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L L Kontsevich

Publications and source records attributed to L L Kontsevich.

7 recordsLinked to original sources
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↗