Biomedical subjects

Harold E Bedell

Publications and source records attributed to Harold E Bedell.

At least 19 recordsLinked to original sources

PubMed2007

Spatial and temporal properties of the illusory motion-induced position shift for drifting stimuli.

The perceived position of a stationary Gaussian window of a Gabor target shifts in the direction of motion of the Gabor's carrier stimulus, implying the presence of interactions between the specialized visual areas that encode form, position, and motion. The purpose of this study was to examine the temporal and spatial properties of this illusory motion-induced position shift (MIPS). We measured the magnitude of the MIPS for a pair of horizontally separated (2 or 8deg) truncated-Gabor stimuli (carrier=1 or 4cpd sinusoidal grating, Gaussian envelope SD=18arc min, 50% contrast) or a pair of Gaussian-windowed random-texture patterns that drifted vertically in opposite directions. The magnitude of the MIPS was measured for drift speeds up to 16deg/s and for stimulus durations up to 453ms. The temporal properties of the MIPS depended on the drift speed. At low velocities, the magnitude of the MIPS increased monotonically with the stimulus duration. At higher velocities, the magnitude of the MIPS increased with duration initially, then decreased between approximately 45 and 75ms before rising to reach a steady-state value at longer durations. In general, the magnitude of the MIPS was larger when the truncated-Gabor or random-texture stimuli were more spatially separated, but was similar for the different types of carrier stimuli. Our results are consistent with a framework that suggests that perceived form is modulated dynamically during stimulus motion.

Distance Perception↗

PubMed2006

The attenuation of perceived motion smear during combined eye and head movements.

The extent of perceived motion smear was compared for targets that underwent similar velocities of retinal image motion during the vestibulo-ocular reflex (VOR) in the dark, the visually enhanced VOR (VVOR), VOR suppression, and fixation. Compared to the extent of perceived motion smear during fixation, observers reported significantly less smear when the target moved either in the same direction or against the direction of the head movement during the VVOR and VOR. We also confirmed a previous finding that perceived smear is attenuated asymmetrically during VOR suppression, with attenuation occurring primarily for targets that move against the direction of the observer's head motion. The results support the hypothesis that the visual system employs extra-retinal signals that accompany eye and head movements to reduce the perception of motion smear for targets that move physically in the opposite direction of eye and/or head movements.

Eye Movements↗

PubMed2006

Beating the beat: reading can be faster than the frequency of eye movements in persons with congenital nystagmus.

PURPOSE: Rapid serial visual presentation (RSVP) of text has been reported to foster higher reading rates than presentation in a continuous text (CT) format, possibly because scanning eye movements are minimized. We investigated how this might be relevant for persons with congenital nystagmus (CN). METHODS: We evaluated whether reading rates differ in persons with CN for RSVP versus CT presentation of single sentences under otherwise similar conditions. In a second experiment, we presented unrelated words to observers with CN in RSVP format while measuring their eye movements to determine whether reading can occur during the high-velocity, nonfoveating periods of the CN wave form. Both sentences and random words were selected from the MNRead corpus and displayed at 2x, 4x, or 8x the threshold word size on a 21-inch computer monitor. RESULTS: Subjects with CN have virtually equivalent maximum reading speeds of 449 and 448 words per minute, respectively, for RSVP and CT presentation of sentences. Typically, reading rates were faster than the frequency of CN, which suggests that subjects could read during the nonfoveating periods of the nystagmus waveform. This finding was confirmed using random words that, unlike those in sentences, cannot be inferred from contextual cues. Examination of eye movements recorded during reading indicated that random words are read correctly with 47% to 65% accuracy (depending on word size) during the nonfoveating periods of the CN waveform. CONCLUSION: A clinical implication of these results is that reading performance in persons with CN should be facilitated by large text sizes that remain legible during a greater fraction of the CN waveform.

Adolescent↗

PubMed2006

Distance and near visual acuity in infantile nystagmus.

PURPOSE: Infantile nystagmus (IN) has been reported to decrease with convergence. However, previous studies reported equivocal results regarding a corresponding improvement in acuity with near viewing. The aim of this study was to determine whether visual acuity improves with near viewing in patients with IN. METHODS: In the first experiment, visual acuities were measured using clinical test charts at standard test distances of 3 or 6 m and 40 cm and using S Charts at 3.75 m and 40 cm. In the second experiment, visual acuities were measured using a Bailey-Lovie chart at distance and a Lighthouse modified ETDRS near card held by each subject at his or her preferred working distance. S-chart acuities were obtained again at 3.75 m and 40 cm for comparison. Horizontal eye movements were recorded using infrared limbal reflection for 20 of the 34 subjects in the first experiment and for all 20 subjects in the second experiment. RESULTS: The S-chart acuities measured at distance and near were almost all within 0.1 logMAR (logarithm of the minimum angle of resolution) in experiments 1 and 2. Clinically measured acuity averaged nearly one line better at 40 cm than at distance in experiment 1, but the mean difference between near acuity using the ETDRS card and distance acuity using the Bailey-Lovie chart was less than one letter in experiment 2. No consistent relationship existed between the changes in visual acuity with viewing distance and the subject's eye movements. CONCLUSION: Despite a reduction of nystagmus at near distances in many patients with IN, the visual acuity at near does not improve significantly. These results imply that visual acuity in patients with IN is determined primarily by sensory limitations rather than by the moment-by-moment characteristics of these patients' eye movements.

Adolescent↗

PubMed2005

Pooling signals from vertically and non-vertically orientation-tuned disparity mechanisms in human stereopsis.

To understand the role that orientation-tuned disparity-sensitive mechanisms play in the perception of stereoscopic depth, we measured stereothresholds using two sets of random-dot stimuli that produce identical stimulation of disparity mechanisms tuned to vertical orientation but dissimilar stimulation of disparity mechanisms tuned to non-vertical orientations. Either 1 or 1.5D of astigmatic blur was simulated in the random-dot images presented to both eyes, using two axis configurations. In the parallel-axis conditions, the axis of simulated astigmatic blur was same in the two eyes (0, 45 or 135 o[rientation]deg). In the orthogonal-axis conditions, the axes of astigmatic blur were orthogonal in the two eyes (LE: 180, RE: 90; LE: 90, RE: 180; LE: 45, RE: 135; and LE: 135, RE: 45). Whereas the stimulation of disparity mechanisms tuned to near-vertical orientations should be similar in the oblique parallel- and orthogonal-axis conditions, the stimulation of non-vertically tuned disparity mechanisms should be dissimilar. Measured stereothresholds were higher in the orthogonal compared to the parallel-axis condition by factors of approximately 2 and 5, for 1 and 1.5D of simulated oblique astigmatic blur, respectively. Further, for comparable magnitudes of simulated astigmatic blur, stereothresholds in the (LE: 180, RE: 90 and LE: 90, RE: 180) conditions were similar to those in the (LE: 45, RE: 135 and LE: 135, RE: 45) conditions. These results suggest that the computation of horizontal disparity includes substantial contributions from disparity mechanisms tuned to non-vertical orientations. Simulations using a modified version of a disparity-energy model [Qian, N., & Zhu, Y. (1997). Physiological computation of binocular disparity. Vision Research, 37, 1811-1827], show (1) that pooling across disparity mechanisms tuned to vertical and non-vertical orientations is required to account for our data and (2) that this pooling can provide the spatial resolution needed to encode spatially changing horizontal disparities.

Depth Perception↗

PubMed2005

Stereothresholds for moving line stimuli for a range of velocities.

This study examined the influence of lateral target motion on the stereothresholds for bright vertical lines at a range of velocities. Stimuli were presented for 200 ms with horizontal velocities from 0 to 12 deg/s. Observers' horizontal eye movements were recorded on additional trials, and confirmed that the velocity of retinal image motion closely matched the velocity of the stimulus. In three auxiliary experiments, stereothresholds were measured (1) after equating the detectability of targets that moved at different velocities, (2) for moving and stationary stimuli with durations between 20 and 200 ms, and (3) for stationary stimuli presented at eccentricities of 0.6 and 1.2 deg. The results indicate that stereothresholds are unaffected by velocities up to approximately 2 deg/s, but worsen in proportion to the velocity at higher speeds. The results of our auxiliary experiments demonstrate that the increase in stereothresholds during image motion cannot be attributed primarily to a reduction in the detectability of the stimulus, a decrease in the effective exposure duration, or non-foveal viewing. We conclude that the elevation of stereo thresholds during lateral motion is consistent with a shift in the sensitivity of the visual system toward lower spatial frequencies as a result of motion blur.

Depth Perception↗

PubMed2005

Asymmetry of perceived motion smear during head and eye movements: evidence for a dichotomous neural categorization of retinal image motion.

We measured perceived motion smear when retinal image motion was created either by a physically moving object or by movement of the eyes or head. Consistent with previous reports, the extent of perceived motion smear during an eye or head movement is less than that produced by physical object motion when the eyes are stationary. Moreover, perceived smear is substantially smaller when the motion of the retinal image is in the same direction as the eye or head movement compared to when image motion is in the opposite direction. These results imply that extra-retinal signals associated with eye and head movements contribute to a reduction of perceived motion smear, thereby fostering perceptual clarity. We hypothesize that the visual system uses a simple dichotomous strategy in applying these extra-retinal signals, based only on the direction of retinal image motion with respect to the ongoing eye or head movement.

Brain↗

PubMed2005

Attenuation of perceived motion smear during the vestibulo-ocular reflex.

Previous studies indicated that less motion smear is perceived when a physically stationary target is presented during voluntary eye movements than when similar retinal-image motion occurs during steady fixation. In this study, we assessed whether the perception of motion smear is attenuated also during the involuntary vestibulo-ocular reflex (VOR). Normal observers matched the length of perceived smear in two experimental conditions that were designed to produce similar trajectories of retinal image motion. In the fixation condition, a small bright target was presented for a duration of 50-200 ms in rightward or leftward motion, while the observer remained stationary and maintained fixation. In the VOR condition, the target moved along with the observer, who underwent full-body rotation around a vertical axis in darkness. Horizontal eye movement recordings during VOR trials allowed us to calculate the velocity of retinal image motion on each VOR trial. The principal result was that the extent of perceived motion smear was significantly less during VOR than fixation trials, particularly for target durations of 100 ms or longer. These findings support the conclusion that extra-retinal signals during the involuntary VOR contribute to a reduction of perceived motion smear.

Analysis of Variance↗

PubMed2005

Asymmetrical vertical phorias in normal subjects: the influence of unbalanced illumination.

PURPOSE: One goal of this investigation was to determine whether asymmetrical vertical hetereophorias, which are qualitatively similar to a dissociated vertical deviation (DVD), occur commonly in normal people. A second goal was to establish whether the magnitude or direction of vertical phoria depends on the illumination of the occluded eye, as would be expected if vertical eye position were influenced by a dorsal light reflex. METHODS: Vertical phorias were estimated in 30 normal subjects using a Maddox rod in conjunction with a dark and an illuminated occluder. RESULTS: On average, subjects exhibited a hypophoria of less than 1 min arc with both the dark and the illuminated occluder. Asymmetrical vertical phorias that were consistent with previously published criteria for a minute DVD were found in eight subjects when a dark occluder was used and in four subjects when an illuminated occluder was used. The proportions of subjects whose vertical phorias were consistent with a DVD did not differ significantly under the two conditions of occlusion. CONCLUSIONS: Asymmetrical vertical phorias that mimic a minute DVD do not occur commonly in normal people. Our finding that vertical phorias do not change systematically when an illuminated instead of a dark occluder is used suggests that normal subjects exhibit little or no vestige of the dorsal light reflex.

Adult↗

PubMed2004

Attenuation of perceived motion smear during vergence and pursuit tracking.

When the eyes move, the images of stationary objects sweep across the retina. Despite this motion of the retinal image and the substantial integration of visual signals across time, physically stationary objects typically do not appear to be smeared during eye movements. Previous studies indicated that the extent of perceived motion smear is smaller when a stationary target is presented during pursuit or saccadic eye movements than when comparable motion of the retinal image occurs during steady fixation. In this study, we compared the extent of perceived motion smear for a stationary target during smooth pursuit and vergence eye movements with that for a physically moving target during fixation. For a target duration of 100 ms or longer, perceived motion smear is substantially less when the motion of the retinal image results from vergence or pursuit eye movements than when it results from the motion of a target during fixation. The reduced extent of perceived motion smear during eye movements compared to fixation cannot be accounted for by different spatio-temporal interactions between visual targets or by unequal attention to the moving test spot under these two types of conditions. We attribute the highly similar attenuation of perceived smear during vergence and pursuit to a comparable action of the extra-retinal signals for disjunctive and conjugate eye movements.

Convergence, Ocular↗

PubMed2004

Differential latencies and the dynamics of the position computation process for moving targets, assessed with the flash-lag effect.

To investigate the dynamics of the position computation process for a moving object in human vision, we measured the response to a continuous change in position at a constant velocity (ramp-response) using the flash-lag illusion. In this illusion, flashed and moving objects appear spatially offset when their retinal images are physically aligned. The steady-state phase of the ramp-response was probed using the "continuous-motion" (CM) paradigm, in which the motion of the moving object starts long before the occurrence of the flash. To probe the transient phase of the ramp-response, we used the "flash-initiated cycle" (FIC) paradigm, in which the motion of the moving object starts within a short time window around the presentation of the flash. The sampling instant of the ramp-response was varied systematically by changing the luminance or the presentation time of the flashed stimulus. We found that the perceived flash misalignments in the FIC and CM paradigms were approximately equal when sampling of the ramp-response occurred after a relatively long delay from the onset of motion and, were significantly different when sampling of the ramp-response occurred at a relatively short delay. The systematic variations in the perceived misalignment between the moving and flashed stimuli as a function of stimulus parameters are compared to the predictions of our differential latency and to alternative models of position computation.

Humans↗

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

Suprathreshold intrinsic dynamics of the human visual system.

Intrinsic high-frequency neural activities have been observed in the visual system of several species, but their functional significance for visual perception remains a fundamental puzzle in cognitive neuroscience. Spatiotemporal integration in the human visual system acts as a low-pass filter and makes the psychophysical observation of high-frequency activities very difficult. A computational model of retino-cortical dynamics (RECOD) is used to derive experimental paradigms that allow psychophysical studies of high-frequency neural activities. A reduced-parameter version of the model is used to quantitatively relate psychophysical data collected in two of these experimental paradigms. Statistical analysis shows that the model's account of the variance in the data is, in general, highly significant. We suggest that psychophysically measured oscillations reflect intrinsic neuronal oscillations observed in the visual cortex.

Action Potentials↗

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↗