Biomedical subjects

H E Bedell

Publications and source records attributed to H E Bedell.

At least 19 recordsLinked to original sources

PubMed1995

The effect of flicker on foveal and peripheral thresholds for oscillatory motion.

This study evaluated the influence of superimposed luminance flicker on the detection of oscillatory motion. Thresholds for oscillatory motion were determined in the fovea and at 2, 6 and 25 deg in the right field for a small luminous target with and without sinusoidal luminance flicker. At the fovea, flicker modulation up to 80% at frequencies from 1.5 to 9 Hz had no effect on motion detection, except for oscillatory motion at a frequency of 8 Hz, for which thresholds were elevated by about 0.2 log units. In the periphery, flicker elevated motion thresholds up to 0.3-0.4 log units at low and moderate frequencies of oscillation at all locations tested. However, both foveal and peripheral motion thresholds were unaffected by flicker when the luminance of the target was reduced. The absence of a robust effect of target flicker on motion thresholds may be accounted for in part by the comparison of activity across a large population of motion-detecting neurons with different direction preferences. Another contributing factor may be the existence of foveal velocity- and position-detecting mechanisms with similar sensitivities.

Female↗

PubMed1995

A target in real motion appears blurred in the absence of other proximal moving targets.

For exposure durations longer than about 40 msec, a field of dots in sampled motion has been reported to appear less smeared than predicted from the visual persistence of static displays. This reduction of perceived smear has been attributed to a motion "deblurring" mechanism. However, it has been long recognized that an isolated target moving continuously in a dark field appears to be extensively smeared. To reconcile these apparently contradictory observations, we investigated the effect of dot density on the extent of perceived smear for a single moving dot and for fields of dots with densities ranging from 0.75 to 7.5 dots/deg2. Bright targets were presented in continuous motion against a photopically illuminated background field. The results reconcile previous conflicting observations by showing that the length of perceived smear decreases systematically with dot density for exposure durations longer than about 50 msec. In three additional experiments, we arranged the spatial configuration of the targets to evaluate whether motion deblurring results primarily from a motion compensation mechanism (such as integration within the spatiotemporally oriented receptive fields of putative motion mechanisms) or from inhibition exerted by spatiotemporally adjacent targets. The results show that the activation of motion mechanisms is not a sufficient condition for motion deblurring and that the reduction of perceived smear requires the presence of spatiotemporally adjacent targets. Taken together, these findings suggest that motion deblurring results primarily from masking exerted by spatiotemporally proximal targets.

Afterimage↗

PubMed1995

Reading rates with artificial central scotomata with and without spatial remapping of print.

People with central field defects resulting from age-related macular degeneration (ARMD) read very slowly. In this study, oral reading rates were determined for unrelated sequences of words in samples of normal young and old subjects with simulated central scotomata of 2 degrees, 4 degrees, and 8 degrees. Scotomata were stabilized at the fovea of the right eye by electronic feedback of eye position, monitored using a SRI dual-Purkinje Eyetracker. Reading rates were determined by jumping print after each stationary presentation through an increasing number of character spaces on different trials. This procedure mimicked the sequence of retinal images produced during the saccades and fixations of normal reading, but without requiring subjects to make accurate eye movements. In Experiment 1, the letter size that yielded the optimal reading rate was found to increase systematically with scotoma size. However, the optimal reading rate decreased more or less linearly as the scotoma size increased. Experiment 2 showed that the optimal reading rate was obtained for essentially the same duration of text presentation, regardless of scotoma size. Experiment 3 investigated the effect of spatial remapping, in which print obscured by the scotoma was stretched electronically to reappear at the scotoma margin. Compared to a nonremapped control condition, spatial remapping produced small but significant increases in reading rate for both 4 degrees and 8 degrees scotomata. Across experiments, average reading rates were faster for the young than the old subjects. Overall, the results define how reading rate is expected to decrease for central scotomata of different sizes and suggest that spatial remapping of print may improve reading rates in patients with ARMD.

Adolescent↗

PubMed1992

Psychophysical and saccadic information about direction for briefly presented visual targets.

For saccades, the difference between desired and actual eye movement (constant error), as well as the variability in amplitude on repeated trials (variable error), presumably represent a combination of errors in processing target position (sensory error) and errors in execution (motor error). To examine whether the saccadic system uses the same information about target position as visual perception, subjects made saccades to and psychophysical judgments about the location of targets presented for 17-200 msec. Mean saccadic amplitude markedly decreased and inter-trial variability increased when saccadic targets were presented briefly and followed by a spatial mask. Judged target position (from psychophysical vernier and bisection tasks) also shifted to lesser eccentricities and was more variable. Although qualitatively alike, changes with duration in saccadic constant errors were larger than could be accounted for by the psychophysical results, suggesting similar but separate processing of position information for the saccadic and perceptual systems. Differences in the sizes of collicular receptive fields that preferentially respond to targets at short and long durations can account qualitatively for the observed changes in saccadic amplitude as well as the common occurrence of saccadic undershoots.

Adult↗

PubMed1992

Variation of congenital nystagmus with viewing distance.

Involuntary oscillations of the eyes in persons with congenital nystagmus often dampen in intensity (amplitude x frequency) with near viewing. This study examined how changes in viewing distance affect the duration and position variability of foveation periods (intervals of low eye velocity when the target's image is at or near the fovea), as these parameters generally correlate better with acuity than nystagmus intensity. To do so, the horizontal positions of both eyes were recorded by infrared limbal tracking in 4 subjects with congenital nystagmus and 4 normals during monocular and binocular fixation at 200, 40, and 20 cm. At nearer distances, position variability (the standard deviation of horizontal eye position) increased in normal subjects, as well as during foveation periods in the subjects with nystagmus. Other parameters of the nystagmus varied idiosyncratically with viewing distance. In none of the subjects with nystagmus was visual acuity (measured psychometrically) better at 40 cm than at distance. The results show that characteristics of congenital nystagmus do not necessarily improve and, in some instances worsen, with near viewing.

Distance Perception↗

PubMed1992

Rapid- and slow-velocity vergence eye movements.

Previously we have measured rapid-velocity vergence responses to targets at different distances that provided no disparity or accommodative stimulation. To evaluate the possibility that this rapid-velocity vergence occurs during saccades, the latencies of eye movement between two long dim luminous rods were compared under two conditions. Rapid-velocity vergence with an average latency of approximately 300 ms was elicited when subjects alternately viewed horizontal rods at distances of 38 and 78 cm, and with a vertical separation of 5.2 degrees. Horizontal saccades with a comparable latency were measured when the rods were equidistant and oriented vertically. The correlation between the mean latencies of vergence and saccadic movements was 0.97, suggesting that the two movements occurred together. In a second experiment vergence responses were measured when the subject looked between a bright vertical line on a screen at 76 cm and a second pair of lines (vertically displaced between 0.15 degrees and 3 degrees) with crossed disparity to simulate a target at 38 cm. Slow-velocity vergence often occurred alone when the vertical separation between targets was small; rapid-velocity vergence intruded when the separation between the targets was larger. The results can be accounted for if proximity and disparity stimulation act through a single vergence controller, the output of which produces slow- or rapid-velocity vergence depending upon whether the saccadic system is concurrently active.

Adult↗

PubMed1992

Sensitivity to oscillatory target motion in congenital nystagmus.

Despite incessant to and fro motion of their eyes, persons with congenital nystagmus rarely report illusory motion of the visual world (oscillopsia). To determine whether insensitivity to retinal image motion prevents oscillopsia, thresholds for detecting horizontal and vertical oscillatory motion of a luminous dot target were measured in subjects with congenital idiopathic nystagmus and compared to normals. Thresholds for unreferenced and referenced oscillatory motion were elevated substantially in subjects with nystagmus over the range of motion frequencies tested (0.25-12 Hz), particularly for motion parallel to nystagmus eye movements. However, motion thresholds were increased comparably in normal subjects when the target underwent continuous sinusoidal or ramp motion to simulate the retinal image motion that occurs with nystagmus eye movements. These results indicate that sensitivity to motion in persons with congenital nystagmus is similar to normal as long as the ongoing motion of the retinal image, produced by the eye movements, is taken into account. Therefore, insensitivity to motion does not explain the lack of reported oscillopsia in subjects with congenital nystagmus. Perceptual adaptation is suggested as a possible mechanism that contributes to the prevention of oscillopsia.

Adolescent↗

PubMed1992

Sensitivity to temporal luminance modulation in congenital nystagmus.

Uniform field temporal contrast sensitivity functions were compared for 10 subjects with congenital nystagmus (seven idiopathic, three with albinism) and 10 normal observers. Sensitivity to luminance modulation did not differ significantly from normal at any temporal frequency tested except 0.5 Hz, at which the subjects with nystagmus had slightly higher sensitivity. In conjunction with other recent findings, their essentially normal temporal contrast sensitivity suggests that persons with nystagmus process retinal information continuously, rather than selectively during only only one phase of the nystagmus.

Adolescent↗

PubMed1991

Interrelations between measures of visual acuity and parameters of eye movement in congenital nystagmus.

The authors assessed relationships between visual acuity and the amplitude, frequency, intensity, and duration of foveation periods in a retrospective study of 32 patients. Twenty-four patients had congenital idiopathic nystagmus, and eight patients had nystagmus and albinism. Visual acuity was determined for Landolt ring optotypes and, as the extrapolated high-frequency cutoff of the contrast sensitivity function, for horizontal and vertical gratings. No significant correlation existed between acuity and any measured eye movement parameter; however, optotype acuity was related to the magnitude of astigmatic refractive correction, both in patients with idiopathic nystagmus and in albinos. In a subgroup of patients with idiopathic nystagmus whose astigmatic refractive error was -1.50 D or less, nystagmus intensity (amplitude x frequency) correlated significantly with acuity for optotypes (r = 0.71), but not for gratings. Although resolution for vertical gratings was correlated with astigmatic refractive correction, the ratio of resolution for gratings parallel and orthogonal to the meridian of nystagmus was not. Thus, the belief that poorer acuity in patients with substantial astigmatism is attributable to an optically induced meridional amblyopia is supported only partly by these results. The authors concluded that among patients with congenital nystagmus, the influence of eye motion on visual acuity is not readily predicted either from the parameters of nystagmus that they evaluated or from the comparison of resolution for horizontal and vertical gratings.

Albinism, Ocular↗

PubMed1990

Proximal vergence and perceived distance.

Proximal vergence was measured haploscopically in response to a change in physical distance of a large screen of blurred horizontal lines. These responses were compared to proximal vergence measurements made using a clinical method. Perceived distance of the horizontal stripes was estimated according to the perceived size of a projected afterimage. For 18 normal observers the change in vergence measured haploscopically (in prism diopters per meter angle change in test distance) averaged 2.59 (SD = 1.44) binocularly and 2.11 (SD = 1.22) monocularly. Clinical measures averaged 2.88 (SD = 1.57) binocularly and 2.10 (SD = 1.27) monocularly. Haploscopic and clinical measures of proximal vergence correlated only when the clinical technique was modified by viewing through pinholes. Taking estimated perceived distance into consideration only partially reduced the differences between proximal vergence measured haploscopically and clinically as well as binocularly and monocularly. Both haploscopic and clinical measures of proximal vergence were sizeable, indicating that this vergence component can contribute significantly to the normal vergence response.

Accommodation, Ocular↗

PubMed1990

Relations between accommodation and vergence in darkness.

Prior studies reported that the resting postures of accommodation and vergence are uncoupled in darkness, calling into question whether accommodation-vergence synkinesis persists when both systems are deprived of stimulation. We tracked the postures of accommodation and vergence of 20 normal subjects for 15 min in darkness, using a Vernier optometer and dichoptic nonius lines. Consistent with previous reports, the mean resting postures of accommodation and vergence correlated weakly (r = 0.19); spontaneous fluctuations of accommodation and vergence in darkness also exhibited little correlation (median r = 0.15). However, resting vergence postures could be predicted (r = 0.65) from the clinically measured distance phoria and the resting posture of accommodation, when subjects' AC/A ratios were taken into account. We conclude that the relaxation of accommodation and vergence to separate resting postures in darkness is compatible with the persistence of synkinesis; independent noise in the two systems may reduce apparent coupling under open-loop conditions.

Accommodation, Ocular↗

PubMed1990

Directionalization of visual targets during involuntary eye movement.

Perceived visual direction with respect to one's self (egocentric direction) depends upon the retinal location of a target's image (its oculocentric direction) and concurrent information about the position of the eyes in the head. Information about eye position is presumably obtained from efference copy signals. However, in order to explain illusory target motion that can occur during reflexive eye movements (e.g., post-rotary nystagmus), these signals have been suggested to accompany only voluntary oculomotor responses. In the experiment reported here, manual pointing was used to assess the perceived direction of targets flashed during optokinetic afternystagmus, an involuntary movement of the eyes that occurs in darkness following optokinetic stimulation. Egocentric directionalization was essentially veridical, indicating that accurate eye position information is available during optokinetic afternystagmus. A model is proposed that accounts for illusory target motion during involuntary oculomotor responses by the cancellation of efference copy information about these eye movements with signals of oppositely directed head motion.

Eye Movements↗

PubMed1990

Fixational drift and nasal-temporal pursuit asymmetries in strabismic amblyopes.

This study evaluated to what extent inaccurate and asymmetric smooth pursuit in strabismic amblyopic eyes is attributable to abnormally high-velocity eye drifts that these eyes exhibit during monocular fixation. Smooth pursuit gains (peak eye velocity/peak target velocity) were determined in the amblyopic and nonamblyopic eyes of 11 strabismics for nasalward and temporalward motion; the target oscillated across 6 degrees of the horizontal meridian at frequencies ranging from 0.0625 to 1 Hz. In general, pursuit gains were higher for nasalward than temporalward motion, for both amblyopic and nonamblyopic eyes. Correction for each eye's mean velocity of fixational drift eliminated this nasal-temporal pursuit asymmetry for most of the nonamblyopic eyes, but not for the amblyopic eyes. Compared to the nonamblyopic eyes, corrected pursuit gains of the amblyopic eyes averaged about 0.2 lower nasalward and about 0.4 lower temporalward, but substantial variation occurred among individuals. We suggest that the overall reduction of pursuit gain in strabismic amblyopic eyes (after correction is made for fixational drift bias) stems from the use of a nonfoveal (eccentric fixation) locus for tracking; the further reduction of temporalward gain may result from a nasal-temporal asymmetry in processing motion signals.

Adult↗

PubMed1990

The oculomotor reference in humans with bilateral macular disease.

Previous studies have shown that most patients with bilateral macular disease use a single retinal area during fixation which is typically located near the edge of the central scotoma. Through frame-by-frame playback of videorecordings showing the target's image on each patient's fundus, we evaluated the sequences of eye movements the patients make during fixation and refixation saccades. We show that although most of our patients with bilateral macular disease had a preferred fixation area, only one third exhibited additional oculomotor behaviors, indicating a complete rereferencing of their eye movements to the preferred area. Such behaviors include the maintenance of the target's image within a circumscribed retinal area during fixation; consistent imaging of the target at the same retinal area as a result of refixation saccades; and the complete absence of "foveating" saccades. Spatial precision of eye movements (fixation and the endpoints of saccades) of patients with a nonfoveal oculomotor reference tends to be scaled to the eccentricity of the preferred fixation area. Saccadic latencies were elevated in patients with bilateral macular disease; elevation was pronounced when the duration of disease was 2 yr or less, but was evident even in patients whose eye movement patterns were consistent with a single nonfoveal reference. We conclude that, over a period of years, a shift of the oculomotor reference from the fovea to a nonfoveal locus is possible in patients with bilateral macular disease. The shift should facilitate the visual rehabilitation of such patients.

Adaptation, Physiological↗

PubMed1989

Visual localization of briefly presented peripheral targets.

Reported here are the results from two experiments designed to investigate Mateeff and Gourevich's (1983, 1984) claim that adult observers make large constant errors when judging the direction of briefly presented peripheral targets, with respect to a continuously visible scale. Experiment 1 involved a virtually exact duplication of Mateeff and Gourevich's paradigm. Experiment 2 involved adjusting the position of a light emitting diode (LED) so as to match the direction of a LED previously flashed in the periphery. No significant constant errors were found in either experiment.

Adult↗

PubMed1989

Contrast sensitivity for letter and grating targets under various stimulus conditions.

Measurement of visual acuity for letters of different contrasts has been suggested as a clinical way to evaluate contrast sensitivity in patients with vision abnormalities. If variable-contrast letter acuity provides information similar to the contrast sensitivity function (CSF), then comparable effects should be seen in stimulus manipulations which simulate decreased vision. Using both our own and published data, we compared the effects of diffusive blur, dioptric blur, and eccentric viewing on contrast sensitivity for letter and grating targets. A diffuser placed close to the eye reduces contrast sensitivity fairly evenly across all spatial frequencies, with similar results for letters and gratings. However, dioptric blur reduces sensitivity substantially more to letters than to comparably fine gratings. Eccentric viewing also produces a larger sensitivity loss for letters than for gratings. Because some stimulus manipulations produce dissimilar changes in contrast sensitivity for letters and gratings, it is questionable whether the results of one measure can be used to draw inferences about the other. It is proposed that local or relative phase discrimination has an important role in explaining the different responses to letter and grating targets.

Adult↗

PubMed1989

Extraretinal information about eye position during involuntary eye movement: optokinetic afternystagmus.

Despite importance for theories of perception, controversy exists as to whether information is available to the perceptual system about involuntary as well as voluntary eye movements. We measured the perceived direction of targets flashed briefly in an otherwise dark field during the primary phase of optokinetic afternystagmus (OKAN), an involuntary eye movement that persists in darkness following optokinetic stimulation. Perceived direction was measured by unseen pointing in one experiment and by pointing made under visual control in a second experiment. Pointing was essentially veridical in both experiments, indicating that accurate extra-retinal information about eye position (presumably, as efference copy) exists for OKAN. Illusory motion of visual targets, which can occur during involuntary oculomotor responses, therefore cannot be attributed to a lack of efference-copy signals for such eye movements.

Attention↗

PubMed1989

Magnitude and velocity of proximal vergence.

Until recently proximal vergence was considered to play only a minor role in the eye alignment changes that occur when looking between distant and near targets. We measured the magnitude and velocity of proximal vergence using infrared limbal sensing to record vergence responses between two untextured luminous horizontal rods which lacked disparity and accommodative cues. The magnitude of proximal vergence responses averaged 3.9 degrees for convergence and 3.8 degrees for divergence, about 70% of the total vergence "demand." Peak velocities for proximal convergence and divergence averaged 69 and 53 deg/sec, substantially faster than the velocities of comparably sized disparity or accommodative vergence responses. Its rapid velocity makes proximal vergence well suited to initiate the eye alignment changes between distant and near targets.

Accommodation, Ocular↗