Search PubMed⌕ Search

Biomedical subjects

L L Kontsevich

Publications and source records attributed to L L Kontsevich.

15 recordsLinked to original sources
PubMed2001

Stereoprocessing of cyclopean depth images: horizontally elongated summation fields.

The study evaluated how the detection efficiency varies with the length and width of Gabor wavelets of depth ripple in random element patterns. For local (one-cycle) wavelets, there was no anisotropy with respect to wavelet orientation, implying equal efficiency for processing shear versus compression disparities at threshold. The processing for larger patterns of depth ripple did not correspond to a fixed summation field but varied in size with spatial frequency, and in shape with orientation of depth ripples, up to four cycles of a horizontal bar at 0.5 cy/deg, or 8 degrees of visual angle. The presence of such extended summation fields in only one orientation is incompatible with the idea of local attentional processing, with a single disparity channel or with an adaptive mechanism that could accommodate to any form of disparity image. Thus, these results suggest the presence of a multichannel hypercyclopean level of processing specialized for horizontal depth contours, whose only disparity information is in their surface texture.

Attention↗
PubMed2000

Relative contributions of sustained and transient pathways to human stereoprocessing.

It has been proposed [Hubel & Livingstone (1987) Journal of Neuroscience, 7, 3378-3415] that stereopsis is mediated solely by magnocellular pathway in primates. This hypothesis was evaluated for humans in psychophysical experiments with dynamic random-noise stimuli, based on the sustained/transient relationship of behavior mediated by the two divisions of the LGN [Merigan & Maunsell (1993) Annual Review of Neuroscience, 16, 369-402]. The stereoscopic limits show that stereoscopic system is more sensitive to sustained random-dot stimuli than to transient ones. Quantitative modeling of the result implied a weak role for magnocellular input, suggests that human stereopsis is more strongly influenced by parvocellular input through the LGN.

Adolescent↗
PubMed1999

Bayesian adaptive estimation of psychometric slope and threshold.

We introduce a new Bayesian adaptive method for acquisition of both threshold and slope of the psychometric function. The method updates posterior probabilities in the two-dimensional parameter space of psychometric functions and makes predictions based on the expected mean threshold and slope values. On each trial it sets the stimulus intensity that maximizes the expected information to be gained by completion of that trial. The method was evaluated in computer simulations and in a psychophysical experiment using the two-alternative forced-choice (2AFC) paradigm. Threshold estimation within 2 dB (23%) precision requires less than 30 trials for a typical 2AFC detection task. To get the slope estimate with the same precision takes about 300 trials.

Bayes Theorem↗
PubMed1999

Nonlinearities of near-threshold contrast transduction.

The existence of analytic threshold nonlinearities was probed with 2AFC incremental threshold functions for both local and extended test patterns on stationary matched pedestals of the same and opposite sign. In contrast to the facilitation effect with same-sign pedestals, sensitivity with opposite-sign pedestals first deteriorated up to the mask detection level, abruptly improved and then deteriorated again. Analytic solutions for the transducer function with additive noise were derived to account for the incremental data in all conditions. The results for positive difference-of-Gaussian (DoG) stimuli (whose increment made the central spot lighter) and for 10 c deg-1 Gabor stimuli were consistent with accurate hard-threshold behavior with best-fitting d' powers from 17 to 358. The 10 c deg-1 data further implied that contrast gain control was operating throughout the subthreshold range. The results for negative DoGs (whose increment corresponds to the darkening of the central spot) and 2 c deg-1 Gabor profiles were consistent with mild nonlinearities having d' powers of 1.6-3. Significant differences between the nonlinearities for positive and negative DoGs indicate that only a small portion, if any, of the near-threshold nonlinearity could be attributed to uncertainty. Our analysis suggests that, with low spatial frequency gratings, detection was based on those bars that become darker; with high-frequency gratings, on the bars that become brighter.

Adult↗
PubMed1999

Distraction of attention and the slope of the psychometric function.

The influential uncertainty model [J. Opt. Soc. Am. A 2, 1508 (1985)] attributes nonlinear contrast sensitivity near threshold to the inability of the observer to discriminate between the signal from stimulated locations and the noise from nonstimulated locations. We introduce an alternative interpretation, the distraction model, to describe the behavior of an observer who knows exactly which location was stimulated but may miss the test stimulus because attention has been distracted by irrelevant (noise) signals. For any stimulus sample, the observer is assumed to be certain of whether this sample is relevant or irrelevant to the stimulus. The non-linear effects predicted by the distraction model without uncertainty are similar to those predicted by the uncertainty model.

Attention↗
PubMed1998

Defaults in stereoscopic and kinetic depth perception.

This study presents three findings concerning the mechanisms of depth perception. First, the shape of the three-dimensional percept evoked by two-frame motion is defined solely by the rotation component around an axis in the frontoparallel plane; the visual system assigns a default value to this rotation component to arrive at a unique solution. Second, when the visual axes of two eyes are almost parallel, the visual system uses a default vergence value to reconstruct stereoscopic depth. Third, the default vergence and default rotation angles are highly correlated across subjects. This correlation implies that the two modalities share a common scaling default at an internal level.

Cues↗
PubMed1998

How much of the visual object is used in estimating its position?

Localization accuracy for a wide Gaussian bar (sigma 1 degree) was essentially invariant with sampling density over a wide range. Luminance contrast, on the contrary, had a profound effect on localization accuracy. This difference suggests that the position is estimated from a few (3-4) samples from all those available in the image. A subsequent experiment confirmed that limiting the display to four samples did not impair localization accuracy. Computer simulations show that the result cannot be explained by the peak or centroid rule for position. The results imply that the visual system uses only a few samples to interpolate the luminance profiles, regardless of how many samples are available in the image.

Contrast Sensitivity↗
PubMed1997

Direction selectivity of synaptic potentials in simple cells of the cat visual cortex.

Direction selectivity of synaptic potentials in simple cells of the cat visual cortex. J. Neurophysiol. 78: 2772-2789, 1997. The direction selectivity of simple cells in the visual cortex is generated at least in part by nonlinear mechanisms. If a neuron were spatially linear, its responses to moving stimuli could be predicted accurately from linear combinations of its responses to stationary stimuli presented at different positions within the receptive field. In extracellular recordings, this has not been found to be the case. Although the extracellular experiments demonstrate the presence of a nonlinearity, the cellular process underlying the nonlinearity, whether an early synaptic mechanism such as a shunting inhibition or simply the spike threshold at the output, is not known. To differentiate between these possibilities, we have recorded intracellularly from simple cells of the intact cat with the whole cell patch technique. A linear model of direction selectivity was used to analyze the synaptic potentials evoked by stationary sine-wave gratings. The model predicted the responses of cells to moving gratings with considerable accuracy. The degree of direction selectivity and the time course of the responses to moving gratings were both well matched by the model. The direction selectivity of the synaptic potentials was considerably smaller than that of the intracellularly recorded action potential, indicating that a nonlinear mechanism such as threshold enhances the direction selectivity of the cell's output over that of its synaptic inputs. At the input stage, however, the cells apparently sum their synaptic inputs in a highly linear fashion. A more constrained test of linearity of synaptic summation based on principal component analysis was applied to the responses of direction-selective cells to stationary gratings. The analysis confirms that the summation in these cells is highly linear. The principal component analysis is consistent with a model in which direction selectivity in cortical simple cells is generated by only two subunits, each with a different receptive-field position and response time course. The response time course for each of the two subunits is derived for four analyzed cells. Each derived subunit is linear in spatial summation, suggesting that the neurons that comprise each subunit are either geniculate X-cells or receive their primary synaptic input from X-cells. The amplitude of the response of each subunit is linearly related to the contrast of the stimulus. The subunits are nonlinear in the time domain, however: the response to a stationary stimulus whose contrast is modulated sinusoidally in time is nonsinusoidal. The principal component analysis does not exclude models of direction selectivity based on more than two subunits, but such higher-order models would have to include the constraint that the extra subunits form a smooth continuum of interpolation between the properties derived from the two subunit solution.

Animals↗
PubMed1995

The nature of the inputs to cortical motion detectors.

Recently, Jagadeesh, Wheat and Ferster [(1993) Science, 262, 1901-1904] presented intracellular recordings from direction-selective simple cells in primary visual cortex and provided an analysis to support the idea that synaptic summation in simple cells is linear. New analysis presented in this study reveals that: (1) the number of subunits contributing to the analyzed simple cell inputs is two; (2) the subunits are nonlinear in the time domain; (3) each subunit linearly integrates the luminance across the receptive field being, thus, linear to local contrast; (4) the waveforms of the subunit signals are linearly modulated by local contrast at the subunit loci unless the contrast changes its sign; (5) the synaptic summation in the simple cell is linear; (6) nonlinearity of even harmonics has sufficient information for retrieving of relative spatial phase of the subunits and reconstruction of the exact temporal profiles of the subunit signals.

Animals↗
PubMed1995

Binocular disparity processing with opposite-contrast stimuli.

Stereoscopic perception of relative depth with reversed-contrast half images differs in several important respects from stereopsis with matched-contrast half images. Thus, reversed-contrast images show no correlated shift in visual direction, indicating that the sensory-fusion mechanism ignores opposite-sign edges; one experiment addressed this aspect of the problem. Mainly, this was a quantitative study of opposite-contrast stereopsis, in which stereoacuity was measured as a function of bar width by means of narrow-band stimuli. Acuity was about an order of magnitude worse for reversed-contrast than for matched stimuli, but the ability to see valid (disparity-dependent) depth was not altogether lost even with wide (1 cycle deg-1) reversed-contrast bars. It is generally believed that depth with opposite-contrast stimuli is mediated by interaction between binocular stimuli components that have the same sign of contrast. Perceived depth was measured as a function of disparity and thus one of the predictions of that 'same-sign hypothesis' was tested experimentally; then, the magnitude of same-sign components was manipulated within the reversed-contrast stimuli, and thus the general prediction of the same-sign hypothesis was tested. The results show conclusively that the same-sign hypothesis cannot account for opposite-contrast stereopsis; its mechanism remains unknown.

Contrast Sensitivity↗
PubMed1995

Mechanisms of stereoscopic processing: stereoattention and surface perception in depth reconstruction.

Consideration of the range of phenomena from studies of human stereopsis suggests that a five-stage model is required to provide a complete account of the processes involved, within which any stereoattention mechanism must operate. The information from the disparity field of the optical projections to the two eyes (stage 1) goes to a set of parallel Keplerian arrays of disparity detectors, each array selective for a different spatiotemporal property of the visual images (stage 2). Global interactions produce a cyclopean depth image that is cleaned of the spurious ghost images in the Keplerian arrays (stage 3) and that may then be processed for its (hypercyclopean) from elements (stage 4). Finally, there must be a stage of integration of the stereoscopic depth cues with monocular and kinesthetic depth cues to form the overall map of perceived distance (stage 5). The fact that multiple cyclopean surfaces may be perceived as transparent implies that the stereoscopic system is not limited by a singular-surface constraint. However, it is unclear whether multiple surfaces can be seen simultaneously or whether only one surface is seen at a time by a selective-attention process, with the others perceived as a purely inchoate (qualitative) depth impression. New experiments on cueing of ambiguous stereocorrugations by singular flat planes suggest that selective stereoattention is a powerful mechanism. In fact, the results show that attention can be focused not just in horopteral planes but in a variety of depth configurations. Moreover, this attention focus may act as a tracking mechanism to allow perception of smooth cyclopean stereomotion, which has a frequency response up to approximately 5 Hz (in contrast to the approximately 15 Hz limit for detecting planar disparity shifts as jerky appearance and disappearance effects). Finally, the spatial limits of stereosurface reconstruction are explored with cyclopean targets to show some interesting asymmetries of the surface-wrapping process that may represent object-oriented constraints on depth reconstruction.

Attention↗
PubMed1994

Analysis of stereothresholds for stimuli below 2.5 c/deg.

We analyze published data on disparity detection thresholds for a wide range of conditions. This type of detection changes behavior dramatically at the spatial frequency of 2.5 c/deg; above this frequency threshold remains constant while below it threshold grows at a uniform rate. Many other types of threshold, such as upper disparity limits for depth perception and threshold amplitudes for stereo and monocular motion, show similar behavior. These data lead to the postulate that there are no foveal stereo channels peaking below 2.5 c/deg, so that foveal stimuli in the whole range below 2.5 c/deg are processed by a single channel tuned to this frequency. Consequently, disparity detection thresholds at frequencies below this frequency are controlled by the single parameter of effective contrast in the 2.5 c/deg channel, whose output depends jointly on the contrast and spatial frequency of the stimuli. We develop this idea to explain the relations between spatial and contrast tuning functions for disparity thresholds. To validate our conclusions, we describe an experiment with difference-of-Gaussian stimuli over a range of interocular widths and contrast differences. For a dichoptic width ratio of 2:1, the dichoptic contrast ratio required to minimize disparity detection thresholds was 1:4, just as predicted by the model.

Contrast Sensitivity↗
PubMed1993

Pairwise comparison technique: a simple solution for depth reconstruction.

A new technique dramatically simplifies the analysis of matching and depth reconstruction by extracting three-dimensional rigid depth interpretation from pairwise comparisons of weak perspective projections. This method provides a simple linear criterion for testing the correctness of correspondence for a pair of images; the method also provides a description of a one-parameter family of interpretations for each pair of images that satisfies this criterion. We show that if at least three projections of a volumetric object are known, then a three-dimensional (3D) rigid interpretation can be inferred from pairwise comparisons between any one of these images and other images in the set. The 3D interpretation is derived from the intersection of corresponding one-parameter families. The method provides a common computational basis for different processes of depth perception, for example, depth-from-stereo and depth-from-motion. Thus, a single mechanism for these processes in the human visual system would be sufficient. The proposed method does not require information about relative positions of eye(s) or camera(s) for different projections, but this information can be easily incorporated. The method can be applied for pairwise comparison within a single image. If any nontrivial correspondence is found, then several views of the same object are present in the same image. This happens, for example, in views of volumetrically symmetric objects. Symmetry facilitates depth reconstruction; if an object possesses two or more symmetries, its depth can be reconstructed from a single image.

Algorithms↗