R. V. Novikova

Sensitivity to cross-like figures in the cat striate neurons

The responses and orientation tuning in 48 out of 62 (77.4%) neurons of the cat striate cortex (area 17) significantly, but with different sign, changed at stimulation by specific cross-like figure flashing in receptive field as compared with single light bar of preferred orientation. Neurons of the first group (19 units from 62, 30.6%) were found to increase the responses by 3.3 times if stimulated by a certain cross-like figure, specific for each cell configuration and orientation. Under the same conditions, neurons of the second group (29 or 46.8% revealed a three-fold decrease of responses and all tuning characteristics worsened. Among them 8% of total number of cells showed bimodal or double orientation tuning when stimulated by some configurations of crosses due to an angle specific inhibition. Dependence of the revealed effects on excitatory convergence from neurons with different orientation tuning, on inhibitory influences from end-stop and side-zones of receptive field, as well as possible functional implication of the first group neurons for an angle and line-crossing detection are discussed.

Selective and invariant sensitivity to crosses and corners in cat striate neurons

Many neurons (56/174, or 32.2%) studied in the cat striate cortex (area 17) increased significantly (by 3.3 times on average) their responses under stimulation by cruciform or corner figures of specific or non-specific shape and orientation flashing in receptive field as compared with single light bar of preferred orientation. Most of these neurons (71.4%) were found to be highly selective to both the shape (the angle between the figures lines) and orientation of these figures. In the neuronal selection studied we have also found all possible types of invariance of the cross and corner tuning to orientation and/or shape of these figures. We found neurons with selectivity to the form of the figures and invariance to their orientation and, on the contrary, units invariant to shape but selective to orientation. Some cells were found invariant to both the form and orientation of the cruciform or corner figure but highly sensitive to appearance of any such figure in the receptive field. Two main hypotheses about the mechanisms of selective sensitivity to crosses and angles can be considered. They are as follows: an excitatory convergence of two units with different preferred orientations, and intracortical inhibitory interactions. The cells with double orientation tuning for a single bar are found relatively rarely (about 20%), thus making the first suggestion the most unlikely. This circumstance is of special importance since it provides evidence against the hierarchic formation of the higher-order cortical units from a set of lower-order cells that is still under discussion. The units with high sensitivity to cross or corner seem to be ideally suitable for their selection, rather than to serve as classical orientation detectors only.

Interrelation of tuning characteristics to bar, cross and corner in striate neurons

Characteristics of responses and background activity, as well as of tuning to a single bar orientation and to cross or corner shape and orientation have been compared in one third (561174) of neurons in the cat striate cortex. Shortening of the response latency to cross vs bar, to corner vs bar and to corner vs cross was revealed in most of the units studied. Direct correlation between the response and tuning characteristics for bar, cross and corner was revealed: units with better tuning to one type of stimulus were typically better tuned to the other types of stimuli. At the middle cortical depth (700-1200 microm from the surface) we found a reliable improvement of response magnitude and latency, cross/bar response ratio and selectivity of tuning in comparison with more superficial and deeper layers. Although we could not find a direct correlation between characteristics of tuning to figures and the type of the receptive field (simple, complex or hypercomplex), our data pointed to a lower cross/bar ratio and selectivity of tuning in the units with small receptive fields. The functional implication of neuronal sensitivity to cross and corner and possible meaning of correlation between their functional characteristics are discussed.

Tuning for Cruciform Stimuli in Visual Cortex Neurons of Cat

In the primary visual cortex of an immobilized awake cat, nearly one-third of the neurons studied (8 out of 22) were found to respond to flashing cruciform light stimuli 1.5–4 times better than to single stimulations with the strips of preferred orientation. It is suggested that such neurons can detect angles and line intersections.

Double orientation tuning of neurons in the primary cortex of cat at different levels of alertness

Orientation tuning (OT) of 225 cat neurons of the primary visual cortex (field 17) to the flashing of a light bar in the discharge centers of their receptive field (RFs) were investigated. It was found that 43% of the cells investigated were monomodally tuned, i.e., were primarily detecting horizontal and vertical orientations. The remaining 57% of the neurons exhibited double OT, i.e, exhibited, in addition to a main preferred orientation (PO), an additional preferred orientation (aPO) at a right or acute angle to the main orientation (the mean angle between the two OT maxima equalled 71.4±2.4°). In bimodal cells, the additional maximum of OT was comparable in magnitude to the main maximum (averaging 0.7±0.03 of the PO) in half the cases. The orientational properties of the main and additional maxima were almost indistinguishable. Under light or moderate anesthesia, approximately half the neurons with double OT became monomodal; at the same time, a small fraction of monomodal cells (12%) manifested double OT. Under anesthesia, the angle between two the preferred orientations decreased, while the ratio of amplitude characteristics remained unchanged. Monomodal neurons frequently exhibited simple RFs and OTs unaffected by anesthesia. Neurons with double OT, on the other hand, exhibited simple and complex types of RFs just as often and their OT changed under the influence of anesthesia. It is suggested that neurons with double OT can function as detectors of angles and angles of intersecting lines; such angles, together with line orientation, are important attributes of images. In contrast, monomodal neurons may provide a benchmark for a stable reference system of orientation coordinates. The interaction of the two neuronal systems mentioned may allow effective analysis of image attributes at the level of the primary visual cortex.

Orientation selectivity of visual cortical neurons at different stimulus intensities in cats

Orientation selectivity of 24 neurons in area 17 of the visual cortex at different intensities of test bars of light, flashing against a constant light background in the center of the receptive field, was investigated in acute experiments on immobilized cats. Five neurons were invariant in orientation tuning to stimulus intensity (contrast): Although the magnitude of the response and acuteness of orientation selectivity were modified, preferential orientation was unchanged. More than half of the cells studied (13) were classed as noninvariant, for their preferential orientation was significantly shifted by 22–90° with a change in contrast. Small shifts of the peak of orientation selectivity, not statistically significant, were observed for the other neurons. Invariant neurons, unlike noninvariant, were characterized by preferential horizontal and vertical orientation, a lower frequency of spontaneous and evoked discharges, and the more frequent presence of receptive fields of simple type. The mechanisms of the change of orientation selectivity during contrast variation and also the different use of the two types of cells in orientation detection operations are discussed.

Selective sensitivity of the cat striate neurons to cruciform and angular figures of various orientations

Selectivity and invariability of tuning were studied in 51 neurons of the primary visual cortex (area 17); cruciform and angular figures (CF and AF, respectively) of different configurations and orientations were presented in their receptive fields. Twenty-three neurons, or 45% of the studied cells, demonstrated selective sensitivity to these figures. Their responses considerably (2.38±0.36 times, on average) increased, as compared with those evoked by presentation of a single bar of preferred orientation. In the examined group, 2 cells demonstrated sensitivity both to the CF and AF. A wide range of detector properties related to the CF and AF analysis was found in the analyzed neuronal population. Detectors of configuration of these figures are described. Selective sensitivity to the angle between branches of these figures was observed in 17 neurons, and responses of 2 neurons among them showed invariability to orientation of these figures. Four cells were selective for orientation and were insensitive to configuration, and 4 other cells showed no specific sensitivity to either of these properties, but were sensitive to the appearance of a CF itself in their receptive field (these cells were regarded as invariant detectors of crossing nodes). Data inconsistent with the hierarchic principle of detection of the above properties are presented. Possible mechanisms and functional significance of selective sensitivity of striate neurons to the CF and AF are discussed.

Effects of local adaptation of receptive field on sensitivity of the cat visual cortex neurons to cruciform images

The responses to flashing single light bars of different orientation and to cruciform images (CI) were compared in 9 neurons of the cat striate cortex possessing high specific sensitivity to CI, during local adaptation of various receptive field (RF) zones. In most neurons, a two- to threefold reduction in the response to CI with a constantly present bar of optimum or orthogonal orientation, if compared with a response to the figure consisting of two flashing bars, was found. Responses to the CI including an adaptation bar were often increased, if compared with those observed at usual orientation tuning. The role of a cross-orientation inhibition in the formation of a selective sensitivity to CI in the neurons of the visual cortex is discussed.

Effect of Nembutal narcosis on dynamics of orientation tuning of cat visual cortex neurons

The dynamics of orientation tuning (OT) were investigated in acute experiments on immobilized locally anesthetized cats during response development in 40 neurons of the primary visual cortex before and after Nembutal injection. The range of OT scanning decreased in 53.8% of neurons after Nembutal administration (on the average, by 53.4±5.1°; P<0.001); the phenomenon disappeared completely in some neurons. After Nembutal anesthesia, scanning in 20.5% of units either increased or started up in cases of its absence. The scanning range remained constant in 25.6% of neurons. The mentioned changes in the scanning range were consistently more accentuated in cells for which the preferred orientation, as estimated by standard criteria, was shifted under narcosis than in cells invariant to general anesthesia. In the latter group, units with an unchanged scanning range occurred four times more often at all stages of the experiment as against the group of “unstable” neurons.

CORRELATIONS BETWEEN THE PROPERTIES OF VISUAL CORTEX NEURONS IN THE CAT

In acute experiments on immobilized cats 13 functional characteristics of 96 visual cortex neurons were investigated. By means of regression, cluster, and multivariate analyses, these could be divided into two subgroups with varying degrees of correlatedness. Cells of the first subgroup were more frequently characterized by their relatively central location in the visual receptive field, while those of the second subgroup were more often found at the periphery. A significant correlation was found between 11 of the properties investigated. In each subgroup, cells with more centrally localized small receptive fields had, in comparison with neurons of the peripheral visual projection, short latent periods, lower thresholds, phasic response, and brief summation; their responses varied widely in intensity, and they had greater differential sensitivity, and were distinguished by high-frequency discharges. Significant correlation coefficients between the factors studied fluctuated between 0.21 and 0.99; moreover, there were almost twice as many significant relationships in the first subgroup of neurons as in the second. The possible mechanisms of correlations between the properties of the visual cortex neurons are discussed, as well as the reasons why they differ in cells of the two subgroups, the cortex, and the lateral geniculate body.