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I A Shevelev

Publications and source records attributed to I A Shevelev.

At least 37 records · Page 2Linked to original sources
PubMed1998

Orientation tuning and receptive field structure in cat striate neurons during local blockade of intracortical inhibition.

The contribution of intracortical inhibition to orientation tuning in the cat striate cortex (area 17) was studied by responses to flashing light bars of different orientations and lengths in 68 single-units before and during microiontophoretical application of bicuculline, a GABAA antagonist, A three-fold increase in the background activity (13.3 +/- 1.3 vs 4.4 +/- 0.5 imp/s) and 4.4-fold increase in the maximal discharge frequency (264.4 +/- 22.3 vs 60.6 +/- 5.3 imp/s) was found in 96.0% of the cells studied during microiontophoresis. In most units all characteristics of orientation tuning significantly changed during application of bicuculline: i) tuning width increased in 76.3% of cells from 52.7 +/- 2.8 degrees in control to 85.2 +/- 4.6 degrees, ii) tuning selectivity diminished in 63.6% of cells by a factor of 1.5, and iii) tuning quality dropped in 68.5% of cases by a factor of 2.5. The threshold ejection current of bicuculline for widening of tuning was in 2/3 of the cells in the range from +10 to +40 nA (+31.0 +/- 4.5 nA) and the maximum effect was obtained in 3/4 of units with +30(-) + 100 nA (+67.1 +/- 6.0 nA). Unmasking of additional excitatory inputs to the studied cells due to blockade of the inputs from inhibitory interneurons in considered as the main mechanism of the described bicuculline effects.

Animals↗
PubMed1998

Selective and invariant sensitivity to crosses and corners in cat striate neurons.

Many neurons (56/174, or 32.2%) studied in the cat striate cortex (area 17) increased significantly (by 3.3 times on average) their responses under stimulation by cruciform or corner figures of specific or non-specific shape and orientation flashing in receptive field as compared with single light bar of preferred orientation. Most of these neurons (71.4%) were found to be highly selective to both the shape (the angle between the figure's lines) and orientation of these figures. In the neuronal selection studied we have also found all possible types of invariance of the cross and corner tuning to orientation and/or shape of these figures. We found neurons with selectivity to the form of the figures and invariance to their orientation and, on the contrary, units invariant to shape but selective to orientation. Some cells were found invariant to both the form and orientation of the cruciform or corner figure but highly sensitive to appearance of any such figure in the receptive field. Two main hypotheses about the mechanisms of selective sensitivity to crosses and angles can be considered. They are as follows: an excitatory convergence of two units with different preferred orientations, and intracortical inhibitory interactions. The cells with double orientation tuning for a single bar are found relatively rarely (about 20%), thus making the first suggestion the most unlikely. This circumstance is of special importance since it provides evidence against the hierarchic formation of the higher-order cortical units from a set of lower-order cells that is still under discussion. The units with high sensitivity to cross or corner seem to be ideally suitable for their selection, rather than to serve as classical orientation detectors only.

Animals↗
PubMed1997

Fast thermal waves spreading over the cerebral cortex.

Fast thermal waves spreading over the cerebral cortex were found in the white rat by infrared neuroimaging (thermoencephaloscopy) in the range of 3-5 microm. Thermal waves appeared under visual stimulation (the probability of their appearance = 0.92), and under background conditions (probability of appearance = 0.42). Typically, they moved during the period from 15 s before to 25 s after each light flash that was presented rhythmically (1.5-3 per min). The waves spread over the cerebral cortex along the limited number of typical trajectories, which are specific for the hemisphere of their origin (ipsilateral or contralateral with the side of stimulation). The waves were never recorded in a dead animal or in a thermal standard. The amplitude of the thermal waves was in the range of 0.005-0.1 degrees C, with the extent of the pathway of 2-56 mm, length of 10-15 mm, duration of 1.2-11.4 s, and velocity of 1-33 mm/s. In about half of the cases the waves appeared in the contralateral visual cortex (areas 17 and 18a), spread to the midline and crossed to the ipsilateral hemisphere (areas 17, 18a and 7). Local waves moving along a circular trajectory and rotating around the central part of the contralateral area 17 were also revealed, as well as wave rotation around the dorsal (mainly parietal) cortical fields. Possible biophysical and neurophysiological mechanisms and the functional significance of the revealed effects are discussed.

Animals↗
PubMed1996

Recognition of direction of uniform and accelerated visual motion and EEG alpha wave phases.

The perception of visual motion in seven subjects was studied comparing motion towards or away from the fixation point in the left or right hemifield. The light target was moved either at a constant velocity or positively or negatively accelerated to compensate for the magnification factor of the visual cortex. We compared probability and latency of motion recognition when it was asynchronous or synchronized to different phases of the alpha wave of the EEG recorded over the occipital cortex. If the motion accelerated away from the fixation point and was synchronized with the alpha wave it was more likely to be perceived whereas if it was towards the fixation point it was less likely to be detected. However, perception of the constant velocity motion was not changed by locking it to the alpha wave phase. These results support the hypothesis that the scanning waves of excitation spread over the visual cortex periodically and that they are locked to the alpha component of the EEG.

Adult↗
PubMed1995

Sensitivity to cross-like figures in the cat striate neurons.

The responses and orientation tuning in 48 out of 62 (77.4%) neurons of the cat striate cortex (area 17) significantly, but with different sign, changed at stimulation by specific cross-like figure flashing in receptive field as compared with single light bar of preferred orientation. Neurons of the first group (19 units from 62, 30.6%) were found to increase the responses by 3.3 times if stimulated by a certain cross-like figure, specific for each cell configuration and orientation. Under the same conditions, neurons of the second group (29 or 46.8% revealed a three-fold decrease of responses and all tuning characteristics worsened. Among them 8% of total number of cells showed bimodal or double orientation tuning when stimulated by some configurations of crosses due to an angle specific inhibition. Dependence of the revealed effects on excitatory convergence from neurons with different orientation tuning, on inhibitory influences from end-stop and side-zones of receptive field, as well as possible functional implication of the first group neurons for an angle and line-crossing detection are discussed.

Animals↗
PubMed1994

Time-slice analysis of inhibition in cat striate cortical neurones.

A prominent feature of visual cortical neurones is orientation specificity. The underlying mechanisms and the possible contributions of intracortical inhibition are continuously the subject of intense debate. We analysed orientation tuning in relation to time after stimulus onset and the effects of blockade of GABAergic inhibition with microiontophoretically applied bicuculline in single cells in area 17. The majority of the cells had sharp and temporally stable orientation specificity; however, changes of receptive field structure, widening of orientation tuning and changes in preferred orientation were observed between 30 and 150 ms after stimulus onset when inhibition was blocked. This indicates that under normal conditions in most in most striate cortical cells intracortical inhibition sculptures receptive field structure and keeps orientation tuning narrow and constant in time.

Animals↗
PubMed1994

Double orientation tuning in the cat visual cortex units.

Orientation tuning of 271 neurons of the cat visual cortex (area 17) was studied with a light bar flashing in the receptive field. Under different conditions, 27-57% of units were found to have double-orientation tuning: they demonstrated the main preferred orientation and an additional preferred orientation. The statistical reliability and reproducibility of additional preferred orientation were shown. The quality of orientation tuning in the second maximum did not differ statistically from the first one. The angle between preferred orientation and additional preferred orientation was either 90 degrees (29% of cases) or an acute one (60.1 +/- 3.1 degrees, 71% of cases). The ratio of discharge frequency in responses to additional preferred orientation and preferred orientation was equal to 0.74 +/- 0.05. Neurons with double-orientation tuning clearly preferred 67 degrees and 157 degrees, while monomodal units preferred 0 degrees and 90 degrees. Probability of the double-tuning increased under bar lengths of near 3 degrees and near 10 degrees and with increase of stimulus/background contrast. At the same time some neurons displayed double-orientation tuning only with relatively low stimulus/background contrast. The proportion of units with double-orientation tuning was lowered by about 1.5-times under general Nembutal narcotization as compared with local anesthesia of the animal. In about one-third of units simultaneous stimulation by two flashing lines crossing in the receptive field center under an angle specific for the cell, evoked a response from 1.5 to four times larger than to the preferred orientation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Thermoimaging of the brain.

A new method of neuroimaging is proposed that is based on the recording of weak changes in infra-red radiation from the brain. Thermoencephaloscopy (TES) uses a combination of thermovision and digital image processing techniques and allows one to make a dynamic investigation of the thermal fields of the cerebral cortex through an unopened skull. Today the method is characterized by: (i) differential thermosensitivity of up to 0.002 degrees C; (ii) instrumental temporal resolution 40 ms (25 thermomaps/s); (iii) time of the temperature measurement in the part of an object that correspondes to one pixel on digital thermomap is equal to 2.4 microseconds; (iv) instrumental spatial resolution of up to 70 microns/pixel; and (v) the thermomap size is 10880 pixels (128 x 85). TES allows detection of an appearance and study of topography and dynamics of the small (from 140 to 300 microns) and precisely localized zones of activation or deactivation of the cerebral cortex that are modally and regionally specific with respect to stimuli modality and parameters and to the state of an animal. Up to now such studies were performed with white rats, rabbits, cats, monkeys and humans under different kinds of sensory stimulation, motor tasks, conditioning (associative learning). Fast thermowaves spreading over the brain cortex along specific trajectories were revealed under sensory stimulation. The mechanisms of the recorded phenomena are discussed: (i) joule heating produced by ion currents through membranes of activated neurones; (ii) metabolic thermoproduction; (iii) thermal convection by the local cerebral blood flow; and (iv) thermodipole formation in the cortical foci with changed lCBF.

Animals↗
PubMed1993

Dynamics of orientation tuning in the cat striate cortex neurons.

Dynamics of orientation tuning was studied by a flashing light bar in 233 single units of cat primary visual cortex. Orientation tuning was plotted by the criterion of spike number in successive fragments of responses with a step of 10 or 20 ms. Preferred orientation and the width of orientation tuning were measured for each of such curves. It was found that successive dynamic change of preferred orientation during response was typical for 63% of units under study, while such change of orientation tuning width was typical for 93%. Neurons with or without shift in preferred orientation differed in their response characteristics: latency and duration of discharge, its frequency and response rise time, as well as quality of orientation tuning. Possible functional significance of two groups of cortical units, "scanners" (with systematic preferred orientation shift) and "timers" (without it), is discussed in accordance with the idea of spatiotemporal orientation coding in the primary visual cortex.

Animals↗
PubMed1992

Temperature topography of the brain cortex: thermoencephaloscopy.

Thermoencephaloscopy (TES) - a new method of functional imaging of the cerebral cortex by its infrared radiation was advanced and developed since 1984. Improved thermovision and image processing techniques allow 2D, contact-free, dynamic and non-invasive recording of background and evoked cortical activity through an unopened skull. Activated (heated) and deactivated (cooled) zones of the cerebral cortex are revealed. The temporal resolution of TES is 40 msec (25 maps/sec), the spatial resolution is up to 70 x 70 microns/pixel. The diameter of the smallest recordable active region of the cortex is 200-300 microns. The minimal time needed for a session used for averaging of 4-9 responses varied from 40 sec up to 18 min. TES allows to detect the position, size and sequence of operation of precisely located specific cortical zones, and to measure their dynamics before, during and after sensory and direct cortical stimulation, motor acts and conditioning (associative learning). TES-effects were recorded in rats, rabbits, cats, monkeys and humans. Waves were found spreading over the cortex with a speed of up to 30 mm/sec along trajectories specific for the sensory modality and the site of stimulation. Some pathological processes in the brain are detectable by TES: experimental tumours and epileptic foci. There are many sources for local heating: neural activity, local metabolism of units, local cerebral blood flow and thermoconductivity in the activated zones of the cortex. Thermoencephaloscopy is a dynamic, non-invasive, contact-free method with a relatively high temporal and spatial resolution and sensitivity. It can be a useful tool in basic neuroscience and medicine.

Acoustic Stimulation↗
PubMed1992

Dynamics of responses of V1 neurons evoked by stimulation of different zones of receptive field.

The dynamics of receptive fields of 73 neurons in area 17 of cat visual cortex were studied using the temporal slice method. Three-dimensional maps of the receptive fields were plotted using the criterion of spike number in successive fragments (step 10 or 20 ms) of responses to 100 local flashes presented at different parts of the receptive field in random order. The size and configuration of such dynamically recorded receptive fields were then estimated. This allowed us to reveal the dynamic reorganization of all receptive fields 20-400 ms after stimulation. A small zone of responses appeared in the receptive field after initial latency, then it widened, received definitive configuration, and after that decreased and disappeared. The effect was reproducible under repeated estimations. The relationships between receptive field and the previously described orientation tuning dynamics, as well as between dynamics of receptive fields and their summation zones, mechanisms and possible functional meaning of the revealed effects for signal processing in the primary visual cortex are discussed.

Animals↗
PubMed1992

[Double orientation tuning of neurons of the primary visual cortex of the cat at various levels of alertness].

Orientation tuning (OT) of 225 visual cortex neurons was studied in immobilized cats by their responses to flashing light bars. It was found that 43% of neurons had monomodal OT and preferred the horizontal and vertical orientations, while 57% of neurons had double OT, i.e. they had main preferred orientation (PO) and additional PO (aPO). The mean angle between PO and aPO was equal to 71.4 +/- 2.4 degrees. In half of the cases the second maximum of OT was equal to the first one (mean: 0.7 +/- 0.03 from PO). Orientation characteristics of PO and aPO were practically identical. Under light and middle levels of narcosis half of the neurons with double OT became monomodal and 12% of monomodal neurons receive bimodal OT. Monomodal neurons had more often simple RF and invariance of OT to narcosis. The neurons with double OT had simple and complex RF equally often and their OT changed in narcosis. It is supposed that the neurons with double OT can be an angle and line cross detectors. Monomodal neurons may be the stable bench mark system of orientation coordinates. Interaction of these neuronal systems permits conducting an effective analysis of image features in the primary visual cortex.

Anesthesia, General↗
PubMed1991

EEG alpha-wave in the visual cortex: check of the hypothesis of the scanning process.

In computer-controlled experiments the recognition by seven human observers of tachistoscopically presented geometrical figures of different size (from 0.5 to 9 angular degrees) or of different eccentricity (from 3 up to 16 degrees) in the visual field was studied. The onset of figures presentation coincided with different phases of the EEG alpha-wave in the occipital region. According to the criterion of an increase of recognition probability, an inverse dependence was revealed between the distance of the figures contour from the gaze (up to 9 degrees) and the succession of phases of alpha-wave. Small or more centrally localized figures were significantly better recognized when presented at relatively earlier phases of EEG alpha-wave, while bigger or relatively more peripherally localized figures - at earlier phases. At 16 degrees form the gaze no reliable dependence of recognition on the alpha-wave phases was revealed. The data obtained are discussed in connection with Pitts and McCulloch (1947) hypothesis about a periodical (with alpha-wave frequency) scanning wave spreading over the visual cortex. Possibility of a synchronous excitability fluctuation in the whole visual cortex with alpha-rhythm frequency that imitated the spreading process is also discussed. Data obtained and simulation of the mentioned possibilities confirmed the first explanation and thus confirmed Pitt's and McCulloch's ideas on the EEG alpha-wave as a reflection of the scanning process in the visual cortex.

Adult↗
PubMed1991

[Orientation tuning of the visual cortex neurons in the cat before and after nembutal injection].

In acute experiments on immobilized cats orientation tuning of 40 neurons in the primary visual cortex (field 17) has been studied before and after nembutal injection. It was found that the preferred orientation (PO) of half of neurons remained constant after injection, while another half of neurons revealed a reliable shift of PO (by 53.6 +/- 8.0 degrees on the average) during 1 min-3 hours. Neurons with constant PO preferred horizontal and vertical orientations more often, while neurons with unstable PO had wider distribution of their POs. "Stable" neurons had more narrow and selective orientation tuning before and after narcotization than the "unstable" ones. After injection of nembutal, the width of orientation tuning changed in a half of neurons; this was more often expressed in "unstable" neurons (68%), than in the "stable" ones (38%). After narcotization the mean background discharge frequency decreased 5.5 times on the average in all neurons, while the evoked frequency of discharges--1.5 times.

Animals↗
PubMed1990

Dynamic infrared functional mapping of the cerebral cortex.

Although many neuroimaging methods (computer tomography, H+ and isotope clearance, [14C]deoxyglucose utilization, potential-sensitive dyes, electroencephalogram and magnetoencephalogram-mapping, nuclear magnetic resonance tomography, single photon emission computer tomography, and positron emission tomography) are now being used to map brain structure and function, there still exists a need for a new approach that is functional, dynamic, remote, and noninvasive, and that also has reasonable sensitivity and spatial and temporal resolution. Such a method is described here. Thermoencephaloscopy (TES), based on thermovision and digital image processing techniques, is a method of neuroimaging consisting of recording through the intact skull the very weak changes in infrared radiation that are connected with brain activity. The parameters of the method are: temperature sensitivity of 0.002 degrees C, instrumental spatial resolution of 70 microns pixel-1, and instrumental temporal resolution of 40 ms (enabling up to twenty-five maps to be produced per second). The distribution over the rat cerebral cortex of local, multiple, and modally and regionally specific thermoresponses to various sensory stimuli, direct cortical stimulation, and associative learning (conditioning), have been studied, as well as the spreading of thermowaves over the brain cortex. The underlying mechanisms which form the basis of the technique of TES, such as joule heating, local changes in brain metabolism and local cerebral blood flow, as well as the formation of thermodipoles, are discussed.

Animals↗
PubMed1990

[The dynamics of the orientation adjustment of the neurons in the cat visual cortex under the action of sombrevin].

The dynamics of orientation tuning of 59 neurons in the primary visual cortex of cat was studied by the method of temporal slices before and after narcotization by sombrevine. It was found that in 2/3 of neurons a dynamic shift of the preferred orientation (from 22 up to 157 degrees) was observed during their response. After injection of sombrevine this shift reliably diminished in 45% of neurons, in some cases up to its complete disappearance, while in 30% of cases this shift increased. Differences in dynamics of orientation tuning under narcosis in two groups of units, as well as the mechanisms of the effect are discussed.

Animals↗
PubMed1989

[The effect of sombrevin on the orientation adjustment of the visual cortex neurons in the cat].

Change in the orientation tuning of 58 visual cortex neurons, evoked by light sombrevine anaesthesia were studied in acute experiments on immobilized cats. After the sombrevine injection a reliable change of the preferable orientation by 47.6 +/- 5.6 degrees took place in 60% of cells, while in the remaining cells it was stable in all stages of anaesthesia. Stable neurons preferred the horizontal and vertical orientations, in the unstable ones this preference was less expressed. The stable neurons possessed higher qualities of orientation detection than the unstable ones. The width of the orientation tuning changed reliably by 65.2 +/- 6.7 degrees on the average in 55% of neurons, while in 31% of neurons the tuning acuteness was worse, but in 24% it was sharper. The frequency of background discharges under anaesthesia in 2/3 of investigated neurons was reduced by 58% and that of the evoked discharges by 35%. The preferable orientation, the width of the tuning and the frequency of the neuron discharge as a rule recovered in 30 min after the beginning of the anaesthesia.

Animals↗