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V D Glezer

Publications and source records attributed to V D Glezer.

At least 19 recordsLinked to original sources
PubMed2005

Separate training of hemispheres to perform visual discrimination in conditions of blocking of interhemisphere transmission by masking.

Subjects were trained to discriminate three figures presented in the left field of vision and three other figures presented in the right field of vision. In these conditions, the two hemispheres usually show identical learning to discriminate the sets of stimuli because the hemispheres in healthy humans can exchange information. In the present study, training was performed in conditions in which, during presentation of stimuli, the opposite visual hemifield was covered by a mask. After training, the recognition of all six figures was compared by presenting them to the left and right visual fields. Each hemisphere recognized figures presented to the cognate hemifield but completely or very nearly failed to recognize figures learned by the other hemisphere. The mask would thus appear to block (completely or partially) the transmission of information from one hemisphere to the other. Thus, it was possible to train the hemispheres separately to recognize different sets of images in healthy subjects.

Discrimination Learning↗
PubMed2003

[Recognition of visual images by separate hemispheres in conditions of masked blocking of interhemispheric transmission].

The subjects learned to recognize three figures presented in the left visual hemifield and three figures presented in the right visual hemifield. During presentation of a stimulus, the contralateral hemifield was overlapped by a mask. After the training, recognition of all six figures presented in the right and left visual hemifields, was compared. Each hemisphere recognizes figures which were learned in the corresponding visual hemifield, but the recognition of figures learned in the opposite visual hemifield was poor. Thus, the ability of the hemispheres to act separately in recognizing different sets of visual images, was established.

Dominance, Cerebral↗
PubMed1997

Linear and nonlinear properties of simple cells of the striate cortex of the cat: two types of nonlinearity.

In a proportion of simple cells of the striate cortex, the weighting functions of the receptive fields (RFs) had more periods than could be established by mapping using responses to light bars and dark bars. In these multiperiodical cells, side subfields do not respond to single bars, as they have lower weights than central zones and the excitation is under the threshold of impulse response if a single bar is applied. This fact has been established by different methods: conditioning and testing stimuli, grating patches, and inverse Fourier transform of the amplitude-phase characteristic, combining them in one cell. We assume that this type of nonlinarity can be used in analyzing the image, as it acts as a spatial-frequency filter of the area overlapped by the RF. The responses to complex gratings composed by two sinusoidal gratings of different frequency, contrast, and phase shift were compared with the sum of the responses to the gratings when they were presented separately. The results show that the principle of superposition holds a reasonable approximation even if the response is evoked from the side subzones. Some simple cells have nonlinear properties beyond the classic zone of RF (2nd type of nonlinearity). Linear cells have a tendency to be localized in layer 4 of striate cortex, cells with a nonlinear surround in layers 2, 3, 5, and 6. The significance of both types of nonlinearities in simple cells is discussed.

Animals↗
PubMed1989

Spatial organization of subfields in receptive fields of cells in cat striate cortex.

Spatial organization of receptive fields (RF) of cells in cat striate cortex was investigated with moving and flashing light and dark bars and with grating-patterns of a varying number of cycles. It was shown that the maximum number of subfields in a simple cell is equal to eight or the number of periods in weighting function is equal to four. Quantitative comparison of the data with the results of seven other studies allows us to suggest that the number of periods in linear component of some complex cells is close to this value. The discrepancies between the results of different authors in estimation of the number of subfields are explained by the experimental data.

Animals↗
PubMed1989

Harmonic basis functions for spatial coding in the cat striate cortex.

The number of subregions in the activity profiles of simple cells varies in different cells from 2-8; that is, the number of cycles in the weighting function varies from 1-4. The distribution of receptive-field (RF) sizes at eccentricities of 0-6 deg are clustered at half-octave intervals and form a discrete distribution with maxima at 0.62, 0.9, 1.24, 1.8, 2.48, and 3.4 deg. The spatial frequencies to which the cells are tuned are also clustered at half-octave intervals, forming a discrete distribution peaking at 0.45, 0.69, 0.9, 1.35, 1.88, 2.7, 3.8, and 5.6 cycles/deg. If we divide the RF sizes by the size of the period of the subregions, then the average indices of complexity (really existing) or the number of cycles in the weighting function form (after normalization) the sequences: 1, 1.41, 2.0, 2.9, 4.15. The relation between the bandwidth of the spatial-frequency characteristic and the optimal spatial frequency is in accordance with predictions of the Fourier hypothesis. The absolute bandwidth does not change with the number of cycles/module. This means that inside the module the absolute bandwidth does not change with the number of the harmonic. The results allow us to suggest the following. A module of the striate cortex, which is a group of cells with RFs of equal size projected onto the same area of central visual field, accounts for the Fourier description of the image. The basis functions of the module are composed of four harmonics only, irrespective of size and position of the module. Besides linear cells (sinusoidal and cosinusoidal elements), the module contains nonlinear cells, performing a nonlinear summation of the responses of sinusoidal and cosinusoidal elements. Such cells are characterized by an index of complexity which is more than the number of cycles in the weighting function and by marked overlap of ON and OFF zones. The analysis of organization suggests that the cells can measure the amplitude and phase of the stimulus.

Animals↗
PubMed1986

[Principle of uncertainty in vision].

When investigating the dependence of the bandwidth of the spatial frequency characteristic (delta F) of the receptive fields of the cat striate cortex on the size (D) of the receptive field it was shown that delta FD = 1.2. Thus in the system of elements performing the processing of information in the visual cortex the inaccuracy of the signal representation in the spatial frequency and in space is related by an uncertainty principle. The size of the constant evidences that the weighting functions of elements are sinusoids or cosinusoids modulated by squarewave impulse rather than by the Gabor elements. The performing of uncertainty principle and the size of the constant are arguments in favour of the hypothesis that the receptive fields of the visual cortex organize a quasilinear basis.

Animals↗
PubMed1984

[Relation between the functional organization of receptive fields of the lateral geniculate body of the cat and visual stimulus contrast].

The functional organization of receptive fields in the lateral geniculate body of cat was studied depending on the contrast of visual stimuli. In receptive fields which reorganize with contrast in the central zone of summation obtained at low contrast there appears an additional inhibitory ring at high contrast. The role of this phenomenon for processing of high spatial frequencies at high contrast is discussed.

Animals↗
PubMed1983

[Mechanism of responses to movement in the visual cortex neurons unresponsive to light flicker].

The responses to moving and stationary stimuli in cat's striate cortical units were studied. Two stationary light bars, one located in the receptive field centre, and another--in the periphery, were used. The sequence of presentation and the time-interval between the stimuli varied thus making the presentation of a pair of stationary bars an analogue of a moving stimulus. Responses occurred in the neurons previously unresponsive to stationary stimuli when two stationary stimuli were presented successively in certain order. A model is proposed explaining the occurrence of responses to moving stimuli in the neurons, unresponsive to stationary ones.

Animals↗
PubMed1983

[Relation between the spatial and spatial-frequency characteristics of receptive fields of the cat visual cortex].

Spatial (the size and the eccentricity) and spatial-frequency (optimal frequency and the bandwidth) characteristics of receptive fields of the cat visual cortex were studied. The linear and quasilinear receptive fields of equal size were shown, in accordance with predictions of the piecewise Fourier analysis, to constitute a module in every field of which the complexity index (ratio of the field size vs. the number of its optimal frequency periods) equaled the optimal frequency multiplied by a coefficient constant for a given module. Five modules were found with field sizes 2.6 degrees; 3.8 degrees; 5.2 degrees; 6.2 degrees and 7.0 degrees moving toward the visual field periphery as the modules increased in size. According to the predictions, the bandwidth decreases in the reversed proportion to the field size in a fixed complexity index. The data obtained corroborate the hypothesis implying that the receptive fields perform a piece wise quasi--Fourier expansion of an image.

Animals↗
PubMed1983

[Study of the mechanism of directionality of the receptive fields of the visual cortex in the cat].

The responses of directional-sensitive striate cortex neurons to two light bars one of which was located in the centre of the receptive field and the other--in the inhibitory zone were investigated in unanaesthetized cats. The order of presentation and the time intervals between the stimuli varied; so the presentation of the pair of stationary bars was an analog of a moving stimulus. It was shown that the inhibitory off-zone located on the side of the preferred direction of movement is characterized by an early inhibitory phase followed by a phase of disinhibition and by the second inhibitory phase. The presentation of a pair of stationary bars with different time intervals between them elicited inhibition when the central zone stimulation coincided with the inhibitory phases and facilitation when it coincided with the disinhibitory phase. For the inhibitory off-zone located on the side of the null direction no disinhibitory phase was found. The significance of the time characteristics of the inhibitory zones for the appearance of directional sensitivity in striate neurons is discussed.

Animals↗
PubMed1982

[Structure of the complex receptive field of the visual cortex in the cat].

This most common type of a complex receptive field was analyzed whose response contains modulated and unmodulated components. Application of the mask covering a half of the field increased (according to the filter theory) the bandwidth of the field as a spatial frequency filter due to appearance or enhancement of the response at lateral low and high frequencies. In this case the modulated components of response from each half of the field are out of phase. Analysis of this fact together with responses to thin light and dark bars permitted describing the structure of the field as consisting of linear and nonlinear subsystems converging on the complex field neuron. The linear system comprises several pairs of on- and off-subfields of corpus geniculatum. On- and off-subfields in the pair are spatially overlapped and converge on a neuron of the linear subsystem with opposite signs. The nonlinear system comprises only on- or off-subfields. Other types of complex fields are formed by various combinations of subsystems. The results evidence that the complex field is a grating spatial frequency filter.

Animals↗
PubMed1982

[Neurophysiological correlates of visual perception].

Structural organization of simple and complex receptive fields of the visual cortex was studied. The bidimensional weight function of the field determining its main properties as a filter of spatial frequencies (the space-frequency and the orientation sensitivity), was investigated. Two types of modules are discussed: the cylinders of cortical neurons whose receptive fields are oriented towards a single area of the visual field and adjusted for different spatial frequencies and orientations. One type of the modules contains neurons whose receptive fields describe the amplitude and phase of a visual signal and give the Fourier-description of a portion of picture; the second type's receptive fields describe the amplitude alone and give the power spectra. Comparison with psychophysical data suggests that the second type serves for texture description. Inhibition in the receptive field induced by frequencies which are lateral in respect to the optimal frequency and by orientations which are perpendicular in respect to the optimal orientation, reveals the mutually inhibiting influences among the module's neurons. Significance of this type of organization for visual perception is discussed.

Humans↗
PubMed1981

[Structure of the simple receptive field of the cat visual cortex].

Comparison of the cat simple receptive field (No. 17) responses to sinusoidal gratings and to thin light and dark bars showed that the excitatory and inhibitory on- and off-zones are composed by on- and off-subfields of corpus geniculatum, converging to the cortical neurons. Each zone comprises a pair of opponent subfields -- excitatory and inhibitory. Such an organization reveals the linear properties of the fields. However a real simple field is a nonlinear system because of available deviations from such an ideal organization. The deviations are manifested in mutual displacement of the subfields, inhomogeneity of the subfields and absence of an antagonistic subfield in the zone. Phasic and tonic subfields may be present simultaneously even within the same field.

Animals↗