Biomedical subjects

I A Shevelev

Publications and source records attributed to I A Shevelev.

At least 19 recordsLinked to original sources

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↗

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↗