Color properties of the receptive fields of visual cortex neurons in the squirrel

The colour-sensitive properties of the visual cortex neurons were studied in the squirrel. All the neurons responded to achromatic, green and blue visual stimuli; responses to red stimuli were slight or absent. According to responses to patterned visual stimuli the neurons were classified as non-selective, directionally-selective and orientation-selective (simple and complex). No colour-opponent properties were revealed in any of these neuronal groups: neuronal responses were qualitatively the same to achromatic, green and blue stimuli, and the receptive fields organization was independent of the stimulus colour. These data are discussed with respect to the fact of presence of colour-opponent cells in the retina and lateral geniculate nucleus of squirrel.     

Orientation selectivity of synaptic potentials in neurons of cat primary visual cortex

Neurons of the visual cortex of the cat were penetrated with intracellular electrodes and postsynaptic potentials evoked by visual stimuli recorded. By alternately polarizing the cell with steady current injected through the recording electrode, IPSPs and EPSPs could be recorded and analyzed independently. Hyperpolarizing current suppressed IPSPs and enhanced EPSPs by moving the membrane potential toward the IPSP equilibrium potential. Depolarizing the cell toward the EPSP equilibrium potential enhanced IPSP. The responses to electrical stimulation of the LGN, where EPSPs and IPSPs could be distinguished easily by virtue of their characteristic latencies and shapes, were used to set the current injection to the appropriate level to view the two types of synaptic potential. EPSPs were found to be well oriented in that maximal depolarizing responses could be evoked at only one stimulus orientation; rotating the stimulus orientation in either direction produced a fall in the EPSP response. IPSPs were also well tuned to orientation, and invariably the preferred orientations of EPSPs and IPSPs in any one cell were identical. In addition, no systematic difference in the width of tuning of the two types of potential was seen. This result has been obtained from penetrations of over 30 cortical cells, including those with simple and complex receptive fields. It is concluded that orientation of cortical receptive fields is neither created nor sharpened by inhibition between neurons with different orientation preference. The function of inhibition evoked simultaneously with excitation by optimally oriented stimuli has yet to be determined, though it is likely to be the mechanism underlying other cortical receptive field properties, such as direction selectivity and end-stopping.     

Characteristics of the responses of cat visual cortex neurons to photic stimulation of different areas of their receptive fields]

The functions of intensity, thresholds of reactions, thresholds of inhibitory effects and differential sensitivity to intensity of local light stimulation of central and different peripheral regions of receptive fields of 96 units in the 17th field of visual cortex were investigated in immobilized and nonanesthetized cats in dark adaptation. Receptive fields of cells had wide threshold and superthreshold reliefs (3-30 degrees). Some of them had V-shape reliefs, while others--reliefs with alternating zones of high and low excitability. As a rule, sensitivity of excitatory and inhibitory inputs was maximal in the centre of the receptive field. In the investigated population of cells the inhibitory inputs were more standard in sensitivity and on the average of lower excitability. The threshold relief of the receptive fields was markedly narrowed under light adaptation mainly due to a decrease in the peripheral inputs sensitivity. The number of low-threshold units, the differential brightness sensitivity and sensitivity of the inhibitory system increased in the visual cortex as compared to the lateral geniculate and retinal units. The mechanisms of receptive field formation in the visual cortex and their plasticity depending on the adaptation level, the role of excitatory and inhibitory inputs in these effects and behavioral significance of the obtained data are discussed.     

Laminar distribution of neurons with different types of receptive fields in the visual cortex of the rabbit

232 neurons of rabbit visual cortex were classified as cells with simple (34.1%), complex (16.4%), hypercomplex (18.5%), non-oriented (21.1%) receptive fields and other (9.9%). Some quantitative characteristics of cellular responses (background activity, velocity and tuning of orientation selectivity) correlated with these receptive field properties. Cells with non-oriented receptive fields were predominant in layer IV and occurred very rarely in layer VI. Cells with simple receptive fields were found in all layers, but were predominant in layer VI. Cells with complex receptive fields occurred with greater frequency in layer V and VI and less commonly in layer IV. Cells with hypercomplex receptive fields occurred frequently in layers II + III and IV but very rarely in layers V and VI. The rate of the background activity of layer II + III cells was the lowest and that of layer V cells--the highest. Tuning of orientation selectivity of simple and complex cells was narrower in layers II + III and V than in layers IV and VI.     

Heterogeneity of a neuron population with complex receptive fields in the visual cortex of costs]

Visual responses of the striate complex cells to stimuli orientation, direction and velocity of movement were studied in awakening, unanesthetized cats. "Complex" cells were divided into four groups according to the response characteristics which were obtained using a stationary slit, moving light spot and moving oriented stimulus. The first group units response characteristics suggest the presence of the orientation selectivity mechanism in their receptive field organization, the fourth group--the direction selectivity mechanisms, the second and third groups--the presence of both mechanisms. It is supposed that there are two separate mechanisms for coding the orientation and direction of the stimulus movement in the neuronal structures of the visual cortex.     

Generation of the receptive fields of subpial cells in turtle visual cortex

The visual cortex of turtles contains cells with at least two different receptive field properties. Superficial units are located immediately below the pial surface. They fire in response to moving bars located anywhere in binocular visual space and to two spots of light presented with different spatiotemporal separations. Their location in the cortex suggests that superficial units correspond to a distinct class of inhibitory interneurons, the subpial cells, that are embedded in geniculocortical axons as they cross the visual cortex of turtles. This study used a detailed compartmental model of a subpial cell and a large-scale model of visual cortex to examine the cellular mechanisms that underlie the formation of superficial units on the assumption that they are subpial cells. Simulations with the detailed model indicated that the biophysical properties of subpial cells allow them to respond strongly to activation by geniculate inputs, but the presence of dendritic beads on the subpial cells decreases their sensitivity and allows them to integrate the inputs from many geniculate afferents. Simulations with the large-scale model indicated that the responses of subpial cells to simulated visual stimuli consist of two phases. A fast phase is mediated by direct geniculate inputs. A slow phase is mediated by recurrent excitation from pyramidal cells. It appears that subpial cells play a major role in controlling the information content of visual responses.     

Neuronal connections underlying orientation selectivity in cat visual cortex

How do neurons of the visual cortex acquire their acute sensitivity to the orientation of a visual stimulus? The question has preoccupied those who study the cortex since Hubel and Wiesel¹ first described orientation selectivity over twenty-five years ago. At the time, they proposed an elegant and enduring model for the origin of orientation selectivity. Fig. 1A, which is adapted from their original paper and which contains the essence of their model, is by now familiar to most students of the visual system and to many others besides. Yet the model, and the central question that it addresses, is still the subject of intense debate. Competing models have arisen in the intervening years, along with diverse experiments that bear on them.     

Dynamic and static features of the receptive fields of neurons of the lateral suprasylvian area of the cat cortex

Dynamic and static properties of neurons were investigated in lateral suprasylvian area of cat. It was shown that neurons with different dynamic characteristics could have the receptive fields with similar static organization. Thus strict correlation between static and dynamic organization of the receptive fields of suprasylvian neurons were not observed. The majority of black-sensitive neurons appear to have off-fields. The contrast reversal test of moving stimuli revealed that the generation of responses both to the light and to the black stimuli was performed by the same part of the receptive field and was not conditioned by the spatially separate on- and off-discharge centers in the receptive field.     

Organization of the substructure of the receptive fields of neurons of the lateral suprasylvian area of the cat cortex

The substructure of receptive fields of the lateral suprasylvian area neurons was investigated in cats. It was shown that the majority of receptive fields investigated were organized of subelements with different qualitative characteristics according to their responses to moving visual stimuli. With the unmasking method of stimulation, small amplitudes of the motion evoked, as a rule, directional responses, whereas with the masking method the same amplitude of the motion produced nondirectional responses. Some receptive fields of the investigated neurons responded vigorously to the motion of borders of the visual stimuli. The heterogeneity in organization of the substructure of the receptive fields was explained by the convergence of different inputs to the investigated neurons.     

Structure of receptive fields of visually sensitive neurons of the cat hippocampus

Structure of receptive fields of visually sensitive neurons in areas CA1 and CA3 of the dorsal hippocampus was investigated in alert cats with the brain-stem pretrigeminal section. The receptive field (RF) structure of 76 hippocampal neurons was analyzed by methods of scanning the RF by moving stimuli and mapping all their surface by a stationary flashing spot. According to presented data the neurons were classified into three groups: neurons with homogeneous structure of the RF (54%), with nonhomogeneous (28%) and neurons more sensitive to stimulus motion (18%) than to a stationary flashing light. Experiments have shown that responses of hippocampal neurons are highly specific ones. Thus, 9% of neurons with the nonhomogeneous RF structure have shown specific responses to variation of the contrast and contours of moving stimuli. The presented results show that hippocampal visually driven neurons have well developed mechanism for processing visual sensory information and apparently this quality ensures participation of the limbic system in visually controlled behavior of the animal.     

Relation between the size of the receptive fields of field 21 neurons of the cat cerebral cortex and eccentricity

The interrelationship between the size of receptive fields and the eccentricity in areas 21, 19 and 7 of both hemispheres was studied on immobilized cats. Correlation between the receptive fields size and the eccentricity in area 21 in the both hemispheres was low (R = 0.15; P greater than 0.05). In areas 19 and 7 the correlation of the size fields and eccentricity was higher: for area 19 R = 0.9; P less than 0.0001 and for area 7 R = 0.5; P less than 0.01. The fact determined before that the receptive fields size in the left hemisphere are larger than in the right hemisphere was confirmed.     

Temporal characteristics of visual receptive fields in primary visual cortex and medial superior temporal cortex areas

We mapped the receptive fields of 49 cells from primary visual cortex and 19 cells from medial superior temporal cortex in two awake monkeys. The receptive field structures we obtained lasted a mean time of 32.7 ms in primary visual cortex and 38.4 ms in medial superior temporal cortex, showing no statistical difference. This result suggests that both areas have the same time requirements for processing visual information. In primary visual cortex, 100% of cells had conformed the receptive field structure at 65 ms pre-spike, whereas in medial superior temporal cortex it occurred at 150 ms. In both areas, cells with shorter response latencies had receptive field structures with longer durations. This may indicate that cells tend to synchronize their output to other areas.     

Activity-dependent receptive field changes in the surround of adult cat visual cortex lesions

Extracellular single cell spike activity was recorded in the visual cortex of anaesthetized adult cats at identical sites before and 2 days after focal excitotoxic lesions induced by injections of ibotenic acid. In the surround of the lesions (up to 5 mm from the border of the lesion), the average postlesion receptive field (RF) sizes were not different from the prelesion RFs. However, RFs of neurons with increased postlesion excitability were slightly enlarged; such neurons were mainly found close to the anterior border of the lesion (< or = 1 mm). After applying a visual training procedure for 1 h to the postlesion RFs (repetitive, synchronous stimulation of a part of the RF and the neighbouring unresponsive part of the visual field), there was a small (0.4-0.8 degrees ) but significant and specific increase of RF size in about half of the tested neurons. This RF enlargement was similar to that observed with the same training procedure in the visual cortex of normal cats. Thus, small RF changes can be induced by visual stimulation within one hour in normal cells as well as in cells at the border of cortical lesions. Any differences between normal and lesioned animals appear to be related to lesion-induced changes of excitability.     

Neurons of the rabbit visual cortex with simple and complex visual fields

The receptive filelds of orientation-selective neurons were studied in the rabbit visual cortex. Two mechanisms of the selectivity were found: the mutual inhibition between on- and off-regions of the receptive field (simple type) and the orientation-dependent inhibition within the uniform region of the receptive field. The non-selective neurons could exhibit the lateral inhibition within the uniform region of the receptive field also. It is supposed that both "simple" and "complex" cells originate from the non-selective units.     

Spatial and temporal selectivity in the suprasylvian visual cortex of the cat

We recorded from single units in the medial and lateral banks of the posterolateral suprasylvian visual cortex (PMLS/PLLS) of the cat. The responses to drifting high-contrast gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency, were examined quantitatively for a sample of cells, whose receptive fields covered a wide range of eccentricities. The optimum spatial frequencies (average about 0.2 cycles/deg) were low compared to the values reported for striate cortex but similar to those for area 18. The mean spatial bandwidth (about 2 octaves) was slightly broader than that of cells in other cortical visual areas. The cut-off spatial frequencies ("acuities") covered a wide range, from 0.05 to 2.1 cycles/deg, similar to those of cells in area 18. Responses to drifting sinusoidal gratings were usually dominated by an unmodulated elevation of discharge, although some modulation occurred at the temporal frequency of drift, especially at low spatial frequencies. Modulated responses were relatively stronger in PMLS than in PLLS. For those cells that responded to flashed stimuli, stationary, contrast-modulated gratings presented at different spatial positions typically evoked small responses at the fundamental frequency (dependent on spatial phase) and a larger component at the second harmonic of temporal frequency, with no overall "null-position." The optimum spatial frequency was usually higher than would be predicted by simple summation within the dimensions of the receptive field. Thus, neurons in PMLS and PLLS, like complex cells in areas 17 and 18, behave nonlinearly and their spatial selectivity is determined by "subunits" smaller than their receptive fields. The range of preferred temporal frequencies ranged from less than 2.5 Hz to more than 10 Hz. In their temporal selectivity neurons in PMLS resembled cells in area 17, with little attenuation at low temporal frequencies, whereas there was a tendency for cells in PLLS to prefer higher temporal frequencies, as is common in area 18.     

Effect of picrotoxin on the receptive fields of neurons of the vibrissal projection area in the cerebral cortex of the cat

Effects of microelectrophoretic injections of picrotoxin on the neuronal receptive fields were studied in the vibrissae projection zone of S1 area in the cerebral cortex of anaesthetized cats. The sensitivity of neurons to the direction of vibrissae bending was completely abolished after picrotoxin influence. These facts show a great importance of intracortical inhibitory processes in the receptive field organization.     

Interrelation among properties of visual cortex neurons in the cat

In acute experiments on immobilized cats 13 functional characteristics of 96 visual cortex neurons were investigated. Regressional and cluster analyses were used to divide these neurons into two subgroups with different density and degree of connections between characteristics. The receptive fields of cells of the first subgroup were localized relatively centrally in the visual field, those of the second subgroup were localized more often on the periphery. A valuable correlation was found in the half of the studied characteristics. In each subgroup the more centrally localized cells with small receptive fields had relatively shorter latencies, lower thresholds, shorter temporal summation, wider intensity range and greater differential sensitivity; their responses were phasic, with high-frequency discharges. The density of valuable correlation of the characteristics varied from 0.21 to 0.99. The amount of these correlations in the first subgroup was two times higher than in the second one. The possible mechanisms of the correlation between the properties of the visual cortex neurons are discussed, as well as their differences in two subgroups and in the cortex and LGB.