Biomedical subjects

Vanitha Sampath

Publications and source records attributed to Vanitha Sampath.

6 recordsLinked to original sources

PubMed2005

Experience-expectant development of contour integration mechanisms in human visual cortex.

Extended contours are a common feature of natural images. Most previous studies have considered contour integration as a two-dimensional process of linking like-oriented elements along their common orientation axis. Yet contours exist in a three-dimensional world, and one might therefore ask about the relationship between contour integration and binocular vision. Using an event-related potential assay of contour integration, we demonstrate that patients with strabismic amblyopia show a relative insensitivity to Gabor-defined contours in their dominant eyes, all of which had normal acuity. These deficits were not seen in the dominant eyes of patients with anisometropic amblyopia without strabismus, but were present in the amblyopic eyes of patients with either strabismus or anisometropia. Deficits were also found in both eyes of strabismus patients who had normal visual acuity in each eye, but who had strongly reduced or absent stereopsis. These results suggest that the maturation of contour detection mechanisms depends at least in part on the presence of normal binocular interaction during a developmental critical period.

Amblyopia↗

PubMed2005

Development of sensitivity to texture and contour information in the human infant.

Texture discrimination and bounding contour extraction are essential parts of the object segmentation and shape discrimination process. As such, successful texture and contour processing are key components underlying the development of the perception of both objects and surfaces. By recording visual-evoked potentials, we investigate whether young infants can detect orientation-defined textures and contours. We measured responses to an organized texture comprised of many Gabor patches of the same orientation, alternated with images containing the same number of patches, but all of random orientation. These responses were compared with a control condition consisting of the alternation between two independently random configurations. Significant difference potentials were found as early as 2-5 months, as were significant odd harmonics in the test conditions. Responses were also measured to Gabor patches organized either as circles (all patches tangent to an imaginary circular path) alternated with pinwheels (all patches having a fixed orientation offset from the path). Infants between 6 and 13 months also showed sensitivity to the global organization of the elements along contours. Differential responses to our texture and contour stimuli and their controls could only have been generated by mechanisms that are capable of comparing the relative orientation of 2 or more patches, as no local information at a single patch distinguished the random and organized textures or the circle and pinwheel configurations.

Adult↗

PubMed2003

Development of the spatial organization and dynamics of lateral interactions in the human visual system.

Psychophysical thresholds and neuronal responses for isolated stimuli are strongly modified by nearby stimuli in the visual field. We studied the orientation and position specificity of these contextual interactions using a dual-frequency visual-evoked potential technique in developing human infants and adults. One set of small, oriented stimulus elements (targets) was tagged with a temporal frequency f1 of 4.52 Hz. The addition of an abutting second set of similar patches (flankers) tagged at f2 = 2.58 Hz had three effects: (1) The flankers reduced the second and fourth harmonic responses to the targets. This reduction was independent of flanker orientation or position and age. (2) The response to the combination of targets and flankers also contained nonlinear interaction terms (1f1 +/- 1f2) that were tuned for flanker orientation and position in adults, but only for flanker orientation in infants 8-31 weeks of age. (3) Nonlinear interaction terms recorded at 2f1 +/- 2f2 were large and untuned for flanker orientation and position in adults but were nearly absent in the youngest infants. The three forms of nonlinear interaction, thus, have differences in sensitivity to flanker orientation and position and differential growth trends, indicating that they are generated by different mechanisms. These three forms of interaction could serve different functional roles. The first process provides a nonselective gain control that is fully functional in early infancy. The second process, which develops slowly, is selective for the specific form of the stimuli. The third process, which is also immature, pools across orientation.

Adolescent↗

PubMed2003

Stereoscopic depth perception from oblique phase disparities.

In order to understand the role of oblique retinal image disparities in the perception of stereoscopic depth, we measured the depth perceived from random dot stereograms in which phase disparities were introduced in a selected band of stimulus orientations. A band of orientation was defined by a center orientation that ranged from 7.5 (near vertical) to 82.5 o[rientation]deg and by a bandwidth that was defined as the difference between the highest and the lowest orientation in the band. The bandwidths tested were 15, 30 and 45 odeg. A constant phase disparity of 90 p[hase]deg was introduced in all of the oriented spatial frequency components within the orientation band and the perceived depth of each stimulus was matched using a small square binocular probe. For each bandwidth, perceived depth increased with an increase in the center orientation up to approximately 60 odeg. This suggests that the human stereovision system derives a large proportion of information about perceived stereoscopic depth from oblique phase disparities. Simulations using an energy model of stereoscopic depth perception indicate that oblique phase disparities are unlikely to be processed by neural mechanisms tuned to near-vertical orientations within the stimulus. Our results therefore suggest that oblique retinal disparities are initially detected as oblique phase disparities by binocular mechanisms tuned to oblique orientations. Because the perceived depth from oblique phase disparities is consistent with the trigonometrically determined equivalent horizontal disparities, we presume that the information from oblique phase disparities is included in the visual system's computation of the horizontal retinal disparity.

Analysis of Variance↗

PubMed2003

Near stereothresholds measured with random-dot stereograms using phase disparities.

BACKGROUND: Clinically, stereothresholds for random-dot (RD) stimuli are measured at near with a typical resolution of 20- to 40-seconds arc. In this article, we describe a method by which stereothresholds are measured using RD stimuli on a conventional computer monitor with sub-picture-element spatial resolution. METHODS: The RD stimuli consisted of individual left and right eye images, viewed haploscopically from 50 cm though orthogonal polarizers. Cross and uncrossed horizontal disparities as small as 6-seconds arc were produced by introducing appropriate phase disparities within the individual spatial frequency components of the RD stimulus. The method of constant stimuli was used to determine the stereothresholds for 20 normal adult observers. RESULTS: The mean stereothreshold across the 20 observers was 24.1 +/- 16.6-seconds arc, with an average trial-to-trial variability of +/- 23%. CONCLUSIONS: Stereothresholds of a few-second arc can be measured accurately from a near distance for RD stimuli, using a conventional computer monitor. A clinical test based on this technique would allow the measurement of global stereothresholds with very high spatial resolution.

Adolescent↗

PubMed2002

Distribution of refractive errors in albinos and persons with idiopathic congenital nystagmus.

We compared retrospectively the distribution of refractive errors in a sample of adolescent and adult albinos (n = 19) with that in persons with idiopathic congenital nystagmus (CN) (n = 46), whose eye movements are similar to those of albinos but whose visual acuity is better. The distribution of spherical-equivalent refractive errors is more broadly distributed and slightly less myopic in albinos than in persons with idiopathic CN. On average, albinos also have more astigmatism (primarily with-the-rule), than persons with idiopathic CN. Unlike the leptokurtic distribution of refractive error that characterizes the normal adolescent and adult population, the distributions of refractive error for albinos and for persons with idiopathic CN exhibit no significant kurtosis. Moreover, neither group of subjects exhibits significant kurtosis for refractive errors in the vertical meridian, which corresponds to the retinal-image orientation with the least motion smear during horizontal nystagmus. The absence of significant leptokurtosis in the refractive-error distributions of young-adult albinos and persons with idiopathic CN suggests that the presence of nystagmus may interfere with normal refractive development.

Adolescent↗