Moving ahead through differential visual latency.
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Humans↗
Biomedical subjects
H E Bedell
Publications and source records attributed to H E Bedell.
Stimulus uncertainty affects velocity discrimination.
Velocity discrimination thresholds were determined for 1 c/deg drifting gratings when uncertainty about the reference velocity was introduced by interleaving stimuli with different reference velocities from trial to trial. When drifting gratings with reference velocities spanning 4 octaves (1-16 deg/sec) were mixed randomly within a series of trials, the velocity discrimination threshold for a 4 deg/sec stimulus increased by more than a factor of 3. The threshold elevation decreased as the range of interleaved velocities was reduced from 4 to approx. 0.75 octaves, below which velocity interleaving had little effect. In contrast, when gratings that spanned a 4-octave range in spatial frequency were interleaved on successive trials, velocity discrimination for 4 deg/sec was essentially unaffected. Our results indicate that the psychophysical mechanisms underlying velocity discrimination are not spatial-frequency specific, but are turned to the velocity or speed of the stimulus.
Discrimination, Psychological↗
Vernier and letter acuities for low-pass filtered moving stimuli.
Vernier and letter acuities are both susceptible to degradation by image motion. In a previous study, we showed that the worsening of Vernier acuity for stimuli moving up to 4 degrees/s is accounted for primarily by a shift of visual sensitivity to mechanisms of lower spatial frequency. The purposes of this study were to extend the previous results for Vernier acuity to higher stimulus contrast and velocities, and to determine if a shift in spatial scale can similarly explain the degradation of letter acuity for moving stimuli. We measured Vernier discrimination for a pair of vertical abutting thin lines and letter resolution for a four-orientation letter 'T' as a function of stimulus velocity ranging from 0 to 12 degrees/s. Stimuli were presented at 20 times the detection threshold, determined for each velocity. To determine the spatial-frequency mechanism that mediates each task at each velocity, we measured Vernier and letter acuities with low-pass filtered stimuli (cut-off spatial-frequency: 17.1-1.67 c/deg) and analyzed the data using an equivalent blur analysis. Our results show that the empirically determined, equivalent intrinsic blur associated with both tasks increases as a function of stimulus velocity, suggesting corresponding increases in the size of optimally responding mechanisms. This progressive increase in mechanism size can account for the worsening of Vernier and letter acuities with velocity. Vernier discrimination is found to be more susceptible to degradation by various stimulus parameters than letter resolution, suggesting that different mechanisms are involved in the two tasks. We conclude that the elevations in Vernier and letter acuities for moving stimuli are the consequence of a shift of visual sensitivity toward mechanisms of lower spatial frequencies.
Female↗
Motion deblurring in a neural network model of retino-cortical dynamics.
Simulations of a neural network model of retino-cortical dynamics (Oğmen H, Neural Netw 6 (1993) 245-273) are presented. The temporal-step response of the model to a single dot (spatial impulse) consists of three post-retinal phases: reset, feed-forward dominant and feedback dominant. In response to a single moving dot, the model predicts the perception of extensive blur. This extensive blur is proposed to be due to the relative spatial and temporal offsets between transient and sustained signals conveyed from retina to post-retinal levels. In response to a pair of horizontally separated dots moving in the horizontal direction, the model predicts extensive blur for the trailing dot irrespective of dot-to-dot separation. For the leading dot, the model predicts a decrease in perceived blur for long exposure durations when dot-to-dot separations are small. The reduction of perceived blur at long exposure durations for small dot-to-dot separations is proposed to stem from the spatio-temporal overlap between the transient activity generated by the trailing dot and the sustained activity generated by the leading dot. The model also predicts that targets moving at higher speeds generate more blur even when blur is normalized with respect to speed. The mechanism in the model generating this effect is a slow inhibition within the sustained channel. These predictions are compared with recent psychophysical data (Chen S, Bedell HE, Oğmen H, Vis Res 35 (1995) 2315-2328) and are found to be in excellent agreement. The model is used to offer a coherent explanation for several controversial findings published in the literature. This computational study shows that a model without any motion-compensation mechanism can give a good account of motion deblurring phenomenon and supplements our recent experimental study which provided evidence against motion-compensation type models in explaining the motion deblurring phenomenon (Chen S, Bedell HE, Oğmen H, Vis Res 35 (1995) 2315-2328).
Feedback↗
Congenital nystagmus image motion: influence on visual acuity at different luminances.
In persons with congenital nystagmus (CN), the ability to integrate visual information over time can be limited by two factors--the duration of foveation periods and the temporal integration period of the visual system. The purpose of this study was to assess the relative importance of these two factors for visual acuity for targets of different luminances. We measured visual acuity using Landolt C targets at 5 luminance levels (50 to 0.005 cd/m2) in 6 observers with CN, and in 6 normal observers with comparable motion of the retinal image. To allow comparison, normal observers viewed the targets during image motion simulating jerk CN, with "foveation durations" ranging from 20 to 160 ms. In the normal observers, acuity improves as a function of the simulated foveation duration at all luminance levels. However, this improvement is larger and occurs at a faster rate at high than at low luminances. The more gradual improvement in acuity with simulated foveation durations at low luminances is consistent with a prolongation of the temporal integration period, which we estimate to range from approximately 140 to 380 ms over the 4 log unit range in luminance that we tested. In observers with CN, the change in acuity with luminance is similar, but not identical, to that in normal observers when the duration of the foveation periods is matched. We conclude that the integration of visual information may be limited by either the period of temporal integration or the duration of the foveation period in persons with CN, depending upon which is shorter at the luminance level under consideration.
Female↗
Suppression of motion-produced smear during smooth pursuit eye movements.
Humans make smooth tracking eye movements to keep the image of a moving target on the foveal region of the retina and, thereby, maintain acute vision. Although the images of physically stationary background stimuli sweep across the retina during smooth pursuit eye movements, non-pursued targets are usually perceived to be neither moving nor smeared. The lack of perceived movement of background stimuli is generally attributed to a 'cancellation' of the retinal image motion by extraretinal information about the eye movement [1,2]; this information comes primarily from a neural facsimile of the efferent command to move the eyes, augmented by afferent signals from receptors in the extraocular muscles [3,4]. Here, we show that a physically stationary target presented during smooth tracking is perceived to have considerably less smear than a target that moves comparably across the retina, but when the eye is stationary. This result implies that extraretinal signals for pursuit eye movements also contribute to the alleviation of perceived smear for non-tracked, background targets.
Eye Movements↗
Vernier in motion: what accounts for the threshold elevation?
Vernier acuity is susceptible to degradation by image motion. The purpose of this study was to determine to what extent vernier thresholds are elevated in the presence of image motion because of reduced stimulus visibility, due to contrast smearing, or to a shift in the spatial scale of analysis. To test the visibility hypothesis, we measured vernier thresholds as a function of stimulus velocity (0-6 deg/sec), for various levels of stimulus visibility, each normalized to the detection threshold at the respective velocity. Contrary to the prediction of the visibility hypothesis, vernier thresholds worsen as the velocity increases, even when the stimuli are equally visible. To test the shift in spatial scale hypothesis, we determined spatial frequency tuning functions for vernier discrimination and line detection tasks, using a masking paradigm. We measured vernier and line detection thresholds as a function of spatial frequency of a sine-wave mask (0.5-32 c/deg), and for stimulus and mask velocities ranging from 0 to 4 deg/sec. Peak masking for both vernier discrimination and line detection, which indicates the most sensitive band of spatial frequencies for each task, shifts systematically toward lower spatial frequencies as the velocity increases. The progressive increase in spatial scale largely accounts for the worsening of vernier thresholds for moving stimuli. Differences between peak masking for vernier discrimination and line detection were found at 0 and 1 deg/sec, suggesting that different mechanisms mediate the two tasks, at least at low velocities. The masking results are consistent with previous findings that directionally selective motion detectors mediate detection of moving stimuli, but suggest that these detectors do not analyze vernier offsets. We conclude that the elevation of vernier threshold for a moving stimulus is accounted for primarily by a shift of sensitivity to mechanisms of lower spatial frequency, and not by decreased stimulus visibility.
Contrast Sensitivity↗
Temporal-contrast discrimination and its neural correlates.
Reported differences in neuronal contrast processing between the parallel magnocellular (M) and parvocellular (P) visual pathways invite the hypothesis that contrast discrimination in the human visual system is more sensitive at low contrasts and less sensitive at high contrasts, for stimuli modulated at high compared with low temporal frequencies. In the present study, an edgeless temporally modulated uniform field was selected as the stimulus for psychophysical contrast discrimination, and contrast-increment thresholds for pedestal contrasts ranging from 5.5% to 78.2% were determined with a temporal two-alternative forced-choice staircase procedure. The increment thresholds for five normal subjects were adequately fit by power functions with exponents that shifted continuously from about 0.5 (square-root-law behavior) to about 1.0 (Weber's-law behavior) as stimulus temporal frequency increased from 1 to 30 Hz. A neural simulation, with the use of published contrast-response functions of magnocellular and parvocellular neurons, adjusted with an estimate of response variance, produced two distinct 'neural increment-threshold functions' that were similar to the psychophysical results obtained at the highest and the lowest temporal frequencies, respectively. A shift from a relatively more noise-limited neural mechanism to one whose response is predominantly determined by gain is suggested to account for the change of the contrast-increment-threshold function with increasing temporal frequency.
Adult↗
Velocity criteria for "foveation periods" determined from image motions simulating congenital nystagmus.
Foveation periods are brief intervals in the congenital nystagmus (CN) waveform when the image is on or near the fovea and eye velocity is relatively slow. The purpose of this study was to determine how visual acuity depends on image velocity for foveation periods of different durations. Visual acuity was measured using high-contrast, single Landolt Cs in four normal observers during image motion simulating that in jerk nystagmus. The "simulated foveation periods" ranged from 20 to 100 ms in duration and 0 to 16 degrees approximately in velocity. The "critical velocity" was defined as the velocity during simulated foveation periods that produced a just-noticeable worsening of acuity (0.05 and 0.1 logMAR) from that in the zero-velocity condition. Critical velocity increased from approximately 3 degrees approximately for simulated foveation durations between 100 and 40 ms to approximately 5 degrees approximately for a simulated foveation duration of 20 ms. Critical velocities increased further when the targets were presented peripherally or with optical blur, to introduce an additional acuity loss. A consequence of these findings is that less recovery of acuity should be expected when retinal image motion is reduced in individuals with CN if a sensory acuity deficit coexists.
Eye Movements↗
Multifaceted treatment of congenital nystagmus: a report of 6 cases.
Patients with congenital nystagmus (CN) often have associated conditions (e.g., strabismus, high astigmatism, and binocular inefficiency) that also impair visual acuity. Unfortunately, individually used techniques directed at reducing nystagmus have generally produced only slight increases in Snellen acuity. We present case reports of six patients with nystagmus seen in our clinic (University of Houston, College of Optometry) to illustrate the use of problem-specific management to improve visual acuity and/or binocular function. The results suggest that a comprehensive management strategy should include treatment of all associated conditions as well as therapy to improve the characteristics of the nystagmus wave form.
Adolescent↗
Ricco's diameter for line detection increases with stimulus velocity.
The purpose of this study was to determine whether Ricco's diameter, the spatial extent within which sensitivity demonstrates a perfect reciprocity between contrast and area, enlarges as the stimulus velocity increases. Detection thresholds were measured for a single line of length 10 arcmin as a function of linewidth that varied between 0.31 and 21.7 arcmin and for velocity ranging from 0 to 6 deg/s. We fitted the detection threshold versus linewidth data with two power functions of slope 0 and 1 and defined the intersection of these two functions as Ricco's diameter. For an increase in velocity from 0 to 6 deg/s, Ricco's diameter increases in dimension by approximately a factor of 4. Similar results were obtained when Ricco's diameter was estimated by comparing detection threshold of a thin line to that of an edge. The increase in Ricco's diameter with stimulus velocity suggests that the spatial-frequency mechanism that mediates line detection shifts progressively toward lower spatial frequencies for faster moving stimuli.
Contrast Sensitivity↗
Perception of motion smear in normal observers and in persons with congenital nystagmus.
PURPOSE: Despite incessant motion of the retinal image, persons with congenital nystagmus (CN) usually do not report that targets are smeared. The authors investigated whether the brief stationary glimpses of a target that occur during foveation periods in the CN waveform contribute to the alleviation of perceived smear. METHODS: Retinal image motion simulating that in jerk nystagmus was produced in normal observers (N = 10) who monocularly viewed either a 5-minute or a 1 degree luminous disk reflected from a horizontally oscillating mirror. Contrast sensitivities to detect each target and to perceive the presence of motion smear were determined for two simulated CN waveforms; observers also estimated the length and brightness of perceived smear for several suprathreshold target luminances. One waveform was a 7 degrees, 4-Hz ramp that included 120 msec zero-velocity intervals, simulating the foveation periods in the CN waveform. The second waveform lacked the zero-velocity simulated foveation periods. For comparison, estimates of perceived smear for physically stationary targets were obtained from three observers with CN. RESULTS: Normal observers' contrast sensitivities for perceiving smear were nearly identical for the simulated CN waveforms with and without a 120-msec foveation period. Estimated length and brightness of perceived smear for suprathreshold targets increased similarly with luminance for both waveforms. Observers with CN reported substantially less smear than did normal observers. CONCLUSIONS: Glimpses of a stationary retinal image during simulated foveation periods do not attenuate the perception of motion-induced smear in normal observers. In persons with CN, the perception of smear may be reduced by the extraretinal signals that accompany their eye movements.
Contrast Sensitivity↗
"Dumping" of rebound nystagmus and optokinetic afternystagmus in humans.
Rebound nystagmus (RN) is an involuntary movement of the eyes, characterized by slow-phase eye velocity in the direction of previously maintained eccentric gaze. The purpose of this study was to clarify the neural or neuromuscular events that are responsible for the generation of RN. To do so, we examined whether a briefly presented visual stimulus during RN reduces (i.e., "dumps") subsequent eye velocity, compared with the velocity of slow-phase eye movements when no visual stimulus was presented. For comparison, "dumping" was examined also for optokinetic afternystagmus (OKAN), which is generally believed to result from eye-velocity signals stored in a central neural integrator as a consequence of optokinetic stimulation. Results obtained from ten normal observers showed that RN and OKAN both exhibit "dumping": average slow-phase eye velocities were reliably slower after fixation of a 0.6 deg stationary target than on trials when no fixation target was presented. Although RN decayed faster than OKAN in darkness, the magnitude of "dumping" increased similarly with the duration of the visual stimulus (25 ms to 4 s) for both types of eye movement. The results imply that signals from a central velocity-storage mechanism contribute to the generation of RN.
Adult↗
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↗
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↗
Effect of retinal image motion on visual acuity and contour interaction in congenital nystagmus.
This study determined how contour interaction (the degradation of visual acuity by the presence of nearby contours) is affected by the incessant retinal image motion that occurs in observers with congenital nystagmus (CN). Visual acuity was measured for single, high-contrast, black Landolt Cs, presented without and with flanking bars (contour-to-C separation = 1, 2, 5, or 10 multiples of the gap width of the C). Stimuli were presented against either a white or a black surround. For comparison, acuity was also determined in normal observers, with and without motion of the stimulus to simulate the retinal image motion in jerk CN. The results show that the peak magnitude of contour interaction (the maximal degradation in acuity attributable to contour interaction) is significantly larger in the observers with CN than in normals. When acuity targets are presented against a black surround, contour interaction also occurs over a wider spatial extent in the observers with CN. Imposed image motion increases the extent of contour interaction in normal observers, but not sufficiently to account fully for the results of the observers with CN. We suggest that the additional contour interaction found in observers with CN may be attributable to the presence of amblyopia. For a small contour-to-C separation, contour interaction is significantly greater when stimuli are presented against a black rather than a white surround. Consequently, single-letter acuity may be appreciably underestimated clinically when an adjustable window is used to isolate letters on a projected acuity chart.
Form Perception↗
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↗
Stability of oculomotor fixation as a function of target contrast and blur.
Previous studies have shown that changes in target parameters, such as luminance, color, size, and shape exert little effect on the stability of fixation, measured as the standard deviation of eye positions over several seconds of fixation. In this study, we investigated the effect of target contrast and dioptric blur on the stability of fixation in five normal subjects. The horizontal positions of both eyes were assessed by infrared limbal tracking while the subjects fixated binocularly for 20-s intervals on a cross-shaped target at 2 m. Across subjects, the mean stability of fixation on a high contrast target worsened significantly from 8.9 to 12.0 min arc with fixation through increasing amounts (0 to 4 D) of dioptric blur. However, without blur there was no statistically significant effect on fixation stability of varying the target contrast from 7 to 84%. The variability of vergence was not significantly influenced in either condition. Like other target parameters investigated in previous studies, dioptric blur and contrast have relatively inconsequential effects on fixation stability, indicating that the signals used to control fixation are largely independent of stimulus parameters. Practically, the findings suggest that when patients' eye movements are recorded for clinical reasons, the results are unlikely to be affected seriously by the presence of residual refractive error.
Contrast Sensitivity↗