Sparing of coarse stereopsis in stereodeficient children with a history of amblyopia.

Stereoscopic depth perception may be obtained from small retinal disparities that can be fused for single vision (fine stereopsis), but reliable depth information is also obtained from larger disparities that produce double vision (coarse stereopsis). Here we assess the possibility that the early development of coarse stereopsis makes it resilient to the factors that cause amblyopia by comparing performance in children with a history of strabismic, anisometropic, or aniso-strabismic amblyopia and age-matched controls (5-12 years). The task was to indicate whether a cartoon character was nearer or farther away than a zero-disparity reference frame. Test disparities were grouped into fine and coarse ranges based on preliminary assessment of diplopia thresholds. In the fine range, accuracy increased with disparity for both groups, but children in the amblyopia group performed significantly worse than children in the control group, particularly when their amblyopia was associated with strabismus. In the coarse range, accuracy was constant across all disparities for both groups although performance appeared to be poorer in the aniso-strabismic group. These results suggest that, under some conditions, stereopsis for large disparities may be spared when stereopsis for small disparities is disrupted by early visual deprivation. This undetected residual binocular function has important clinical implications given recent efforts to improve amblyopia treatment outcomes by employing binocular treatment protocols.

On the effective number of tracked trajectories in normal human vision.

Z. W. Pylyshyn and R. W. Storm (1988) have shown that human observers can accurately track four to five items at a time. However, when a threshold paradigm is used, observers are unable to track more than a single trajectory accurately (S. P. Tripathy & B. T. Barrett, 2004). This difference between the two studies is examined systematically using substantially suprathreshold stimuli. The stimuli consisted of one (Experiment 1) or more (Experiments 2 and 3) bilinear target trajectories embedded among several linear distractor trajectories. The target trajectories deviated clockwise (CW) or counterclockwise (CCW) (by 19 degrees, 38 degrees, or 76 degrees in Experiments 1 and 2 and by 19 degrees, 38 degrees, or 57 degrees in Experiment 3), and observers reported the direction of deviation. From the percentage of correct responses, the "effective" number of tracked trajectories was estimated for each experimental condition. The total number of trajectories in the stimulus and the number of deviating trajectories had only a small effect on the effective number of tracked trajectories; the effective number tracked was primarily influenced by the angle of deviation of the targets and ranged from four to five trajectories for a +/-76 degrees deviation to only one to two trajectories for a +/-19 degrees deviation, regardless of whether the different magnitudes of deviation were blocked (Experiment 2) or interleaved (Experiment 3). Simple hypotheses based on "averaging of orientations," "preallocation of resources," or pop-out, crowding, or masking of the target trajectories are unlikely to explain the relationship between the effective number tracked and the angle of deviation of the target trajectories. This study reconciles the difference between the studies cited above in terms of the number of trajectories that can be tracked at a time.

The effect of occlusion therapy on motion perception deficits in amblyopia.

There is growing evidence for deficits in motion perception in amblyopia, but these are rarely assessed clinically. In this prospective study we examined the effect of occlusion therapy on motion-defined form perception and multiple-object tracking. Participants included children (3-10years old) with unilateral anisometropic and/or strabismic amblyopia who were currently undergoing occlusion therapy and age-matched control children with normal vision. At the start of the study, deficits in motion-defined form perception were present in at least one eye in 69% of the children with amblyopia. These deficits were still present at the end of the study in 55% of the amblyopia group. For multiple-object tracking, deficits were present initially in 64% and finally in 55% of the children with amblyopia, even after completion of occlusion therapy. Many of these deficits persisted in spite of an improvement in amblyopic eye visual acuity in response to occlusion therapy. The prevalence of motion perception deficits in amblyopia as well as their resistance to occlusion therapy, support the need for new approaches to amblyopia treatment.

On the typical development of stereopsis: fine and coarse processing.

Stereoscopic depth perception may be obtained from small retinal disparities that can be fused for single vision (fine stereopsis), but reliable depth information is also obtained from larger disparities that produce double vision (coarse stereopsis). While there is some evidence that stereoacuity improves with age, little is known about the development and maturation of coarse stereopsis. Here we address this gap by assessing the maturation of stereoscopic depth perception in children (4-14 years) and adults over a large range of disparities from fused (fine) to diplopic (coarse). The observer's task was to indicate whether a stereoscopic cartoon character was nearer or farther away than a zero-disparity reference frame. The test disparities were grouped into fine (0.02, 0.08, 0.17, 0.33, 0.68, 1.0 deg) and coarse (2.0, 2.5, 3.0, 3.5 deg) ranges based on an initial determination of the diplopia threshold for each observer. Next, percent correct depth direction was determined as a function of disparity. In the coarse range, accuracy decreased slightly with disparity and there were no differences as a function of age. In the fine range, accuracy was constant across all disparities in adults and increased with disparity in children of all ages. Performance was immature in all children at the finest disparity tested. We conclude that stereopsis in the coarse range is mature at 4 years of age, but stereopsis in the fine range, at least for small disparities, continues to mature into the school-age years.

The effect of dot speed and density on the development of global motion perception.

The purpose of this study was to investigate the effect of dot speed and dot density on the development of global motion perception by comparing the performance of adults and children (5-6years old) on a direction-discrimination task. Motion coherence thresholds were measured at two dot speeds (1 and 4deg/s) and three dot densities (1, 15, 30dots/deg(2)). Adult coherence thresholds were constant at approximately 9%, regardless of speed or density. Child coherence thresholds were significantly higher across conditions, and were most immature at the slow speed and at the sparse density. Thus, the development of global motion perception depends heavily on stimulus parameters. This finding can account for some of the discrepancy in the current developmental literature. Our results, however, caution against making general claims about motion deficits in clinical populations based on only a single measurement at a specific combination of speed and density.

Quantitative measurement of interocular suppression in children with amblyopia.

In this study we explored the possibility of using a dichoptic global motion technique to measure interocular suppression in children with amblyopia. We compared children (5-16 years old) with unilateral anisometropic and/or strabismic amblyopia to age-matched control children. Under dichoptic viewing conditions, contrast interference thresholds were determined with a global motion direction-discrimination task. Using virtual reality goggles, high contrast signal dots were presented to the amblyopic eye, while low contrast noise dots were presented to the non-amblyopic fellow eye. The contrast of the noise dots was increased until discrimination of the motion direction of the signal dots reached chance performance. Contrast interference thresholds were significantly lower in the strabismic group than in the anisometropic and control group. Our results suggest that interocular suppression is stronger in strabismic than in anisometropic amblyopia.

Loss of positional information when tracking multiple moving dots: the role of visual memory.

Pylyshyn, Z.W. and Storm, R.W. (1988) (Tracking multiple independent targets: Evidence for a parallel tracking mechanism. Spatial Vision, 3(3), 179-197) proposed that human observers could simultaneously track up to five dots when presented with an array of dots moving in a random manner. In contrast, Tripathy, S.P., and Barrett, B.T. (2004) (Severe loss of positional information when detecting deviations in multiple trajectories. Journal of Vision, 4(12):4, 1020-1043, http://journalofvision.org/4/14/4/, doi: 10.1167/4.12.4) showed that when a threshold paradigm was employed, observers' ability to track deviations in straight-line trajectories is severely compromised when attending to two or more dots. In this study we present a series of four experiments that investigates the role of attention and visual memory while tracking deviations in multiple trajectories using a threshold paradigm. Our stimuli consisted of several linear, non-parallel, left-to-right trajectories, each moving at the same speed. At the trajectory mid-point (reached simultaneously by all dots), one of the dots (target) deviated clockwise or counter-clockwise. The observers' task was to identify the direction of deviation. The target trajectory was cued in the second half of the trial either by disappearance of distractors at the monitor's mid-line (Experiment 1) or by means of a change in colour of the target (Experiment 2); in both cases deviation thresholds rose steeply when the number of distractor trajectories was increased from 0 (typical threshold approximately 2 degrees) to 3 (typical threshold>20 degrees). When all the trajectories were presented statically in a single frame (Experiment 3), thresholds for identifying the orientation change of the target trajectory remained relatively unchanged as the number of distractor trajectories was increased. When a temporal delay of a few hundred milliseconds was introduced between the first and second halves of trajectories (Experiment 4), deviation thresholds increased steeply. These results suggest that the persistence of trajectory-traces in visual sensory memory may play an important part in determining thresholds for detecting deviations in trajectories.