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V M Bondarko

Publications and source records attributed to V M Bondarko.

At least 19 recordsLinked to original sources
PubMed2005

Detection of narrow bars when presented in the right and left visual hemifields.

Three series of psychophysical experiments were performed to study the effects of additional images on the contrast thresholds for detection of narrow bars of width 1 arc min and length 1 arc degree presented randomly to the left and right sides of the fixation point with an eccentricity of 4 degrees. The additional images were bars of the same size but with lower contrast, presented to the right and left of the test bar at varying distances, i.e., 1-2 degrees. Different series used different contrast ratios for the test and additional bars. The first series revealed significant predominance of one visual hemifield over the other in performing the bar detection task, though the predominant side varied in different subjects. This predominance disappeared or changed to the opposite side in the second or third series. There was a tendency for the additional images to have asymmetrical influences on the detection threshold for the test bar: additional bars had more extensive inhibitory influences when they were on the peripheral side of the test bar. These data provide evidence for the absence of specificity in hemisphere operation in the detection task. The mechanism of description of peripheral images is discussed.

Contrast Sensitivity↗
PubMed2004

Resolving ability and image discretization in the visual system.

Psychophysiological studies were performed to measure the spatial threshold for resolution of two "points" and the thresholds for discriminating their orientations depending on the distance between the two points. Data were compared with the scattering of the "point" by the eye's optics, the packing density of cones in the fovea, and the characteristics of the receptive fields of ganglion cells in the foveal area of the retina and neurons in the corresponding projection zones of the primary visual cortex. The effective zone was shown to have to contain a scattering function for several receptors, as this allowed preliminary blurring of the image by the eye's optics to decrease the subsequent (at the level of receptors) discretization noise created by a matrix of receptors. The concordance of these parameters supports the optical operation of the spatial elements of the neural network determining the resolving ability of the visual system at different levels of visual information processing. It is suggested that the special geometry of the receptive fields of neurons in the striate cortex, which are concordant with the statistics of natural scenes, results in a further increase in the signal:noise ratio.

Adult↗
PubMed2003

[Contrast detection thresholds during their presentation in right and left hemifields of vision].

We measured contrast detection thresholds for thin black vertical bars (1 min of arc wide, 1 deg long) against a bright background, which were presented eccentrically at 4 deg to the left or right of fixation and flanked on one side by a subthreshold bar having the same dimensions. On each presentation, the stimulus randomly appeared in the left or right visual hemifield. The separation between the test and the flanks varied from 1 to 60-120 min of arc. Different subjects showed better performance in different hemifields. This preference was lost or changed to opposite in the course of training. The central excitatory areas are more or less symmetrical, but inhibitory areas reveal asymmetrical tendency: when flanked bars were more peripheral than the test line, the inhibitory areas were larger. Our results show absence of hemispheric specificity in the line detection task. We discuss the ways of stimulus description in fovea and periphery.

Fovea Centralis↗
PubMed2002

[The resolution and discrimination of images in the visual system].

We analysed the conformity of optic and retinal cones anatomy factors by the two-point test. Obtained by F. Campbell and R. W. Gubish, the point spread function has a width of about 1 arc min. Cones sizes are equal to 0.5 arc min in the fovea. Functional pixel consists of 3-5 cones under the point spread function. Such an organisation in very useful in decreasing the samping noise of receptors. We carried out psychophysical investigations to show a consensus among the optic, receptors', and neuronal levels. In experiments we studied changes of the two-point pattern perception in respect to the points separation, measured the orientation threshold of small size stimuli. Data were compared with optical point-spread function, the hexagonal mosaic of cones, and line spread function of spatial elements, which form spatial frequency channels at the cortical level.

Adaptation, Ocular↗
PubMed1999

Spatial interval discrimination in the presence of flanking lines.

Spatial interval discrimination was studied in the absence or presence of distractors. In the latter case, two flanking lines surrounded two vertical lines delimiting the spatial interval. Using a temporal 2AFC technique with a method of constant stimuli we measured the accuracy of performance (discrimination thresholds) and biases (points of subjective equality) depending on the separations between the target and the flanking lines. For separations less than or comparable to the size of the spatial interval we found both a reduction of precision and the increase of perceived sizes of the spatial intervals: the discrimination thresholds were increased, the size of the spatial interval was overestimated. For larger separations, the size of the spatial interval was underestimated, but the precision of performance was not affected by the presence of flanking lines. We discuss the possible mechanisms underlying spatial interval discrimination in the presence of flanking lines.

Humans↗
PubMed1997

What spatial frequency do we use to detect the orientation of a Landolt C?

We calculated the two-dimensional Fourier spectrum of a Landolt C. For Landolt Cs of orthogonal orientation, the main differences in the amplitude spectrum were found at a low frequency such that 1.3 periods were equal to the size fo the Landolt C, rather than at the high frequency corresponding to the size of the gap, i.e., such that 2.5 periods were equal to the size of the Landolt C. We compared visual acuity assessments obtained with Landolt C optotypes and the cut-off of the contrast sensitivity function measured with sinusoidal gratings. We found that the frequency corresponding to the size of the gap is twice the latter frequency. We suggest that, in fact, this lower frequency can be used to determine the position of the gap.

Fourier Analysis↗
PubMed1990

Similarity between Fourier transforms of objects predicts their experimental confusions.

Two sets of stimuli were presented tachistoscopically to 4 subjects. On each trial, a single stimulus was presented, and the subject was required to identify the stimulus by verbal response. An exposure duration was chosen such that the subject's identification performance fell within a range from faultless identification to chance guessing. The object-identification data of each subject obtained for all stimulus exposures were pooled to form an object confusion matrix. A model of visual processing based on two-dimensional spatial frequency content (Fourier transforms) was used to predict confusions among stimulus pairs. The model properties that appear to be the most essential are those that allow it (1) to account for the obvious dependence of the Fourier transform on the choice of an origin point; and (2) choose the point of origin for each object separately, irrespective of other objects of the set. The point of origin of the reference frame, in which Fourier transforms are performed, is chosen so as to minimize the low-frequency phase component for each object. A high correlation (up to .96) between confusion matrices and model interobject distances was attained. The results demonstrate that such a distance measure gives a good prediction of object confusability.

Adult↗
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↗
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↗
PubMed1980

[Linear and nonlinear properties of cat visual cortex receptive fields].

Impulse responses of the simple fields cat visual cortex were found to be modulated by gratings passing the field. The complex fields proved to be of three types: with modulated responses, unmodulated responses, and with modulated responses against unmodulated background. Amplitude-phase characteristic (APC) measured were inverse Fourier transformed to obtain the field's weighting function. Simultaneously the APC was reconstructed from the responses to edges and bars, with the use of the Fourier transform. Cross-comparison of the reconstructed APC and the WF showed that a RF has some linear properties but, strictly considered, is a non-linear system. Simple fields display the largest degree of linearity. The more complex field is the greater departures from linearity. As linear methods are inadequate for dealing with cortical RFs, their identification was performed in model experiments on a computer. The evidence obtained suggest that the RFs form a system of operators which perform the expansion of the image in non-classical pattern. Such an expansion can be termed quasi-Fourier-description.

Animals↗
PubMedDate not supplied

[Two-dimensional structure of receptive fields of the cat visual cortex].

Two-dimensional spatial-frequency characteristics of receptive fields were obtained when investigating with sinusoidal gratings of different spatial frequency and orientation. The optimal spatial frequency does not change with orientation. Theoretical analysis of the role of limited size of RF shows that RF must be broadband in frequency and orientation. The results show that RF are grating filters.

Animals↗