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Biomedical subjects

Y Yeshurun

Publications and source records attributed to Y Yeshurun.

At least 19 recordsLinked to original sources
PubMed2001

Superresolution in MRI: application to human white matter fiber tract visualization by diffusion tensor imaging.

A superresolution algorithm was applied to spatially shifted, single-shot, diffusion-weighted brain images to generate a new image with increased spatial resolution. Detailed two-dimensional white matter fiber tract maps of the human brain resulting from application of the technique are shown. The method provides a new means for improving the resolution in cases where k-space segmentation is difficult to implement. Diffusion-weighted imaging and diffusion tensor imaging in vivo stand to benefit in particular because the necessity of obtaining high-resolution scans is matched by the difficulty in obtaining them. Magn Reson Med 45:29-35, 2001.

Algorithms↗
PubMed2000

Transient vortex states in Bi2Sr2CaCu2O(8+delta) crystals

A high temporal resolution magneto-optical system is employed to observe the time evolution of the vortex structure in Bi(2)Sr(2)CaCu(2)O(8+delta) crystals after a sudden application of a magnetic field. The magneto-optical images reveal dynamic coexistence of two vortex phases: a quasiordered phase in the sample interior and a transient disordered phase near the sample edges. The border between these two phases, marked by an abrupt change in the gradient of the local induction, moves with time. This motion enables tracing the decay of the transient state and the concurrent growth of the thermodynamic vortex phases. The growth rate is sensitive to the location in the field-temperature phase diagram.

Journal Article↗
PubMed2000

The locus of attentional effects in texture segmentation.

Cuing covert spatial attention can increase spatial resolution. Here we pinpointed the specific locus of this effect using texture segmentation. At the level of visual cortex, texture segmentation theoretically involves passage of visual input through two layers of spatial linear filters separated by a pointwise nonlinearity. By manipulating the textures to differentially stimulate first- or second-order filters of various scales, we showed that the attentional effect consistently varied with the latter. These psychophysical results further support the hypothesis that attention increases resolution at the attended location and are consistent with an effect of attention at stages as early as the primary visual cortex.

Attention↗
PubMed1999

Cortical hypercolumn size determines stereo fusion limits.

The size of a pair of cortical ocular dominance columns determines a basic anatomical module of V-1 which Hubel and Wiesel have termed the hypercolumn. Does this correspond to a basic functional, or psychophysically measurable, module as well? This is the basic question addressed in the present paper. Since the ocular dominance column architecture is presumed to be related to stereo vision, it is natural to assume that hypercolumn size should provide a modular basis for basic phenomena of stereopsis. In previous work, we have suggested that local nonlinear filtering via the cepstral transform, operating on a local window of cortical tissue scaled by hypercolumn size, provides such a modular model of stereopsis. In the present paper, we review this model and then discuss a number of issues related to the biological plausibility and implementation of this algorithm. Then, we present the main result of this paper: we have analyzed a number of experiments related to stereo fusion limits (Panum's area) and to disparity gradient and disparity scaling, and demonstrate that there is a simple unifying explanation for these phenomena in terms of a constant cortical module whose size is determined by a pair of ocular dominance columns. As a corollary, Panum's area must increase according to (inverse) cortical magnification factor. We show that this is supported by all existing experimental data.

Animals↗
PubMed1999

Spatial attention improves performance in spatial resolution tasks.

This study used peripheral precueing to explore the effect of covert transient attention on performance in spatial resolution tasks. Experiments 1 (Landolt-square) and 2 ('broken-line') measured gap resolution and Experiment 3 measured vernier resolution. In all three tasks the target was presented alone in a large number of possible locations, ranging from 1.5-6 degrees of eccentricity in the vertical or horizontal axes. The precue indicated the target location but did not convey information regarding the correct response. Performance decreased as the gap size or the vernier offset size decreased and as target eccentricity increased. Precueing improved performance in terms of RT and accuracy in all three tasks; the eccentricity effect decreased in the cued trials of the gap resolution tasks. These findings support the idea that the performance improvement at attended locations results, to some extent, from an enhanced spatial resolution at the cued location, and not just from distractor exclusion, diminished uncertainty, or decisional factors.

Attention↗
PubMed1998

Attention improves or impairs visual performance by enhancing spatial resolution.

Covert attention, the selective processing of visual information at a given location in the absence of eye movements, improves performance in several tasks, such as visual search and detection of luminance and vernier targets. An important unsettled issue is whether this improvement is due to a reduction in noise (internal or external), a change in decisional criteria, or signal enhancement. Here we show that attention can affect performance by signal enhancement. For a texture segregation task in which performance is actually diminished when spatial resolution is too high, we observed that attention improved performance at peripheral locations where spatial resolution was too low, but impaired performance at central locations where spatial resolution was too high. The counterintuitive impairment of performance that we found at the central retinal locations appears to have only one possible explanation: attention enhances spatial resolution.

Attention↗
PubMed1998

The contribution of covert attention to the set-size and eccentricity effects in visual search.

To reexamine the role of covert attention in visual search, the authors directly manipulated attention by peripherally cueing the target location and analyzed its effects on the set-size and the eccentricity effects. Observers participated in feature and conjunction tasks. Experiment 1 used precues, and Experiment 2 used postcues in a yes-no task under valid-, invalid-, and neutral-cueing conditions. Experiments 3 and 4 used a 2-interval alternative forced-choice visual-search task under cued and neutral conditions. Precueing the target location improved performance in feature and conjunction searches; postcueing did not. For the cued targets, the eccentricity effect for features and conjunctions was diminished, suggesting that the attentional mechanism improves the quality of the sensory representation of the attended location. The conjunction set-size effect was reduced but not eliminated. This questions serial-search models that attribute a major role to covert attention in visual search.

Adult↗
PubMed1998

Spatial size limits in stereoscopic vision.

Stereoscopic vision is extremely precise in detecting minute differences between adjacent depth planes, but quite imprecise in estimating absolute depth. In this paper, we address the issue of the spatial acuity (and not the stereo acuity) of stereopsis. Static RDS (random dot stereograms) stimuli were used to find the spatial grain in which human stereoscopic vision operates. Using psychophysical experiments it was found that foveally, stimuli smaller than 8' cannot be accurately perceived. For other eccentricities, it was found that this threshold is inversely proportional to the Cortical Magnification factor. We interpret this spatial size limit, which is an order of magnitude larger than visual spatial acuity, as an indication that stereopsis is an area based comparison rather than a point process, and discuss the relations between the cortical 'patch' size that corresponds to this 8' limit and Ocular Dominance Columns.

Computer Simulation↗
PubMed1996

Bilateral comparison of occipital lobe sulci: a sulcus identifying algorithm.

A procedure for automatic identification of sulci of cerebral hemispheres in magnetic resonance images is presented. The algorithm departs from information of the spatial location of a given feature (e.g., sulcal contour) in one hemisphere and automatically defines its course in the other hemisphere. The automatic tracing of sulci was successfully performed in the mesial aspect of both hemispheres in 18 normal individuals. The sulcal location in the target hemisphere was first approximated by assuming an affine transformation between hemispheres, and then refined by local edge analysis. The method produced reliable results in comparing the intersulcal areas (the cuneus), sulcal length, their complexity, and the angle between the pariteooccipital and retrocalcarine fissures.

Adolescent↗
PubMed1995

Spatial-gradient limit on perception of multiple motion.

Motion is perceived whenever a subject is presented with an appropriate spatiotemporal visual pattern. Like many other visual tasks, motion perception involves both local and global processing, and thus might be subject to the well-known paradox that arises from the fact that local features and observations form the basis for global perception, but sometimes this global percept can not be easily derived from any single local observation, as is best exemplified by the aperture problem. Globally, dual (transparent) motion can be readily perceived. Spatial limits on the local ability to perceive multiple motion are sought. By using the framework of apparent motion, it is found that dual, orthogonally oriented motion can be perceived only when the dots that constitute the two motions are separated by some spatial limit. For short-range apparent motion, the limit is found to be comparable to D(max), and the visual system cannot perceive more than a single coherent motion in a local "patch" of radius D(max). It was also found that this spatial limit on local-motion perception is not constant, but depends linearly on the spatial organisation of the stimuli, and vanishes for stimuli having reverse contrast. The lower bound on the ability to perceive multiple motion is compared with some well-known bounds in stereopsis, and a cortical columnar architecture that might account for it is proposed.

Adult↗
PubMed1994

Quantification of local symmetry: application to texture discrimination.

Symmetry is one of the most prominent cues in visual perception as well as in computer vision. We have recently presented a Generalized Symmetry Transform that receives as input an edge map, and outputs a symmetry map, where every point marks the intensity and orientation of the local generalized symmetry. In the context of computer vision, this map emphasizes points of high symmetry, which, in turn, are used to detect regions of interest for active vision systems. Many psychophysical experiments in texture discrimination use images that consist of various micro-patterns. Since the Generalized Symmetry Transform captures local spatial relations between image edges, it has been used here to predict human performance in discrimination tasks. Applying the transform to micro-patterns in some well-studied quantitative experiments of human texture discrimination, it is shown that symmetry, as characterized by the present computational scheme, can account for most of them.

Form Perception↗