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I A Shevelev

Publications and source records attributed to I A Shevelev.

At least 91 records · Page 5Linked to original sources
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

Sensitivity of striate neurons to Y-like figures: experiment and simulation.

Under stimulation of the receptive fields (RF) of neurons in the cat area 17 by flashing Y-like figures of different shape and orientation, the sensitivity to these figures was revealed in 72% of the studied cells, while 62% of units were sensitive to cross-like figures as well. Tuning to Y-like figures was typically selective to their shape and orientation, but in some cases it was invariant to these features. Response magnitudes to single bar, Y-like figure and cross were positively correlated. Simulation showed that the disinhibition might be a sufficient mechanism for effective detection of Y-like figures in a classical receptive field.

Corpus Striatum↗
PubMedDate not supplied

Second-order features extraction in the cat visual cortex: selective and invariant sensitivity of neurons to the shape and orientation of crosses and corners.

About 1/3 of neurons (52/174) studied in the cat striate cortex (area 17 or V1) gave a larger (by 3.2 times on average) response to a flashed cross or corner centered in the receptive field (RF) than to a single bar of optimal orientation. Most such neurons (71.4%) were found to be highly selective both to shape (angle between the lines) and to orientation of these figures. In the studied neuronal selection we also found all possible types of invariance of sensitivity to orientation and/or shape of these figures. We found neurons with selectivity to form of the figure and invariance to its orientation and vice versa. Some cells were found invariant both to form and orientation of the cross or corner but highly sensitive to flashing of any such figure in the RF. The role of RF center and surrounding area in sensitivity to cross figures was also studied in 44 additional units. Separated and combined stimulation of these zones revealed in different units summation, antagonism and absence of interaction of these zones by the selectivity index (figure/bar response ratio). Possible mechanisms of the described effects are discussed as well as their functional implication for second-order feature extraction in the visual cortex: selective or invariant sensitivity of neurons to the shape and orientation of the line-crossings.

Animals↗
PubMedDate not supplied

Disinhibition as a mechanism for visual cortex neurons to tune to cross-shaped figures.

A discrete simulation model of a receptive field selectively responding to cross-shaped figures, as seen in 40% of primary visual cortex neurons in the cat, was studied. The model was based on disinhibition of end-stop inhibition in the receptive field by the lateral disinhibition zone. These experiments showed that this mechanism can produce selective or, conversely, invariant tuning to the shape and orientation of cross-shaped figures and could underlie the high sensitivity of neurons to second-order image features.

Animals↗
PubMedDate not supplied

Model studies of the mechanisms of tuning of visual cortex neurons to incomplete cross-shaped figures.

Numerical simulation modeling of the receptive fields of visual cortex neurons able to detect cross-shaped figures with masked central or peripheral areas was performed. Receptive field models of two types were considered: those with antagonistic and cooperative interactions between the center and the periphery. Model neurons with receptive fields with reciprocal (antagonistic) interactions produced greater responses to peripheral or central crosses than to complete crosses. Studies using the model showed that the basis of this type of tuning could be provided by a disinhibition mechanism: blockade of the inhibitory zones in the center or periphery of the receptive field by activation of a lateral disinhibitory zone. A model with cooperative interactions between the center and periphery of the receptive field was also studied, in which responses to complete crosses were summed from the responses to the peripheral and central parts. Tuning of these model receptive fields was comparable to the sensitivity of real visual cortex neurons to the shape, size, and orientation of figures. The properties of model receptive fields (configuration, localization, and weightings of the various zones) allowing simulation of the properties of cat visual cortex field 17 neurons sensitive to the orientation and configuration of incomplete cross-shaped figures were identified.

Models, Neurological↗
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 dynamic localization of a dipole source of the alpha rhythm in the human brain].

In 11 subjects the dynamic localization (with 2-ms step) of the equivalent current dipole of alpha rhythm during travelling alpha waves was studied using one-dipole 3-layered spherical head model and MRI. The dipole was localized in the occipital cortex and during the development of a single alpha wave it shifted in one or another direction while dipole's moment revealed fan-like rotation mainly in sagittal and horizontal planes. The results obtained indicate changing localization of the alpha-rhythm source in the region corresponding to striate cortex. They seem to prove scanning hypothesis that is still under discussion.

Adult↗
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

[Gender distinctions in recognition of partly masked geometric figures].

Visual recognition of entire and partly masked geometric figure by men and women was studied. Probabilities of correct recognition of the entire (control) and partly masked figures (without some parts of their lines or corners) under conditions of their near-threshold tachistoscopic presentation were compared. A gradual decrease in recognition probability with increasing masking was observed. It was more pronounced for figures without some corners than for figures without part of their sides. Reliable gender differences were found: men better than women recognized figures, especially, with masked corners. Possible reasons for gender differences in recognition of geometrical figures are discussed, in particular, the role of striate sensitivity to a single light bar and a line crossing in the observed phenomena.

Adult↗
PubMedDate not supplied

[The visual cortex "on command": 2 modes of operation of neuronal mechanisms (a topical article)].

Orientation tuning (TO) of 152 single units in the 17th area of cat cerebral cortex was studied under different contrasts of the light bar flashed in RF (2.3, 10 and 100) and also under local and general anaesthesia (Sombrevin, 4 mg/kg or Nembutal, 35 mg/kg). An invariance of preferred orientation to contrast or anaesthesia was revealed in 40-50% of cells. Noninvariant cells shifted significantly their preferred orientation on 22-90 degrees. Invariant units more often have a simple RF, more sharp OT and preferred horizontal and vertical orientations. Under the low-contrast conditions or under general anaesthesia, noninvariant neurons shifted their preferred diagonal orientations. Different role of the two neuronal groups in detection of the reper (horizontal and vertical) and other orientations in a normal conditions (high-contrast, alert state) in comparison with their behavior under worse conditions (low-contrast, narcosis or sleep) is discussed.

Animals↗
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

[Simulation of the role of intracortical inhibition as a source of sensitivity to cross-like figures].

Simulation of receptive fields of striate neurons sensitive to cross-like figures under the blockade of intracortical inhibition was performed. It was shown that without the inhibition, a neuron with convergence of signals from two orientation detectors widens its tuning to a cross in such a way that the tuning becomes invariant to the shape and orientation of the cross. Detector of a cross whose orientation is based on the disinhibition mechanisms becomes a bar orientation detector under conditions of inhibition blockade. Another scheme of receptive field is advanced, in which the inhibitory zones mask tuning to cross-like figures, but the blockade of inhibition unveils such sensitivity. We specified the features of the receptive fields (shape, localization, and weight of their zones), which, being applied in simulation, imitate properties of the real striate neurons sensitive to shape and orientation of a cross-like figures.

Animals↗
PubMedDate not supplied

[Role of filtration in localization of the evoked potential dipoles in the human brain: experiment and simulation].

3D dipole tracing with 1 ms step of visual evoked potentials recorded from 40 electrodes was performed under exposition of crosses in 5 healthy human subjects. The data on dipole displacement were compared with prediction of the simulation study on distortion of dipole localization by the signal filtration in the low-frequency band. These predictions were experimentally confirmed: the effect depends on the degree of filtration (0.1 or 0.5 Hz) and on the latency of EP waves. Localization and strength of P1 dipoles were not changed under filtration, while for later components--N2 and especially P3--they changed significantly. For the improvement of these distortions time constant of the amplification tract must be some times longer than the time of the dipole activity.

Brain↗
PubMedDate not supplied

[Trajectories of alpha rhythm dipoles shifting over the human brain cortex].

Dynamic study of 3D localization of the equivalent current dipoles (ECD)--sources of the EEG alpha rhythm in the human brain was performed in seven subjects with closed eyes using a one-dipole model. An exact localization of ECDs was obtained by combination of EEG and MRI mapping that allowed tracing of ECD shifts over the cortex with 4 ms step. Our data confirmed localization of these ECDs mainly in the occipital cortex and revealed their successive shift over this area during generation of each alpha-wave. Typical trajectories of these shifts were revealed and quantitatively compared by the hierarchical cluster analysis. The data obtained directly proved periodical rhythmic alpha-wave spreading process in the human visual cortex and an external control of this process. The data are discussed in terms of the "scanning hypothesis" (Pitts W., McCulloch W.H. Bull. Math. Biophys. 1947. V. 9. P. 127) which predicted a certain functional meaning of the alpha activity for cortical processing of sensory information in the human brain.

Adult↗