upper visual field projected posteriorly, the most nasal part mapped onto the lateral border. Here the projection did not end at the vertical meridian passing through the animal's long axis, but proceeded for at least 10 degrees into the ipsilateral hemifield of vision, so that at least 20 degrees of visual field were represented in both hemispheres. The magnification in area 17 was rather uniform throughout the visual field. In an area lateral to area 17 (18a) the fields were projected in condensed mirror image fashion with respect to the arrangement of area 17. Medial to area 17 a third visual area (area 18) was again related to 17 as a condensed mirror image.

The development of synapses in the visual system of the cat

Synapses have been counted by electron microscopy and neurones by light microscopy through the depth of the visual cortex in a series of cats from 37 days gestation to adulthood. A few definite synapses are present as early as three weeks before birth, but there is then a latent period of four weeks before synapses increase rapidly in number 8-37 days after birth. The synapses occur just above and just below the cell plate at first, but in the adult cat they become evenly distributed in the depth of the cortex. The gradual separation of neurones by neuropil during development precedes a parallel increase in the density of synapses by about one week. The average number of synapses associated with one neurone rises to a peak of about 13,000 at seven weeks after birth. The densities of synapses and of neurones subsequently fall to slightly lower values in adult cats as the glial cells continue to develop. The timing of synaptic development in the visual cortex has been compared quantitatively with that in the L. G. N. and qualitatively with synaptogenesis in the retina. Synapses develop in the L. G. N. and cortex in a parallel fashion, and the L. G. N. precedes the cortex by a short interval of about two days. In the cell plate of the retina a few receptor synapses are present nine days before birth. Inner plexiform synapses are aslo present at this time, but ribbon-containing synapses do not appear until birth. Very few receptors possess outer segments with discs at birth, but five days later disc-bearing outer segments have developed. Thus synaptic development starts before afferent impulses can enter the visual system, but the main increase in synapses in the L. G. N. and cortex takes place four weeks after the start of synapse formation while the visual system is being used.

Periodic excitability changes across the receptive fields of complex cells in the striate and parastriate cortex of the cat

1. Complex cells in cortical areas 17 and 18 of the cat have been studied in response to narrow slits and edges moving across the receptive field in the preferred direction and also to stationary slits of different widths. 2. Average response histograms, recorded as a narrow slit was moved across the receptive field, displayed a periodic series of peaks above a base line level. The response histogram for most area 17 and 18 cells contained five principal peaks; sometimes one or two weaker peaks were present at receptive field borders. The histogram for one cell located at the area 17-18 border showed thirteen distinct peaks. Periodic response patterns were also generated as an extended edge was moved across the receptive field. Plots of cell responses versus slit width for stationary slits of different widths also indicated periodic response pattern. 3. The accuracy of determining the preferred slit orientation was the single most important requirement for demonstrating the periodic response pattern. Significant changes in the appearance of the periodic pattern occurred even upon 5 degrees rotations away from the preferred orientation. 4. Average response histograms were also studied over a wide range of moving slit velocities. The number of peaks across corresponding spacings within the recewptive field remained constant over a range of velocities. Response amplitudes, however, were velocity dependent. Thus the response peaks remain associated with fixed positions within visual space independent of stimulus velocity, even though temporal as well as spatial factors may be involved in response selectivity and the periodic modulation. The most striking periodic response histograms were generated at the velocities which produced the greatest cell firing rates. Area 17 complex cells responded well to velocities of less than 0-5 degrees to 6-0 degrees/sec, but cells in area 18 generally required higher velocities, sometimes as high as 20 degrees--30 degrees/sec, for a good response. 5. Spatial frequencies for the periodic component of the receptive field for area 17 cells in the central visual area covered a range of three octaves up to 5 cycles/degree, and area 18 cells included another octave on the low frequency side. The spatial frequency of a cell was found to be roughly inversely proportional to the receptive field width. Only a small sample of area 18 cells was studied, but these cells tended to represent low spatial frequencies and to respond selectively to high velocity stimuli...

Modification of single neurons in the kitten's visual cortex after brief periods of monocular visual experience

Kittens were deprived of form vision by suturing the lids of both eyes, except for a brief period (1, 6 or 20 hours) on the 29th day when the right eye was opened. 6 space and 20 hours of monocular vision produced a distinct shift in the ocular dominance of visual cortical neurons towards the experienced eye, and an increase in the proportion of cells with obvious orientation selectivity. These modifications in the visual cortex were enhanced by a period of "consolidation": they were somewhat less obvious if recordings were taken immediately after the exposure but were complete 2 days later. Although remarkably little visual experience was needed for these changes, the results contrast with the effects of rearing in an environment of vertical stripes, where only 1 hour of exposure produces much more striking effects. A normal visual environment may have a less powerful organizing influence on cortical neurons than such an environment containing only one orientation.

Progressive changes in kitten striate cortex during monocular vision

Following initial rearing in either total darkness or normal illumination, kittens at different ages were subjected to right-eye closure and various periods of vision through the left eye. After the period of monocular vision, single units in striate cortex were tested for visual responsiveness through each eye. A severe reduction in the proportion of units responsive to the deprived eye occurred over the first few days of monocular vision. Functional abnormalities were variably present after 1 day, marked after 2.5 and 3.5 days, and complete after 10 days. Monocular vision produced very much the same effect on ocular dominance of striate units, provided age and duration of suture were identical, regardless of whether kittens had received prior dark- or light-rearing.

Laminar thermocoagulation of the visual cortex of the rat. I. Interlaminar connections

Lesions were made in the visual cortex of hooded rats by laminar thermocoagulation and later examined using modifications of the Fink-Heimer method. Following lesions limited to supragranular layers a dense, descending projection to layer V was identified, as well as a probable projection to deep layer III, callosal and cortico-cortical projections, but no subcortical projections. The morphological changes in the degenerating systems were examined as a function of survival time. The anatomical basis of functional derangement related to superficial lesions of visual cortex was discussed.

Neural basis of orientation perception in primate vision

Orientational differences in human visual acuity can be related parametrically to the distribution of optimal orientations for the receptive fields of neurons in the striate cortex of the rhesus monkey. Both behavioral measures of acuity and the distribution of receptive fields exhibit maximums for stimuli horizontal or vertical relative to the retina; the effect diminishes with distance from the fovea. The anisotropy in the neuronal population and in visual acuity appear to be determined by postnatal visual experience.