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Biomedical subjects

I A Shevelev

Publications and source records attributed to I A Shevelev.

At least 127 records · Page 7Linked to original sources
PubMedDate not supplied

[Relationship between level of vigilance and changes in the receptive fields of the cat visual cortex].

The activity of 118 neurones of the primary visual cortex (17th field) was studied in unanesthetized cats, immobilized with d-tubocurarine, in a state of calm wakefulness and in strained attention, alarm. The strained attention was elicited by an air-puff directed to the corner of the closed eye, not used for the photic stimulation. Considerable rearrangments of the receptive field of neurones (94%) were observed in the state of strained attention. In the majority of the studied cortical cells (75%) during 10-30 minutes the excitatory centres of their receptive fields became narrower while their inhibitory periphery widened; in the cells with the inhibitory centres of the receptive fields they were larger in 80% of the cases. It is shown that with dark, mesopic and scotopic adaptation the major characteristics of the changes in the receptive fields remain unaltered. It is assumed that these effects have a behavioural value for the organism, because the created rise in the level of alertness is accompanied by a sharpening of the receptive fields, i.e. by a greater ability to a fine analysis and recognition of visual images.

Adaptation, Physiological↗
PubMedDate not supplied

[Formation and functional significance of the dark receptive fields of the cat visual cortex].

Receptive fields (RFs) of single units in the 17th field of the visual cortex of immobilized cat were investigated under dark adaptation. The mean RF size was equal to 67 degrees and varied from 3 degrees up to 120 degrees. The RFs with centres located near gaze were from 3 degrees up to 120 degrees in dia, but with growth of excentricity the number of small RFs decreased, and in the region of 70 to 100 degrees from gaze only RFs with diameters equal to 100 degrees were found. The shape of "dark" RFs was either ellipsoidal (in most cases) or round. Detector properties (orientational, directional, size and velocity selectivity) of the "dark" RFs were significantly less manifest or absent. Under photopic light adaptation the same units reorganized their RFs to well known sizes and configuration. The hypothesis is discussed of the formation of local detector RF in the visual cortex in light adaptation by selective cortical inhibition which is activated in darkness only slightly. This view is an alternative to the commonly-accepted scheme of local cortical RF formation by the hierarchical and selective excitatory convergence.

Animals↗
PubMedDate not supplied

[The estimation of the direction of uniform and nonuniform movement and the alpha-rhythm EEG].

In experiments with 7 human observers the recognition of side and direction of the even and non-even (compensating the magnification factor) light motion in the left or right visual hemifield and to or from the fixation point was studied. The onset of the movement coincided with different phases of the EEG alpha-wave in the occipital region. In a control study stimuli were presented irrespectively to alpha-wave phases. Synchronization of non-even motion with some of the alpha-wave phases lead to an increase of its recognition probability and relative shortening of the response latency for directions from the center of gaze, while opposite directions became worse recognized and evoked relatively long-latency responses. Recognition of the even motion was not changed in experiment in compare with control. The obtained date are discussed in relation to Pitts and McCulloch (1947) hypothesis on a periodical (with alpha-wave frequency) scanning wave spreading over the visual cortex.

Adult↗
PubMedDate not supplied

[Visual illusions and the alpha-rhythm EEG].

During flicker stimulation with the alpha-rhythm frequency through the closed eyes 11 from 12 subjects clearly saw illusory objects (in order of descending probability, ring, circle, three-dimensional spiral or spiral spring, funnel, curved crossed bars). The individual optimal stimulation frequency for producing an illusion was always within the alpha range and was closely correlated with the individual dominant alpha frequency (r = 0.86). A supposition is discussed that the effect obtained is based on the quasistroboscopic fixation of the spreading cortical excitation wave of the alpha-rhythm due to its summation with the isorhythmic afferent diffuse cortical excitation evoked by flashes. This phenomenon enables "freezing" of the alpha wave at a certain instant of its spreading and makes it "visible".

Adult↗
PubMedDate not supplied

[The functional significance of alpha activity in the visual cortex for the recognition of images and movement].

The idea of the EEG alpha-waves as a rhythmically spreading scanning process was tested in 27 subjects. The stimuli presentation was not synchronised with the alpha-wave phase in control. In experimental group, recognition of different-shape figures and size coincided with one of four phases of the EEG alpha-wave in occipital area. Under flicker stimulation through closed eyes with the alpha-frequency, 11 of the 12 subjects perceived distinct illusory visual objects. The optimal stimulation frequency was always within the alpha-range and closely correlated with the individual predominant alpha-frequency. The data obtained corroborate the theory that the alpha-wave in the visual cortex reflects a uniformity of the spreading scanning process which plays a major part in the process of visual recognition.

Adult↗
PubMedDate not supplied

[The relationship of visual illusions to the frequency and phase shift of rhythmic photostimulation synchronized with the EEG alpha wave].

Under flicker stimulation through the closed eyes, synchronized with different phases of the EEG alpha wave, all 18 subjects systematically perceived the illusory visual objects (ring, circle, spiral, or curvilinear grid). Most frequently, they saw a ring or a circle, a little less frequently, a three-dimensional spiral, and at last-a curvilinear grid. The incidence of the ring and spiral illusion was the highest when the stimulation frequency strictly coincided with the individual dominant frequency of the alpha-rhythm. On the contrary, the grid illusion appeared more frequently when the stimulation frequency differed from that of the alpha-rhythm by 1-2 Hz. No significant effect was found of the alpha wave phase, which triggered the photostimulation, on the probability of the illusion perception. The revealed phenomena are probably explained by summation of the spreading alpha wave (the "scanning" hypothesis) with the isorhythmical afferent excitation evoked by flashes due to the quasi-stroboscopic effect.

Adult↗
PubMedDate not supplied

[Visual illusions evoked by rhythmic bursts and the "movement" through the cortex of waves of alpha activity].

In computer-controlled experiments the visual illusions were studied which appeared in human subjects under conditions of diffuse rhythmic photostimulation through the closed eyes with the individual dominant alpha-rhythm frequency. A significant correlation was demonstrated between the type of experienced illusion (or its absence) and the trajectory of the alpha-wave travelling in the prestimulation and stimulation periods.

Adolescent↗