Biomedical subjects

Michael T Ukwade

Publications and source records attributed to Michael T Ukwade.

5 recordsLinked to original sources

PubMed2003

Stereopsis is perturbed by vergence error.

Fixation disparity or vergence noise produce instantaneous vergence errors. These errors are analogous to the imposition of a pedestal disparity, which is known to elevate stereothresholds. In this study, stereothresholds were measured as a function of induced vergence errors in subjects with normal binocular vision. Stereo half-images were viewed in the dark through a custom mirror haploscope. Vergence constant error and vergence variability were induced by introducing horizontal disconjugate position offsets in a pair of moveable mirrors within the haploscope, resulting in forced vergence demands of 6(Delta) base-in to 12(Delta) base-out. In addition, vergence variability was simulated by producing oscillatory disconjugate retinal image motion via motion of the moveable mirrors. The motion of the mirrors was either sinusoidal (frequency=2-6 Hz) or random, with peak-to-peak amplitudes of 0 degrees -0.5 degrees per eye. Stereopsis worsened systematically with induced or simulated VV that exceeded approximately 1.5' disparity. The results were similar regardless of whether the vergence error was induced by forced vergence or was simulated by periodic or random disconjugate retinal image motion. Stereothresholds were invariant with the frequency of disconjugate oscillation, within the range of frequencies and amplitudes tested. Hence, the simulated vergence velocity is not the essential factor that limits stereopsis within Panum's fusional area. The results indicate that the stereothreshold is elevated if the vergence error exceeds a critical value.

Adult↗

PubMed2003

Stereothresholds with simulated vergence variability and constant error.

Stereothresholds are elevated by vergence constant error (fixation disparity), vergence noise, or both. This study investigated the separate and combined effects of simulated vergence constant error and variability on stereothresholds in four normal observers. Targets were 30 arc min bright vertical lines presented separately to the two eyes for 150 ms in darkness. Vergence constant error, simulated as a pedestal disparity, was induced by altering the screen positions of the stereo half-images relative to a previously visible binocular fixation point. Vergence variability was simulated simultaneously by disconjugate motion (amplitude=0-0.5 deg per eye; frequency=2 or 4 Hz) of a pair of scanning mirrors in a Wheatstone stereoscope that was used to present the images to each eye. Various combinations of pedestal disparity and simulated vergence variability produce equivalent stereothresholds whenever the calculated mean deviation (sigma/instantaneous vergence errors//N) of the stimulus from the fixation plane is the same. In particular, stereothresholds are optimal for mean deviations up to approximately 1.4 arc min and then increase according to a power function with an exponent of 0.61. The results imply that vergence constant errors and vergence variability impair stereothresholds because of the resulting mean deviation from the horopter.

Adult↗

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

Orientation discrimination and variability of torsional eye position in congenital nystagmus.

Thresholds for discriminating the orientation of unreferenced horizontal and vertical lines were measured in subjects with congenital nystagmus (CN) and normal observers and compared to the variability of torsional eye position. Orientation thresholds were determined for horizontal and vertical lines between 0.7 degrees and 5.6 degrees in length, that were presented binocularly for 20-1280 ms. The variability of torsional eye position was assessed using the magnetic search coil technique. Orientation thresholds improved with line length and stimulus duration in both groups of observers. Some of the subjects with CN exhibited poorer than normal thresholds, particularly when the length of the line was short. In addition, orientation discrimination in the subjects with CN was consistently anisotropic, with significantly lower thresholds for horizontal than vertical lines. The standard deviations of torsional eye position were larger in the subjects with CN than in normal observers. However, orientation thresholds were poorer than expected from the variability of torsional eye position in normal observers, and better than expected on the basis of torsional variability in some of the subjects with CN. These results imply that torsional variability does not limit normal orientation thresholds and that torsional eye movements in CN are compensated partially by extraretinal signals.

Adult↗