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A S Tikhomirov

Publications and source records attributed to A S Tikhomirov.

At least 19 recordsLinked to original sources
PubMed2006

The time course of disinhibition of visual cortex neurons and sensitivity to cross-shaped figures.

The relationship between the responses of 74 neurons in field 17 of the cat cortex to presentation of cross-shaped figures flashing in their receptive fields and the asynchronicity with which the lines of the figures were presented were investigated. The cross sensitivity of neurons was studied with simultaneous, leading, and delayed activation of the disinhibitory zone of the receptive field in relation to the time at which its major excitatory and end-stopping inhibitory zones were stimulated. Two types of temporal interaction were identified between the receptive field zones determining cross sensitivity. In cells of the first type (14 of 23 cells), the response was maximal in conditions of simultaneous stimulation of the major and disinhibitory zones of the receptive field; neurons of the second type (nine of 23 cells) showed the opposite temporal relationship. Digital simulation showed that cross sensitivity in neurons of the first type was supported by disinhibition of end-stopping inhibition, while in neurons of the second type it depended on a combination of disinhibitory and convergence mechanisms.

Action Potentials↗
PubMed2005

Dynamic changes in the tuning of striate neurons to the shapes of cross-shaped figures.

Time slice analysis was used to study the dynamics of tuning to the shapes of cross-shaped figures flashing in the receptive fields of 83 neurons in the primary visual cortex (field 17) of the cat brain. Tuning was assessed in terms of the numbers of spikes in the overall response and its sequential 20-msec fragments. Only 11.7% of neurons produced reproducibly developing spike responses to a given shape (defined as the angle between the lines), i.e., had a preferred cross-shaped figure. In the remaining cases (88.3%), tuning of neurons to the shape of the cross showed dynamic changes. In 7.2% of cases, changes in the preferred shape of the cross occurred monophasically; changes were biphasic in 27.0% of cases, while in the remaining 54.1% of cases, the dynamics in changes in the preferred cross shape were undulatory. The tuning of receptive field zones is assessed as the cause of these effects and their difference from the previously observed dynamics of preferred orientations of single bars and cross-shaped figures; the functional significance of these effects is also discussed.

Action Potentials↗
PubMed2004

[Temporal course of disinhibition of the visual cortex neurons and their sensitivity to cross-like figures].

In acute experiments with narcotized and paralyzed cats, we studied responses of 74 striate neurons to cross-like figure under synchronous and asynchronous presentation of its lines. The aim of the study was to characterize the temporal course of interaction between three RF zones: main excitatory, end-inhibitory, and side disinhibitory ones. Previously we have found that this interaction is responsible for sensitivity to a cross in near 3/4 of cat striate cells with such sensitivity. In neurons with sensitivity to a cross, we found two types of temporal interaction between zones of RF. In the 1st type cells (14/23), the response significantly increased if the disinhibitory and the main excitatory zones of RF were stimulated simultaneously. Neurons of the 2nd type (9/23) revealed opposite temporal function: synchronous activation of RF zones evoked a minimal response. Simulation shows that the 1st type of behavior is connected with disinhibitory mechanism, while that of the 2nd type--with combination of this mechanism with convergence of orientation detectors of the previous functional level.

Animals↗
PubMed2003

[Dynamics of tuning to the shape of a cross-like figure in striate neurons].

Dynamics of tuning to the shape of cross-like figure flashed in receptive field was studied in 83 striate neurons by the method of temporal slices. Tuning was estimated by the total number of spikes in the response and by this number in successive fragments of the response with 20 ms steps. It was found that only in 11.7% cases neurons showed stable tuning to the same shape of the preferred figure (an angle between its lines), in other cases (88.3%) during response generation this tuning changes being one-phase (7.2%) or two-phase (27.0%), or undulatory (54.1%). Different dynamical reorganization of receptive field zones is discussed as a possible mechanism of the revealed effects as well as their correlation with previously described dynamics of tuning to orientation of a single bar and a cross in striate cells.

Action Potentials↗
PubMed2001

Tuning to Y-like figures in the cat striate neurons.

Sensitivity to symmetric or asymmetric Y-like figures and crosses of different shapes and orientations flashed in the receptive field was studied in 101 neurons of the cat striate cortex (area 17) and compared with their orientation tuning to a single light bar. Selective sensitivity to the Y-like figure (figure/bar response ratio more than 1.25) was found in 78/101 neurons (77.2% of cases) and to the cross-in 54/101 units (53.4%). In 62.5% of neurons with sensitivity to both figures, sensitivity to the Y-like figure was higher than to a cross. Tuning to Y-like figure was typically (60%) selective to both its shape and orientation. The remaining Y-like selective neurons exhibited invariant tuning to orientation and/or shape of the figure. The preferred angles between two lines of Y-like figures were distributed in the range of 22.5-157.5 degrees with slight preference to 90 degrees, while crosses of 45 degrees and 90 degrees angles were preferable. Response magnitudes to a single bar, a Y-like figure and a cross were positively correlated. Possible mechanisms and functional implication of the striate sensitivity to Y-like figures are discussed.

Action Potentials↗
PubMed2001

[Disinhibitory zone of the striate neuron receptive field and its sensitivity to the cross-like shape].

Sensitivity of 84 neurones to presentation of cross-form figures was studied in cats as well as the lever length function prior to and after stimulation of the RF disinhibitory zone. Responses to lever length of the cross-sensitive cells were augmented in simultaneous stimulation of the RF. The data obtained suggest involvement of the end inhibition and its blocking by a side-disinhibitory zone of the RF.

Animals↗
PubMed1999

Interrelation of tuning characteristics to bar, cross and corner in striate neurons.

Characteristics of responses and background activity, as well as of tuning to a single bar orientation and to cross or corner shape and orientation have been compared in one third (561174) of neurons in the cat striate cortex. Shortening of the response latency to cross vs bar, to corner vs bar and to corner vs cross was revealed in most of the units studied. Direct correlation between the response and tuning characteristics for bar, cross and corner was revealed: units with better tuning to one type of stimulus were typically better tuned to the other types of stimuli. At the middle cortical depth (700-1200 microm from the surface) we found a reliable improvement of response magnitude and latency, cross/bar response ratio and selectivity of tuning in comparison with more superficial and deeper layers. Although we could not find a direct correlation between characteristics of tuning to figures and the type of the receptive field (simple, complex or hypercomplex), our data pointed to a lower cross/bar ratio and selectivity of tuning in the units with small receptive fields. The functional implication of neuronal sensitivity to cross and corner and possible meaning of correlation between their functional characteristics are discussed.

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↗
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

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

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

[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

[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

[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↗
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↗