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Biomedical subjects

G A Sharaev

Publications and source records attributed to G A Sharaev.

At least 55 records · Page 3Linked to original sources
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↗
PubMedDate not supplied

Tuning of striate neurons to cross-shaped figures in conditions of local blockade of intracortical inhibition.

The recently observed selective sensitivity to cross-shaped and angular figures was studied in 85 primary visual cortex (field 17) neurons in cats before and after local blockade of GABA(A)ergic inhibition by microiontophoretic application of the GABA antagonist bicuculline. Two opposite effects were seen: half of the neurons studied showed decreases or complete loss of sensitivity to crosses, and a third of the cells showed increases or the appearance of sensitivity to crosses. These data provide evidence for significant roles for intracortical inhibition in providing sensitivity to crosses and intersecting lines in two types of visual cortex neurons, the effects on these two types of neuron being opposite.

Action Potentials↗
PubMedDate not supplied

The disinhibitory zone of the striate neuron receptive field and its sensitivity to cross-like figures.

Acute experiments on immobilized anesthetized cats were used to confirm the suggestion that the sensitivity of many neurons on the primary visual cortex to cross-shaped, angular, and Y-shaped figures may be determined by the presence within their receptive fields of disinhibitory zones, which block end-stopping inhibition. A total of 55 neurons (84 functions, i.e.. on and off responses) were used for studies of sensitivity to crosses, and responses to single bars of different lengths were compared before and after stimulation of an additional lateral zone of the field (the presumptive disinhibitory zone), which was located in terms of responses to crosses. Seventeen of the 55 cells in which increases in the length of a single bar decreased responses, i.e., which demonstrated end-stopping inhibition, showed significant increases in responses (by an average factor of 2.06 +/- 0.16) during simultaneous stimulation of the lateral zone of the receptive field, which we interpreted as a disinhibitory effect on end-stopping inhibition. These data provide the first direct evidence for the role of end-stopping inhibition and its blockade by the disinhibitory zone of the receptive field in determining the sensitivity of some neurons in the primary visual cortex of the cat to cross-shaped figures.

Animals↗
PubMedDate not supplied

[A geometric model of orientation tuning dynamics in visual neurons].

A geometrical model for the dynamics of orientation tuning of visual neurones was proposed, which makes it possible to study the dynamics of configuration, localization, and weight of excitatory and inhibitory subzones of the receptive field. The model reproduces typical patterns of orientation tuning dynamics, observed in neurophysiological experiments on cat visual cortex neurones. The parameters of the model (size and mutual position of excitatory and inhibitory zones of the receptive field, their weight, and dynamics type) were estimated that correspond to the main types of orientation tuning dynamics in natural conditions. It is shown that selective and acute tuning of neurones can be formed and/or sharpened by intracotrical inhibition, while the dynamics of preferred orientation is due to changes in the geometry of the inhibitory subzone of the receptive field.

Animals↗
PubMedDate not supplied

[The tuning of striate neurons to cross-like figures during local blockade of intracortical inhibition].

Many neurons in the cat primary visual cortex reveal sensitivity to cruciform and corner figures as well as to local orientation discontinuities. We investigated this sensitivity in 85 V1 neurons before and after the local blockade of GABA(A)ergic inhibition by microiontophoretic application of bicuculline and observed two opposite effects: a decrease or disappearance of sensitivity to crosses in about half of the neurons and its increase or appearance in about one third of the units. The results indicate substantial and drastically different contribution of intracortical inhibition to sensitivity to line-crossings in the visual cortical neurons. Some possible mechanisms of this difference are discussed.

Animals↗
PubMedDate not supplied

[Relative meaning of lines and corners in geometric figures for their recognition by humans].

Neurons tuned to line-crossings (corners, crosses, Y-like and three-ray star-like figures) of different shape and orientation rather than to a single bar were found in the area 17 of the cat visual cortex. We studied the relative role of lines and corners of 2D and 3D geometrical figures for their recognition by humans. Probability of figure recognition during its tachistoscopic presentation was compared for the whole (control) and partly masked figures. Sides or corners of the figures were masked to varying degrees and probabilities of correct response were compared. The recognition probability successively decreases with increasing extent of figure masking. This decrease is significantly more pronounced for the figures without corners than for the figures without part of the lines. The relatively greater significance of the corners than sides of geometrical figures for human visual recognition and some possible neuronal mechanisms of this effect are discussed.

Adult↗
PubMedDate not supplied

[Dynamics of tuning to orientation of cross-like figures in neurons from the cat visual cortex].

Dynamics of tuning to orientation of flashing light bar and to orientation of cross-like figure was studied by a temporal slices method in 87 neurons of the cat primary visual cortex. Tuning was plotted by spikes number in the entire response and in its successive fragments with a step of 20 ms. It was found that successive dynamic shift of preferred orientation of a bar was typical for 87% units, white such shift of preferred orientation of a cross was met in 75% of cases. Comparison of tuning dynamics for bar and cross allowed to separate units into three groups: the first one (58.6% of cases) with larger dynamic shift of a bar preferred orientation then of a cross (74.9 +/- 5.8 degrees [symbol: see text] 29.8 +/- 4.1 degrees, correspondingly, p < 0.00001), the second group (21.5%) with opposite effect (24.2 +/- 5.2 degrees and 69.2 +/- 10.0 degrees, p < 0.0002) and the third group (19.8%) without significant shift of preferred orientation of bar and cross and without difference in their dynamics. Possible mechanisms of the preferred orientation dynamics and its difference for bar and cross are discussed.

Action Potentials↗
PubMedDate not supplied

[Human visual evoked potentials to bars and cross-like figures].

Temporal and amplitude characteristics of averaged visual evoked potentials to presentation of lines, corners and crosses with different orientation recorded in 19 adult healthy subjects were compared in 34 derivations. In all subjects, the latencies of P1, N2, and P3 were shorter while their amplitudes were higher for crosses than for lines. The effect of lengthening of P1 peak latency from occipital to temporal cortex was mostly pronounced for EPs to a bar, whereas as increase in the P1 amplitude was most evident for a cross-like figure. Correlation of these data with: i) greater magnitude and shorter latencies of responses to crosses vs. bars in a half of cat striate neurons, ii) sensitivity of cells in monkey inferior temporal cortex to star-like figures, and iii) relatively better human recognition of figures with comers than with lines, as well as significance of the effects obtained for detection of image features in different areas of the human visual cortex is discussed.

Adult↗
PubMedDate not supplied

[Spatial characteristics and the dynamics of the activation foci in the cortex during the elaboration of a conditioned reflex (thermographic research)].

In acute experiments on immobilized white rats, by thermovision methods and digital image processing, spatial distribution and dynamics of local heating and cooling of dorsal surface of the cerebral cortex was studied through intact skull during elaboration of a conditioned reflex. Distant and noninvasive thermovision methods allowed to reveal and quantitatively study local cortical thermal reactions in the somatosensory and motor cortex and in the cerebellum in the process of elaboration and testing of conditioned connection. Under discussion are mechanisms, properties and significance of these effects for investigation of learning.

Animals↗
PubMedDate not supplied

[Sensory mapping of the rat cerebral cortex by thermographic methods].

Using thermovision and digital image processing, the dynamics of thermomap in the brain cortex has been studied on albino rats through the opened skull before and during sensory stimulation. Photic, sonic and somatosensory stimuli lead to the onset of local heating foci in the corresponding cortical zones, as well as to local multiple thermoresponses in other areas. Some quantitative parameters of these effects are given and their possible mechanisms are discussed.

Acoustic Stimulation↗