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Biomedical subjects

Y Y Zeevi

Publications and source records attributed to Y Y Zeevi.

At least 19 recordsLinked to original sources
PubMed1989

Localized texture processing in vision: analysis and synthesis in the Gaborian space.

Recent studies of cortical simple cell function suggest that the primitives of image representation in vision have a wavelet form similar to Gabor elementary functions (EF's). It is shown that textures and fully-textured images can be practically decomposed into, and synthesized from, a finite set of EF's. Textured-images can be synthesized from a set of EF's using image coefficient library. Alternatively, texturing of contoured (cartoon-like) images is analogous to adding chromaticity information to contoured images. A method for texture discrimination and image segmentation using local features based on the Gabor approach is introduced. Features related to the EF's parameters provide efficient means for texture discrimination and classification. This method is invariant under rotation and translation. The performance of the classification appears to be robust with respect to noisy conditions. The results show an insensitivity of the discrimination to relatively high noise levels, comparable to the performances of the human observer.

Artificial Intelligence↗
PubMed1989

The role of stimulus structure in spatial hyperacuity.

In spatial hyperacuity the subjects discriminate a stimulus feature relative to a reference, with an accuracy significantly better than the grain of the retinal mosaic. We show that the normalized thresholds have a dichotomous behavior; they are either insensitive to the spatial parameter in the experiment or increase very steeply with it. This behavior is explained by the involvement in the processing of pixel (receptor) accuracy information about the structure of the stimulus. A computational model employing optimal filtering reproduces the experimental data and suggests that processing of spatial hyperacuity tasks in the human visual system is optimal.

Computer Simulation↗
PubMed1988

Preferences and asymmetries in saccadic responses to delayed bihemifield stimuli.

Saccaadic response preferences and latencies were measured using dual targets presented at 5, 10 and 15 deg along the horizontal meridian in either one or both visual hemifields. In the unihemifield condition, subjects exhibited a strong preference (about 95%) in favor of the target presented closer to the original fixation point. Further, the presence of the second target did not increase the response latency to the preferred target. In the bihemifield condition using spatially symmetric targets, directional preference varied across subjects from 57 to 100% (mean = 83%). There was also a significant increase in response latency as compared with responses to either single targets or dual targets presented in one hemifield. This latency increase was observed for responses in both the preferred and nonpreferred directions and was taken as evidence of mutual interaction between the two hemispheres. The response preferences could be eliminated by delaying the onset of the preferred target by about 100 msec in the unihemifield condition and by about 50 msec in the bihemifield condition. Further, the interactions between the effects of the targets on response time in the bihemifield condition were found to be asymmetrical in that a target presented in the preferred hemifield has a greater effect than a target in the nonpreferred hemifield on responses to targets presented in the counterhemifield. The results suggest an asymmetry in hemispheric interaction which has not previously been demonstrated in the visualoculomotor system.

Adult↗
PubMed1987

Changes in the oscillatory potentials of the electroretinogram in glaucoma.

Dark adapted and progressively light adapted electroretinograms (ERGs) were recorded from 34 normals and from 45 glaucoma patients. To enhance the oscillatory potentials (OP) the ERGs were highpass-filtered. The OP were characterized by two indices: their root-mean-square value and the inter-flash interval for which the maximal amplitude was obtained. In most of the glaucomatous OP, 50 of 81 eyes, both indices were abnormal, in 22 one parameter was abnormal and only in 9 were both parameters normal. In 13 of the 14 OP recorded from the opposite ('normal') eye of patients with unilateral glaucoma either one or both of the indices were abnormal. These findings indicate that the damage to the retina in glaucoma may extend more distally than the ganglion cell layer and that subclinical changes might be revealed by OP changes.

Adaptation, Physiological↗
PubMed1986

Interhemispheric coordination is compromised in subjects with developmental dyslexia.

Developmental dyslexics (DDs) and good readers (GRs) were tested on measures of interhemispheric coordination. All subjects (ages 16 to 47) demonstrated normal oculomotor control and visual acuity, prior to testing. Subjects were instructed to track three different point-light source patterns (single stimulus in one hemifield, dual stimuli in one hemifield and a pair of simultaneous, symmetric, bihemifield stimuli [SSBS]), presented in random sequence and arrayed horizontally at +/- 5, +/- 10, and +/- 15 degrees of eccentricity. Tested with unihemifield stimuli, all subjects showed normal saccadic latencies and trajectories. In response to SSBS, all GRs had pronounced directional preference, choosing largely to track one side over the other. In contrast, DDs showed reduced laterality bias (p less than .025). DDs exhibited also significantly longer response latencies to SSBS than to unihemifield stimulation (p less than .01) and differed significantly from GRs (p less than .05).

Adolescent↗
PubMed1986

Importance of global information in vernier acuity.

Noisy vernier configurations were generated by normally distributing the lateral displacements of dot elements forming vertical dotted lines. Observations with noisy configurations were compared with thresholds obtained with noise-free lines. In configurations with gap, we have found, through independent perturbation of the end regions adjacent to the gap, that the entire length of the line was actively processed. In an experiment with noisy abutting lines, the threshold as a function of line length exhibited only a slight upward shift relative to the curve obtained with noise-free configurations. Since it is not possible to interpret all the results through a comparison of local positional information extracted from the stimulus configuration, we conclude that global information is also involved in the processing.

Discrimination, Psychological↗
PubMed1985

Context dependent processing in spatial hyperacuity.

The importance of comparison processes in spatial hyperacuity tasks was investigated by means of a new relative nonalignment discrimination task. Vernier-type configuations nonaligned to either left or right, were presented in succession, and the subject had to decide which configuation exhibited a larger nonalignment. The relative nonalignment thresholds were found to be more strongly dependent on the spatial arrangement of the two lines than the thresholds corresponding to "ordinary" spatial hyperacuity performances. This context dependency is attributed to the active role played in the processing by comparison mechanisms.

Discrimination, Psychological↗
PubMed1984

Vernier acuity with noisy lines: estimation of relative position uncertainty.

Vernier acuity experiments were carried out to obtain estimates of the minimal number of spatial samples extracted, and of the equivalent noise figure characterizing inherent limitations of the visual system in relative position detection tasks. Results are interpreted by modelling the visual system as an ideal detector limited by external noise and internal position uncertainty resulting from structural limitations and neural noise. Accordingly, we estimate that less than ten samples per line are extracted by the observer and that the equivalent noise figure or position uncertainty is of the order of 20 arc s.

Humans↗
PubMed1984

Smooth eye-movement control with secondary visual feedback.

A signal derived from continuous measurement of eye position is displayed on a visual frame of reference, thereby closing a secondary visual feedback (2VFB) loop. The distance between a target and the displayed gaze point provides the subject with an extra, artificial position error. Experiments show that subjects can use the 2VFB to generate smooth eye movements, even in the absence of any smoothly moving independent targets. Under these conditions, both direction and velocity can be brought under voluntary control by the subjects. As a control signal for the smooth-eye-movement mode, the 2VFB is robust and easily manipulated and provides an attractive means for the investigation of the smooth-movement control system in a variety of tasks and under different conditions.

Eye Movements↗
PubMed1979

A model for processing of movement in the visual system.

Processing of spatio-temporal information in the human visual system has been investigated thoroughly during the past decade, but is still far from being properly understood. Moreover, the theory of separation of information by means of sustained and transient channels already at the retinal level is not satisfactory, as experimental results indicate that these two types of channels span a continuum of temporal characteristics. It is however obvious, that the process of pattern recognition and velocity perception calls for their separation at some level of the hierarchy. In this communication, we extend our model of three-dimensional spatio-temporal frequency expansion in the visual system (Gafni and Zeevi, 1977) to show how velocity-information extraction channels, sensitive to direction and velocity exclusively, can be formed by simple summation of signals from well-defined sets of channels representing points in the frequency space. Correspondence of these channels to characteristics of the cortical neurons is discussed.

Depth Perception↗
PubMed1979

Analysis of "integrate-to-threshold" neural coding schemes.

Methods of analysis for some deterministic and stochastic variants of the integrate-to-threshold neural coding scheme are presented. Adaptation phenomena are modeled by means of feedforward and feedback adaptive threshold control. Simulations of sinusoidal and step responses reproduce satisfactorily the qualitative characteristics of adaptation as compared with physiological data. It is postulated that such adaptive threshold control may be accomplished by the release, or conformation change, of molecules involved in the control of excitable-channel dynamics.

Action Potentials↗
PubMed1979

Study of eccentric fixation with secondary visual feedback.

Secondary visual feedback (2VFB) is a visual signal derived from continuous measurement of eye position and provides an extra artificial indication of the point of gaze. 2VFB may be eccentrically displaced and subjects are able to visually superimpose 2VFB onto a visual target signal and thus achieve and maintain eccentric fixation. Initial transient patterns of movement depend upon training but even naive subjects can achieve eccentric fixation within the first 40 s of such a task. Individual strategies and idiosyncratic patterns are exaggerations of normal control and fixational eye movements. The variance of maintained fixation increases with eccentricity and appears to be related to visual acuity as well as to precision of ocular motor control.

Biofeedback, Psychology↗
PubMed1979

Latency of peripheral saccades.

Displaying the point of gaze to the observer in addition to a point target provides a secondary visual feedback (2VFB). Eccentric fixation is achieved using a biased 2VFB to yield an experimentally imposed "eccentric fovea." The target is suddenly moved to a new position and the task is to regain it, in the "eccentric fovea". It is found that the pattern of eye-movement response consistently starts with saccadric foveal exploration of the target, but its latency has twice the duration of a regular voluntary saccade. Practice, however, makes for the shortened latency tending asymptotically to the regular saccadic duration.

Eye Movements↗
PubMed1979

Response of visual mechanisms to stimulus onsets and offsets.

Transient and sustained visual mechanisms were studied with single, flickering bars of various widths. Wide bars were largely detected on the basis of temporal luminance transients whereas thin bars were detected on the basis of the sustained contrast. A rapidly flickering uniform field selectively masked wide flickering bars, which suggests that different mechanisms detect wide versus thin flickering bars. For coarse spatial patterns, stimulus onsets were slightly more visible than stimulus offsets, and the response to onsets and offsets approximately summated.

Darkness↗
PubMed1978

Noise suppression in photoreceptors and its relevance to incremental intensity thresholds.

Quantal noise is the main limiting factor of visual contrast sensitivity only over an intermediate range of light intensities. At low and high intensities receptor and neural noise determine the bound on sensitivity. In this paper we analyze the effect of receptor nonlinearity on the statistics of visual signals and show its role in the suppression of quantal and receptor noise at high light intensities. This provides adequate physical interpretation of physiological data. Extending this analysis to the behavior of the whole visual system, we arrive at the conclusion that such noise suppression provides a unified explanation of psychophysical incremental threshold data over the whole visual range.

Animals↗