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Y Yeshurun

Publications and source records attributed to Y Yeshurun.

13 recordsLinked to original sources
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↗
PubMed1993

Examining the volume efficiency of the cortical architecture in a multi-processor network model.

The convoluted form of the sheet-like mammalian cortex naturally raises the question whether there is a simple geometrical reason for the prevalence of cortical architecture in the brains of higher vertebrates. Addressing this question, we present a formal analysis of the volume occupied by a massively connected network or processors (neurons) and then consider the pertaining cortical data. Three gross macroscopic features of cortical organization are examined: the segregation of white and gray matter, the circumferential organization of the gray matter around the white matter, and the folded cortical structure. Our results testify to the efficiency of cortical architecture.

Animals↗
PubMed1989

Prediction of linear and non-linear responses of MGB neurons by system identification methods.

In sensory physiology, various System Identification methods are implemented to formalize stimulus-response relationships. We applied the Volterra approach for characterizing input-output relationships of cells in the medial geniculate body (MGB) of an awake squirrel monkey. Intraspecific communication calls comprised the inputs and the corresponding cellular evoked responses--the outputs. A set of vocalization was used to calculate the kernels of the transformation, and these kernels subserved to predict the responses of the cell to a different set of vocalizations. It was found that it is possible to predict the response (PSTH) of MGB cells to natural vocalizations, based on envelopes of the spectral components of the vocalization. Some of the responses could be predicted by assuming a linear transformation function, whereas other responses could be predicted by non-linear (second order) kernels. These two modes of transformation, which are also reflected by a distinct spatial distribution of the linear vis-à-vis non-linear responding cells, apparently represent a new revelation of parallel processing of auditory information.

Acoustic Stimulation↗
PubMed1987

Identification of MGB cells by volterra kernels. III. A glance into the black box.

Neuronal systems can be described by their transfer functions, which can be represented by a Volterra series expansion. While the high level of abstraction which characterizes this representation enables a global description, it is problematic, to some extent, in the context of linking the formal representation of the system to its actual structure. The formal representation is unique, yet there are multiple physical realizations of this representation. Separating the system's output into its logical components (linear, cross-linear, and self nonlinear, in this study), and inspecting the relative contribution of these components, might provide a key towards a linkage between the formal and actual representations. Based on results drawn from identification of MGB cells of the squirrel monkey, it is shown that the relative contributions can be described in neurobiological terms such as excitation and inhibition and thus be attributed to actual subsystems.

Animals↗
PubMed1985

Functional organization of the medial geniculate body's subdivisions of the awake squirrel monkey.

The response properties of 138 cells in the medial geniculate body (MGB), of the awake squirrel monkey (Saimiri sciureus), to 7 species-specific vocalizations were studied. Cells were divided into 4 subgroups: 26 in the ventral, 24 in the medial and 46 in the lateral subdivision. Forty-two cells located on the borders between the subdivisions represent the fourth group. No significant differences were found between the subdivisions with respect to their selectivity, nor did cells in any subdivision respond preferentially to any particular vocalization. On the other hand, the response patterns of the ventral and the lateral subdivisions showed significant differences (P less than 0.001, chi 2-test) from those of the medial subdivision. Most of the cells in the medial subdivision (87.5%) responded with similar response pattern to the 7 vocalizations (mainly 'on' or 'sustain'), while most of the cells in the ventral and the lateral subdivisions (61.5% and 69.6% respectively) responded with complex, time-locked and different patterns to the various vocalizations. Cells that exhibited a response characterized as an intermediate between the two types were accumulated mainly on or close to the borders between the medial and the other subdivisions of the MGB. The possible role of each response patterns is discussed with respect to the projection of the subdivisions to the cortex.

Animals↗
PubMed1985

Identification of MGB cells by Volterra kernels. I. Prediction of responses to species specific vocalizations.

The function of CNS sites is frequently explored by an analysis of its input-output relationships. However, such research are often confined to a qualitative and subjective inspection of raw data. System Identification methods can be used to formalize the stimulus--response relations, and one of them, the Volterra approach, is employed here in order to define these relations in the MGB of the squirrel monkey, natural vocalizations being the stimuli. In order to validate the formal representation of the system under study, the predictability power of the model is tested. Having the distances between responses (PSTH) and predicted response quantified, it is found that the predictions made by the model are, in general, "closer" to the actual responses then some arbitrarily chosen responses. It is concluded that there are cells in the MGB that can be characterized by their Volterra kernels, and further research on the cell's functional role can be based on these kernels.

Acoustic Stimulation↗
PubMed1981

Responses of single cells in the medial geniculate body of awake squirrel monkeys.

Response properties of 142 medial geniculate (MGB) cells were investigated in the awake and undrugged squirrel monkey (Saimiri sciureus). Using Jordan's (1973) parcellation of this complex nucleus, cells were assigned to 3 major subdivisions a, b and c MGB and compared for their general characteristics and response properties. b MBG cells had significantly higher rates of spontaneous firing and longer latency periods than a and c MGB cells. With regard to responsiveness to various auditory stimuli, response patterns, and tuning characteristics, cells in all 3 subdivisions were statistically similar and were thus treated as one cell population. About 95% of the cells responded to broadband white noise, steady tone bursts and frequency modulated (FM) tones. Click activated only 69% of the responding cells. Various "through-stimulus" responses comprised about 80% of the responses. Among the tone-sensitive cells, 90% responded with complex patterns, out of which 50% were frequency-dependent. About 62% of the cells (for which tuning properties were determined) were quite broadly tuned (Q10dB less than 2) and had either single or multi-peaked response areas. The other 38% were quite narrowly tuned (Q10dB greater than 2) and had single-peaked, symmetrical or "tailed" response areas. Different inhibitory and excitatory response components of individual cells had different characteristic frequencies and response thresholds. The c MGB, which is tonotopically organized in a latero-medial orientation, appears to be homologous to the cat pars lateralis of the ventral MGB. The tonotopical organization of the b MGB, which is probably homologous to the cat's medial or magnocellular subdivision, is less clear. Most of the cells which were activated by FM tones disclosed "direction sensitivity" with different degrees of pattern complexity. It is suggested that pitch resolution in the MGB is based on spatio-temporal mechanisms.

Acoustic Stimulation↗
PubMed1979

Analysis of recall and recognition in a certain class of adaptive networks.

A general model of adaptive networks which perform recall is analyzed in view of qualitative psychological findings. The assumptions underlying the basic model are few and general in the sense that no specifications of structure or mechanisms of adaptation are imposed. The analysis of the model is towards the addition of various features drawn from the global input-output relations expected from the networks. The memorizing process of recognition is found to be intrinsic to the model, and four of the most prominent relations between the performance of recall and recognition are shown to either exist in model or to be realizable by means of few additional plausible features.

Association Learning↗