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Biomedical subjects

I A Shevelev

Publications and source records attributed to I A Shevelev.

At least 55 records · Page 3Linked to original sources
PubMed1989

[Rapid thermal waves propagated through the cerebral cortex during visual stimulation].

Fast thermowaves spreading over the brain cortex were found and investigated in acute experiments on white rats using a new method of neuroimaging: thermoencephaloscopy (TES) through the intact skull. Thermowaves (amplitude 0.005-0.1 degree C, path 2-56 mm, length 10-15 mm, duration 1.2-11.4s, speed 1-33 mm/s) were evoked by light stimulation. With a high probability (0.92) they appeared during 15s before and 26s after the rhythmical flashes (interval 1.5-3 min). In about a half of cases the wave appeared in the contralateral visual cortex (fields 17 and 18a), spread to the middle-line of the brain and to the ipsilateral hemisphere (fields 17, 18a and 7). Local circle-spreading waves in the contralateral visual cortex and other waves differing in their trajectories and spreading from the ipsilateral visual cortex were also revealed. The possible functional significance of the effect as a reflection of scanning of the cortex by the focus of selective visual attention is discussed.

Animals↗
PubMed1987

[Testing the scanning effect of a range of orientations by visual cortex neurons of the cat].

The preservation of the effect of dynamic scanning of a part of the orientation diapason during development of neuron responses in the primary visual cortex of cat was examined after equalization of their latencies and after estimation of only highly significant fragments of their reactions. It was found that this effect was preserved in 13 neurons out of 17 examined ones: it remained invariable in 7 units of this group and was modified due to a shift of the scanning beginning along the diapason of orientations, to an increase of scanning diapason or to a change of scanning direction. The stabilization of the orientational tuning occurred only in 4 cells after equalization of latencies. The obtained results confirm that the dynamic changes of orientation tuning in the majority of visual cortex neurons are connected with reorganization of the time pattern of the reaction, as suggested by the hypothesis of the spatial-temporal orientation coding in the visual system.

Animals↗
PubMed1987

[Temperature distribution on the surface of the rat brain during direct electric stimulation].

Dynamics of spatial temperature distribution over the dorsal surface of the cerebral cortex was studied through the skull with the thermovision technique on immobilized white rats. Direct cortical stimulation revealed local thermoresponses (up to +0.2 degrees C) under electrodes, in the symmetrical point in the opposite hemisphere as well as in some other cortical zones. The response latency was within the limit of 160 ms, peak latency--2-5 s and relaxation time 2-3 min. Nembutal caused depression of thermoresponses, increase in their thresholds, limitation in size as well as slowing down of the development and prolongation in time.

Animals↗
PubMed1986

[Detection of the activation of the motor cortex of the cat during movement of an extremity using thermomapping of the brain].

In chronic experiments on cats the distribution of temperature on the dorsal surface of the cerebral cortex and its dynamics were studied through unopened skull by the thermovision and digital image processing technique. During the reflex movement of the forelimb and after it focal biphasic thermoreactions in the motor cortex which lasted for up to 30 s and sign-opposite thermoreactions in symmetrical points of both hemispheres were revealed.

Animals↗
PubMed1986

[Parameters of photic stimulation and characteristics of thermoreactions in the cerebral cortex of the white rat].

The dynamics of local thermoresponses in the brain cortex was studied through the unopened scull under patterned light stimulation of the retina in acute experiments on white rats by means of thermovision and digital image processing technique. The dependence of the amplitude, temporal and spatial characteristics of these responses on the size, retinal localization, focusing of the stimulus, its duration, number and frequency of presentation was estimated. The properties, significance and mechanisms of the revealed thermoeffects are discussed.

Animals↗
PubMed1986

[Movement of temperature waves across the cerebral cortex of the white rat].

In acute experiments on immobilized or anesthetized white rats the spatial distribution and dynamics of temperature on the dorsal surface of the cerebral cortex were investigated by the thermovision technique and digital image processing through the intact skull. Under certain conditions the appearance of moving thermowaves that spread over the cortex for 8-30 mm during 2-9 s with a rate of 2-10 mm/s was found. Among the trajectories of these waves the circular one was more common: it began in one hemisphere, spread in one or another direction, then went over to the second hemisphere, moved in the opposite direction and came back to the initial zone. Linear and semicircular trajectories were found as well with or without crossing from one hemisphere to the other. The mechanisms and possible functional significance of the effect are discussed.

Animals↗
PubMed1986

["Timer" neurons and "scanner" neurons in the visual cortex of the cat in response to low-contrast between stimulus and background].

The characteristics of the reactions and the properties of orientation detection in neurons separated formerly into "timers" and "scanners" at a high contrast (100) were studied in experiments on unanesthetized, relaxed cats at a lower contrast level. It was found that all "scanners" preserved their properties and did not pass to the group of "timers". On the other hand, only 40% of the initial number of the "timers" remained in this group. Correlation of the properties of neurons of these groups remained as it was at maximum contrast: in "timers" the reaction began and reached the maximum earlier than in "scanners"; it was of a higher frequency and shorter and its characteristics of the orientation tuning were considerably worse. Neurons which passed from the group of "timers" at the lower contrast possessed characteristics intermediated between those of the "timers" and "scanners", approaching the latter by a number of properties. The obtained data confirm the conclusion about the presence and stability of the activity of the visual cortex neurons--"timers" and "scanners" within a wide range of conditions; the former carry out the function of synchronizers, the latter--the role of orientation filters reforming in time.

Animals↗
PubMed1986

[Dynamic thermomapping of the brain of the rat during sensory stimulation and spreading depression].

Dynamics of spatial distribution of temperature over the dorsal surface of white rat cerebral cortex was studied through the skull with the thermovision technics. Visual, somatosensory and acoustic stimulation revealed both diffuse and regionally specific thermoreactions of the brain as well as multiple local termoresponses. After injection of KCl into the cortex temperature waves spreading over the cortex were obtained. Mechanisms of brain patterned thermoreactions dealing with blood flow and neuronal metabolic thermoproduction are discussed.

Animals↗
PubMed1986

[Differences in the properties of 2 groups of orientation detectors of the visual cortex in the cat].

Functional properties of 149 neurons in field 17 of the visual cortex were studied in immobilized cats. They were divided into two groups. In 37% of neurons the initial and peak latencies of responses changed not more than by 10 ms in the orientation range. They were named "timer"-neurons. 63% of neurons were called "scanners". "Timers" reliably differed from "scanners" in shorter latencies, time of the discharge frequency increase, duration of the response and higher discharge frequency in all stimuli orientations. Scanners had a higher orientation selectivity, they considerably more often scanned the range of orientations in time. Neurons of the two groups had contraphase dynamics of the width of tuning orientation during response while distribution of their preferential orientations supplemented each other. In orientation columns of the visual cortex, both timers and scanners were most often found with a predominance of the latter. The columns consisting of only timers or only scanners were more seldom. The role of differences in properties of two neuronal groups in the cat visual cortex for orientation detection is discussed.

Animals↗
PubMed1985

["Timers" and "scanners" among orientation detectors of the visual cortex in the cat].

Acute experiments on immobilized cats were carried out to check the suggestion that a special class of neurons ("timers") which do not change time properties of responses with variations in light bar orientation exists in the visual cortex. Previously neurons of the visual cortex with a dynamic shift of preferable orientation ("scanners") were revealed. Among 76 neurons 27 (36%) showed different properties: stability of latencies in the orientation range, shorter initial and peak latencies, duration and time of the discharge frequency increase, wider orientational tuning and worse relations between magnitudes of responses to preferred and nonpreferred stimuli. They were considered as representatives of the suggested "timers".

Animals↗
PubMed1985

[Standardness and invariability of the detector tuning of neurons in the orientation column of the visual cortex of the cat].

The variability of neuron orientation tuning in separate columns and the degree of it stability under changes in levels of the contrast between stimulus and background were studied in experiments on unanesthetized, relaxed cats. Several types of orientation columns were revealed: with a relatively high, standard and stable orientation tuning; with a widely changing orientation tuning from neuron to neuron; with invariance to contrast level; with variant orientation tuning; with mixed (invariant-variant) properties. In the columns, standard-nonstandard properties, on the one hand, and properties of invariability-variability, on the other hand, could combine in different ways. More often differences of orientation tuning within a column were observed between neurons of upper and lower cortical layers. Possible distinctions in the functional role of types of orientation tuning in columns described and in mechanisms of formation of detector properties of their neurons are discussed.

Animals↗
PubMed1985

[Interrelation among properties of visual cortex neurons in the cat].

In acute experiments on immobilized cats 13 functional characteristics of 96 visual cortex neurons were investigated. Regressional and cluster analyses were used to divide these neurons into two subgroups with different density and degree of connections between characteristics. The receptive fields of cells of the first subgroup were localized relatively centrally in the visual field, those of the second subgroup were localized more often on the periphery. A valuable correlation was found in the half of the studied characteristics. In each subgroup the more centrally localized cells with small receptive fields had relatively shorter latencies, lower thresholds, shorter temporal summation, wider intensity range and greater differential sensitivity; their responses were phasic, with high-frequency discharges. The density of valuable correlation of the characteristics varied from 0.21 to 0.99. The amount of these correlations in the first subgroup was two times higher than in the second one. The possible mechanisms of the correlation between the properties of the visual cortex neurons are discussed, as well as their differences in two subgroups and in the cortex and LGB.

Animals↗
PubMed1984

Some unusual properties of neurones in the cat posterotemporal cortex.

Visual receptive fields (RF) of 103 single units in posterotemporal cortex (area 21) were investigated in cats immobilized by d-tubocurarine. The method of express-mapping of the RF allows to estimate every 3.3 min its localization, size and configuration. A considerable variability of RF size and form was revealed in 35% of units. In 88% of the unit studied the RFs were invariant with adaptation level. Little dependence of the response on the length of the light bar, but a definite orientational sensitivity was found. Spontaneous displacements of the RF in the visual field are found in 14% of the visually activated cells. Such RF dislocations varied in magnitude from 3.5 upto 25 degrees and were mainly horizontal. The mechanisms of RF variability and displacement in the visual field are discussed in connection with possible influence of the copy of the motor command that introduces corrections in the visual coordinates in area 21 to provide for the constancy of vision.

Animals↗
PubMed1983

[Features of the dynamic reorganization of the receptive fields of visual neurons of the cerebral cortex and lateral geniculate body in the cat in response to changes in the parameters of photic stimulation].

Influence of parameters of local light stimulation of different points of receptive fields on characteristics of previously observed dynamic reorganization of recorded receptive fields of visual cortex and lateral geniculate neurons in time microintervals was studied in details in computer-controlled acute experiments on unanesthetized immobilized cats. The dependence of this process on the background illumination level, on intensity, contrast, area, duration, energy and orientation of a local rectangular or round light stimulus flashing in a random order at 100 points of the tested part of the visual field was determined. The analysis of the data obtained suggests a specificity of the influence of the background illumination level, intensity, area, duration and orientation of the stimulus on the dynamic reorganization of the receptive field. All the parameters were revealed to have a significant nonmonotonic influence on the receptive field parameters. The found effect is discussed for its probable functional significance and a possibility of its participation in process of information coding in the visual system.

Animals↗
PubMed1983

[Orientational tuning of visual cortex neurons to different stimulus intensities in the cat].

The orientation tuning of field 17 neurons of the visual cortex was studied in immobilized and unanesthetized cats under different intensities of test light slits and constant light background. Orientation tuning of five neurons was invariant to stimulus intensity: their preferential orientation did not change. Thirteen cells were variable as during the change of the contrast they showed a statistically significant displacement of orientation tuning from 22 degrees to 90 degrees. Changes in other neurons were not significant. Invariant neurons differed from variable ones in several characteristics. The mechanisms of orientation tuning changes during contrast variations are discussed.

Adaptation, Physiological↗
PubMed1983

[Double orientation tuning of visual cortex neurons in the cat].

Orientation tuning of 148 visual cortex neurons was investigated in immobilized unanaesthetized cats. The light slit of the optimal size flashing in the receptive field was used as a stimulus. It was found that 88 neurons (59%) had double orientation tuning: preferred and additional. Additional orientation was either orthogonal or at a sharp angle to the preferred orientation. In 64% neurons double orientation was found only after a change of the contrast between stimulus and background. This kind of tuning in many neurons appeared only at definite moments after the beginning of the stimulus. Model analysis showed that double orientation tuning might be a consequence of the specific structure of neuronal receptive fields. The functional meaning of double orientation tuning and its role in the detection of visual images signs are discussed.

Animals↗
PubMed1983

[Differences in the dynamics of the visual receptive field and its summation zones in the cat].

In acute computer-controlled experiments on immobilized unanasthetized cats, dynamic reorganization of the recorded receptive field and its summation zone were compared for lateral geniculate body and cortical neurons. Counterphasic development of these processes was observed and investigated in detail, i.e. enlargement and then narrowing of the recorded receptive field as compared with narrowing and then enlargement of the summation zone. Differences of physiological mechanisms of these two processes and their possible functional role are discussed.

Animals↗