Biomedical subjects

Harold E Bedell

Publications and source records attributed to Harold E Bedell.

9 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

Velocity dependence of Vernier and letter acuity for band-pass filtered moving stimuli.

The ability to see fine detail diminishes when the target of interest moves at a speed greater than a few deg/s. The purpose of this study was to identify fundamental limitations on spatial acuity that result from image motion. Discrimination of Vernier offset was measured for a pair of vertical abutting lines and letter resolution was measured using a four-orientation letter 'T'. These stimuli were digitally filtered using one of five band-pass (bandwidth=1.5 octaves) filters with a center frequency between 0.83 and 13.2 c/deg, and presented at velocities that ranged from 0 to 12 deg/s. Filtered and unfiltered stimuli were presented for 150 ms at a constant multiple (4x or 2x) of the contrast-detection threshold at each velocity. For stimuli of low to middle spatial frequency (up to 3.3 c/deg), Vernier and letter acuity for equally detectable targets are essentially unaffected by velocity up to 12 deg/s, i.e., for temporal frequencies of motion (velocity x spatial frequency) up to approximately 50 Hz. For stimuli of higher spatial frequency, acuity remains essentially constant until the velocity corresponds to a temporal frequency of about 30 Hz, and increases thereafter. Both Vernier and letter acuities worsen by approximately a factor of two for each one-octave decrease in filter spatial frequency. Both types of acuities worsen also as the contrast of the stimulus is reduced, but Vernier discrimination exhibits a stronger contrast-dependence than letter resolution. Our results support previous suggestions that a shift in the spatial scale used by the visual system to analyze spatial stimuli is principally responsible for the degradation of acuity in the presence of image motion. The results are consistent with a spatio-temporal-frequency limitation on spatial thresholds for moving stimuli, and not with a temporal-frequency limitation per se.

Discrimination, Psychological↗

PubMed2003

Color and motion: which is the tortoise and which is the hare?

Recent psychophysical studies have been interpreted to indicate that the perception of motion temporally either lags or is synchronous with the perception of color. These results appear to be at odds with neurophysiological data, which show that the average response-onset latency is shorter in the cortical areas responsible for motion (e.g., MT and MST) than for color processing (e.g., V4). The purpose of this study was to compare the perceptual asynchrony between motion and color on two psychophysical tasks. In the color correspondence task, observers indicated the predominant color of an 18 degrees x 18 degrees field of colored dots when they moved in a specific direction. On each trial, the dots periodically changed color from red to green and moved cyclically at 15, 30 or 60 deg/s in two directions separated by 180 degrees, 135 degrees, 90 degrees or 45 degrees. In the temporal order judgment task, observers indicated whether a change in color occurred before or after a change in motion, within a single cycle of the moving-dot stimulus. In the color correspondence task, we found that the perceptual asynchrony between color and motion depends on the difference in directions within the motion cycle, but does not depend on the dot velocity. In the temporal order judgment task, the perceptual asynchrony is substantially shorter than for the color correspondence task, and does not depend on the change in motion direction or the dot velocity. These findings suggest that it is inappropriate to interpret previous psychophysical results as evidence that motion perception generally lags color perception. We discuss our data in the context of a "two-stage sustained-transient" functional model for the processing of various perceptual attributes.

Analysis of Variance↗

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↗

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↗

PubMed2002

Effect of exposure duration, contrast and base blur on coding and discrimination of edges.

We extend a neural network model, developed to examine neural correlates for the dynamic synthesis of edges from luminance gradients (Oğmen, 1993), to account for the effects of exposure duration, base blur and contrast on the perceived sharpness of edges. This model of REtino-COrtical Dynamics (RECOD) predicts that (i) a decrease in exposure duration causes an increase in the perceived blur and the blur discrimination threshold for edges, (ii) this increase in perceived blur is more pronounced for sharper edges than for blurred edges, (iii) perceived blur is independent of contrast while the blur discrimination threshold decreases with contrast, (iv) perceived blur increases with increasing base blur while the blur discrimination threshold has a nonmonotonic U-shaped dependence on base blur, (v) the perceived location of an edge shifts progressively towards the low-luminance side of the edge with increasing contrast, and (vi) perceived contrast of suprathreshold stimuli is essentially independent of spatial frequency over a wide range of contrast values. These predictions are shown to be in quantitative agreement with existing psychophysical data from the literature and with data collected on three observers to quantify the effect of exposure duration on perceived blur.

Contrast Sensitivity↗