Visual search deficits in amblyopia.

Amblyopia is a neurodevelopmental disorder defined as a reduction in visual acuity that cannot be corrected by optical means. It has been associated with low-level deficits. However, research has demonstrated a link between amblyopia and visual attention deficits in counting, tracking, and identifying objects. Visual search is a useful tool for assessing visual attention but has not been well studied in amblyopia. Here, we assessed the extent of visual search deficits in amblyopia using feature and conjunction search tasks. We compared the performance of participants with amblyopia (n = 10) to those of controls (n = 12) on both feature and conjunction search tasks using Gabor patch stimuli, varying spatial bandwidth and orientation. To account for the low-level deficits inherent in amblyopia, we measured individual contrast and crowding thresholds and monitored eye movements. The display elements were then presented at suprathreshold levels to ensure that visibility was equalized across groups. There was no performance difference between groups on feature search, indicating that our experimental design controlled successfully for low-level amblyopia deficits. In contrast, during conjunction search, median reaction times and reaction time slopes were significantly larger in participants with amblyopia compared with controls. Amblyopia differentially affects performance on conjunction visual search, a more difficult task that requires feature binding and possibly the involvement of higher-level attention processes. Deficits in visual search may affect day-to-day functioning in people with amblyopia.

Size matters: Perceived depth magnitude varies with stimulus height.

Both the upper and lower disparity limits for stereopsis vary with the size of the targets. Recently, Tsirlin, Wilcox, and Allison (2012) suggested that perceived depth magnitude from stereopsis might also depend on the vertical extent of a stimulus. To test this hypothesis we compared apparent depth in small discs to depth in long bars with equivalent width and disparity. We used three estimation techniques: a virtual ruler, a touch-sensor (for haptic estimates) and a disparity probe. We found that depth estimates were significantly larger for the bar stimuli than for the disc stimuli for all methods of estimation and different configurations. In a second experiment, we measured perceived depth as a function of the height of the bar and the radius of the disc. Perceived depth increased with increasing bar height and disc radius suggesting that disparity is integrated along the vertical edges. We discuss size-disparity correlation and inter-neural excitatory connections as potential mechanisms that could account for these results.

Behavioral Training as New Treatment for Adult Amblyopia: A Meta-Analysis and Systematic Review.

PURPOSE: New behavioral treatment methods, including dichoptic training, perceptual learning, and video gaming, have been proposed to improve visual function in adult amblyopia. Here, we conducted a meta-analysis of these methods to investigate the factors involved in amblyopia recovery and their clinical significance. METHODS: Mean and individual participant data meta-analyses were performed on 24 studies using the new behavioral methods in adults. Studies were identified using PubMed, Google Scholar, and published reviews. RESULTS: The new methods yielded a mean improvement in visual acuity of 0.17 logMAR with 32% participants achieving gains ≥ 0.2 logMAR, and a mean improvement in stereo sensitivity of 0.01 arcsec-1 with 42% of participants improving ≥2 octaves. The most significant predictor of treatment outcome was visual acuity at the onset of treatment. Participants with more severe amblyopia improved more on visual acuity and less on stereo sensitivity than those with milder amblyopia. Better initial stereo sensitivity was a predictor of greater gains in stereo sensitivity following treatment. Treatment type, amblyopia type, age, and training duration did not have any significant influence on visual and stereo acuity outcomes. CONCLUSIONS: Our analyses showed that some participants may benefit from the new treatments; however, clinical trials are required to confirm these findings. Despite the diverse nature of the new behavioral methods, the lack of significant differences in visual and stereo sensitivity outcomes among them suggests that visual attention-a common element among the varied treatment methods-may play an important role in amblyopia recovery.

A computational theory of da Vinci stereopsis.

In binocular vision, occlusion of one object by another gives rise to monocular occlusions­regions visible only in one eye. Although binocular disparities cannot be computed for these regions, monocular occlusions can be precisely localized in depth and can induce the perception of illusory occluding surfaces. The phenomenon of depth perception from monocular occlusions, known as da Vinci stereopsis, is intriguing, but its mechanisms are not well understood. We first propose a theory of the mechanisms underlying da Vinci stereopsis that is based on the psychophysical and computational literature on monocular occlusions. It postulates, among other principles, that monocular areas are detected explicitly, and depth from occlusions is calculated based on constraints imposed by occlusion geometry. Next, we describe a biologically inspired computational model based on this theory that successfully reconstructs depth in a large range of stimuli and produces results similar to those described in the psychophysical literature. These results demonstrate that the proposed neural architecture could underpin da Vinci stereopsis and other stereoscopic percepts.

da Vinci decoded: does da Vinci stereopsis rely on disparity?

In conventional stereopsis, the depth between two objects is computed based on the retinal disparity in the position of matching points in the two eyes. When an object is occluded by another object in the scene, so that it is visible only in one eye, its retinal disparity cannot be computed. Nakayama and Shimojo (1990) found that a precept of quantitative depth between the two objects could still be established for such stimuli and proposed that this precept is based on the constraints imposed by occlusion geometry. They named this and other occlusion-based depth phenomena "da Vinci stereopsis." Subsequent research found quantitative depth based on occlusion geometry in several other classes of stimuli grouped under the term da Vinci stereopsis. However, Nakayama and Shimojo's findings were later brought into question by Gillam, Cook, and Blackburn (2003), who suggested that quantitative depth in their stimuli was perceived based on conventional disparity. In order to understand whether da Vinci stereopsis relies on one type of mechanism or whether its function is stimulus dependent we examine the nature and source of depth in the class of stimuli used by Nakayama and Shimojo (1990). We use three different psychophysical and computational methods to show that the most likely source for depth in these stimuli is occlusion geometry. Based on these experiments and previous data we discuss the potential mechanisms responsible for processing depth from monocular features in da Vinci stereopsis.

Perceptual asymmetry reveals neural substrates underlying stereoscopic transparency.

We describe a perceptual asymmetry found in stereoscopic perception of overlaid random-dot surfaces. Specifically, the minimum separation in depth needed to perceptually segregate two overlaid surfaces depended on the distribution of dots across the surfaces. With the total dot density fixed, significantly larger inter-plane disparities were required for perceptual segregation of the surfaces when the front surface had fewer dots than the back surface compared to when the back surface was the one with fewer dots. We propose that our results reflect an asymmetry in the signal strength of the front and back surfaces due to the assignment of the spaces between the dots to the back surface by disparity interpolation. This hypothesis was supported by the results of two experiments designed to reduce the imbalance in the neuronal response to the two surfaces. We modeled the psychophysical data with a network of inter-neural connections: excitatory within-disparity and inhibitory across disparity, where the spread of disparity was modulated according to figure-ground assignment. These psychophysical and computational findings suggest that stereoscopic transparency depends on both inter-neural interactions of disparity-tuned cells and higher-level processes governing figure ground segregation.

Disparity biasing in depth from monocular occlusions.

Monocular occlusions have been shown to play an important role in stereopsis. Among other contributions to binocular depth perception, monocular occlusions can create percepts of illusory occluding surfaces. It has been argued that the precise location in depth of these illusory occluders is based on the constraints imposed by occlusion geometry. Tsirlin et al. (2010) proposed that when these constraints are weak, the depth of the illusory occluder can be biased by a neighboring disparity-defined feature. In the present work we test this hypothesis using a variety of stimuli. We show that when monocular occlusions provide only partial constraints on the magnitude of depth of the illusory occluders, the perceived depth of the occluders can be biased by disparity-defined features in the direction unrestricted by the occlusion geometry. Using this disparity bias phenomenon we also show that in illusory occluder stimuli where disparity information is present, but weak, most observers rely on disparity while some use occlusion information instead to specify the depth of the illusory occluder. Taken together our experiments demonstrate that in binocular depth perception disparity and monocular occlusion cues interact in complex ways to resolve perceptual ambiguity.

Monocular occlusions determine the perceived shape and depth of occluding surfaces.

Recent experiments have established that monocular areas arising due to occlusion of one object by another contribute to stereoscopic depth perception. It has been suggested that the primary role of monocular occlusions is to define depth discontinuities and object boundaries in depth. Here we use a carefully designed stimulus to demonstrate empirically that monocular occlusions play an important role in localizing depth edges and defining the shape of the occluding surfaces in depth. We show that the depth perceived via occlusion in our stimuli is not due to the presence of binocular disparity at the boundary and discuss the quantitative nature of depth perception in our stimuli. Our data suggest that the visual system can use monocular information to estimate not only the sign of the depth of the occluding surface but also its magnitude. We also provide preliminary evidence that perceived depth of illusory occluders derived from monocular information can be biased by binocular features.

Perceptual artifacts in random-dot stereograms.

Unrestricted positioning of elements in random-dot stereograms with steep disparity gradients, such as stereo-transparent stereograms depicting overlaid surfaces, can produce perceptual artifacts similar to disparity noise. It is shown that these artifacts hinder the segregation of overlaid surfaces in transparent random-dot stereograms and thus disrupt the perception of stereotransparency. This effect is intensified with increases in the overall element density of the stimuli. We outline the origin of this phenomenon and discuss techniques to prevent such artifacts.

Uses of virtual reality for diagnosis, rehabilitation and study of unilateral spatial neglect: review and analysis.

Unilateral spatial neglect is a disabling condition frequently occurring after stroke. People with neglect suffer from various spatial deficits in several modalities, which in many cases impair everyday functioning. A successful treatment is yet to be found. Several techniques have been proposed in the last decades, but only a few showed long-lasting effects and none could completely rehabilitate the condition. Diagnostic methods of neglect could be improved as well. The disorder is normally diagnosed with pen-and-paper methods, which generally do not assess patients in everyday tasks and do not address some forms of the disorder. Recently, promising new methods based on virtual reality have emerged. Virtual reality technologies hold great opportunities for the development of effective assessment and treatment techniques for neglect because they provide rich, multimodal, and highly controllable environments. In order to stimulate advancements in this domain, we present a review and an analysis of the current work. We describe past and ongoing research of virtual reality applications for unilateral neglect and discuss the existing problems and new directions for development.

Stereoscopic transparency: constraints on the perception of multiple surfaces.

Stereo-transparency is an intriguing, but not well-understood, phenomenon. In the present experiment, we simultaneously manipulated the number of overlaid planes, density of elements, and depth separation between the planes in random dot stereograms to evaluate the constraints on stereoscopic transparency. We used a novel task involving identification of patterned planes among the planes constituting the stimulus. Our data show that observers are capable of segregating up to six simultaneous overlaid surfaces. Increases in element density or number of planes have a detrimental effect on the transparency percept. The effect of increasing the inter-plane disparity is strongly influenced by other stimulus parameters. This latter result can explain a difference in the literature concerning the role of inter-plane disparity in perception of stereo-transparency. We argue that the effects of stimuli parameters on the transparency percept can be accounted for not only by inhibitory interactions, as has been suggested, but also by the inherent properties of disparity detectors.