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I A Shevelev

Publications and source records attributed to I A Shevelev.

At least 73 records · Page 4Linked to original sources
PubMed1983

[Dynamic detection of light intensity by cat visual cortex neurons].

Dynamics of intensity functions of neurons in area 17 of the visual cortex was studied by the method of time slices on unanesthetized cats. The intensity functions of the neurons were estimated from successive fragments of responses. In 70% of cases during 40-200 ms after beginning of the stimulus action the preferred intensity changed from lesser to greater brightness. The possible role of this effect in the intensity of time coding by the visual cortex neurons is discussed.

Animals↗
PubMed1982

Receptive fields of neurons in the cats visual cortex after a change of alertness level.

Exact and reliable changes in receptive fields (RF) of neurons in the primary visual cortex were revealed after the action of extravisual stimuli such as air-puff to the corner of a closed eye which leads to an enhancement of alertness level. Such stimulus evoked a change in the configuration and size of RFs in most investigated units. This lasted from 15 to 30 min and in most cases the size of RFs excitatory zones decreased, while the size of inhibitory surround increased. This effect could be seen at different levels of visual adaptation. Under light and short anesthesia (sombrevin) the main picture of RF change was different: increase of excitatory zones and nearly equal probability of the opposite changes in inhibitory RF parts lasting from 5 to 30 min. The role of intracortical inhibitory systems in the modulation of RF size and form is discussed. The possible behavioral meaning of the observed effects may consist in the fine description of signal properties in the small RFs at high alertness level, while under opposite conditions enlarged RFs can serve to improve signal detection and organization of orienting reaction even to a weak new stimulus.

Animals↗
PubMed1982

[Dynamics of the receptive fields of visual cortex and lateral geniculate body neurons in the cat].

The dynamics of receptive fields of the visual cortex and lateral geniculate neurons were investigated in acute experiments on unanesthetized immobilized cats. For this purpose a computer presented small local flashes in a random order to 100 points of the receptive field and changes in three-dimensional relief of activity within the field were evaluated every 20 ms. Regular dynamic reorganizations of registered receptive fields of all investigated neurons were observed both after switching on and off the light stimulus. After the latency a small region of weak responses appeared in the centre of field. Later on the recorded field enlarged gradually, reaching maximal size at 60-100 ms, and after that began to narrow and disappeared or fell to pieces. The central inhibitory zone of the field had the same dynamics. If the neuron responded to stimulation by generating two bursts of spikes, during the second one the receptive field changed in the same way. The effect was revealed under various conditions of light stimulation. Functional significance of these effects for orientational information coding by visual cortex neurons is discussed.

Adaptation, Physiological↗
PubMed1981

[Dynamics of orientational tuning of visual cortex neurons in the cat].

Orientational tuning of singly units in the primary visual cortex of a nonanesthetized paralyzed cat was studied by usual criteria (number of spikes and maximal frequency of the response) and by "time slices" method. A computer plotted graphs of orientational tuning for successive time intervals with a step of 10 or 20 ms (differential slices) or for prolonged intervals with same step (integral slices). In both cases it was revealed that in all the investigated units orientational tuning markedly changed in time, most often in wideness of characteristic and in 87% of cases in preferred orientation range was detected: preferred orientation systematically moved for 15-112 degrees in successive moments during some tens of ms of the first and the second burst of spikes. A possible functional significance of the data revealed and the neurophysiological mechanisms are discussed.

Animals↗
PubMed1980

[Receptive fields of cat visual cortex neurons when photic stimulation parameters are altered].

Spatial excitability reliefs of receptive fields of visual cortex units were investigated in acute experiments on immobilized cats by changing physical and physiological parameters of light stimulation under dark, mesopic and low photopic adaptation. Transition from dark to low mesopic adaptation results in no changes of the shape and size of the receptive fields, revealed by constant intensity test slits, whereas with transition to low photopic adaptation the receptive fields were narrowed in 72% of cases and acquired an elongated shape while the sharpness of their selectivity to direction of movement and orientation of stimuli increases. At a constant background the receptive fields narrowed with decreasing brightness of the test light slit. The excitability relief in the field estimated by the absolute threshold of the cell response and by the level of intensity needed to get a constant number of discharges in response, considerably narrowed with a rise in the threshold criterion and with transition from dark to light adaptation. At light adaptation, stimulation with physiologically equal intensity revealed narrower reactivity reliefs. than those at dark adaptation. The estimation of the receptive fields, taking into account a possible contribution of light scattering in the eye media and over the screen, shows that in most cases their size cannot be explained by this factor.

Animals↗
PubMed1979

[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.

Animals↗
PubMedDate not supplied

Backward conditioned connection and inhibitory reorganization of the receptive fields of cortical neurons as the basis of subconscious change in the thresholds of visual recognition and detection.

Three groups of facts are compared in this study: the significant adaptive and adaptational modification of the receptive fields of neurons of the visual cortex of the cat, the conditioned, selective, subsensory change in the threshold of perception (detection and recognition) by an individual of a letter in relation to two control letters, and the role of spatially-specialized cortical inhibition in the formation and adaptive modifications of the receptive fields and detector properties of neurons of the visual cortex. The relationship of the described phenomena and the commonality of their mechanisms is discussed.

Adaptation, Physiological↗
PubMedDate not supplied

A model of a neuronal network for detection of single bands and cross-like figures.

A model of a neuronal network is presented which, like a significant proportion of the neurons in the visual cortex, identifies not only the orientation of a single band, but also high selective sensitivity to cross-like figures. The model was used to study the properties of a "cross detector" constructed on the basis of convergence of excitatory connections with different weightings from elements with different orientational tuning. It is shown that a cross detector with monomodal orientational tuning needs an amplifier mechanism (a reverberator).

Neural Networks, Computer↗
PubMedDate not supplied

Functional importance of alpha-activity in the visual cortex during recognition of images and movement.

Twenty-seven studies were carried out on the recognition of the shapes of geometrical figures of different sizes by healthy adults, on the recognition of the direction of movement of a light spot within the field of vision, and of visual illusions produced by rhythmic visual stimulation. Tachystoscopic presentation of figures and the onset of movement were synchronized with different phases of the EEG alpha-rhythm in the occipital region. In controls, stimuli were presented without a shift in the alpha-rhythm. Recognition improved significantly when small figures were presented at relatively late phases of the alpha-wave and when large figures (up to 9 degrees) were presented at relatively early phases. Recognition of the side and direction of apparent movement (in the left or right halves of the visual field and centrifugal or centripetal) depended on the phase of the alpha-wave only for nonuniform (accelerating or decelerating, depending on direction) movement, allowing for the cortical magnification factor. Centrifugal movements in experiments were recognized better than in controls, while centripetal movements were recognized worse, and elicited a relatively long-latency movement response. Diffuse rhythmic light stimulation at the alpha-rhythm frequency produced the illusory percept of a ring or circle in 11 of 12 subjects. The optimal stimulation frequency for this was tightly connected with the dominant alpha-rhythm frequency, with a correlation coefficient of 0.86. The link between these effects and the propagation of the wave process through the visual cortex, as reflected by the EEG alpha-rhythm, is discussed. The data support the hypothesis of Pitts and McCulloch [29], which proposes scanning of the visual cortex by a wave process operating at the frequency of the alpha-rhythm, which reads information from the visual cortex.

Adult↗
PubMedDate not supplied

Image features selected by neurons of the cat primary visual cortex.

The sensitivity of neurons in field 17 of the visual cortex in cats to cross-shaped, Y-shaped, and star-shaped figures flashing in the receptive field was studied. About 40% of the neurons studied (114 of 289) were found to generate large responses (with an average response factor of 3.06 +/- 0.32) to one of the figures flashing in the center of the receptive field, as compared with the responses produced to a single bar in the optimal orientation. Most of these neurons (72%) were selectively sensitive to the shape and orientation of figures; the remainder demonstrated some degree of tuning invariance to these properties. The latent periods of responses to figures were usually shorter than those of responses to bars. Tuning parameters for bars and figures were generally related: neurons with acute orientational tuning to a bar were usually highly selective to both the configuration and the orientation people figures. Separate or combined stimulation with crosses in the center and near periphery of the receptive fields demonstrated summation, antagonism, or the lack of any interaction between these zones in producing sensitivity to crosses. Local blockade of intracortical GABAergic inhibition by microiontophoretic application of bicuculline showed that in one third of the neurons studied, sensitivity to figures was generated or enhanced by inhibition in normal conditions, while one third of cells showed suppression by inhibition, and sensitivity in the remainder was independent of inhibition. These data show that reconsideration of existing concepts of the role of field 17 in selecting only first-order shape features of images (i.e., the orientations of single lines) is needed, since almost half the neurons in the cat primary visual cortex can efficiently detect second-order features (angles and line intersections).

Action Potentials↗