Transfer of perceptual learning of depth discrimination between local and global stereograms

The eRDS v6 Stereotest and the Vivid Vision Stereo Test: Two New Tests of Stereoscopic Vision

Purpose

To describe two new stereoacuity tests: the eRDS v6 stereotest, a global dynamic random dot stereogram (dRDS) test, and the Vivid Vision Stereo Test version 2 (VV), a local or "contour" stereotest for virtual reality (VR) headsets; and to evaluate the tests' reliability, validity compared to a dRDS standard, and learning effects.

Methods

Sixty-four subjects passed a battery of stereotests, including perceiving depth from RDS. Validity was evaluated relative to a tablet-based dRDS reference test, ASTEROID. Reliability and learning effects were assessed over six sessions.

Results

eRDS v6 was effective at measuring small thresholds (<10 arcsec) and had a moderate correlation (0.48) with ASTEROID. Across the six sessions, test-retest reliability was good, varying from 0.84 to 0.91, but learning occurred across the first three sessions. VV did not measure stereoacuities below 15 arcsec. It had a weak correlation with ASTEROID (0.27), and test-retest reliability was poor to moderate, varying from 0.35 to 0.74; however, no learning occurred between sessions.

Conclusions

eRDS v6 is precise and reliable but shows learning effects. If repeated three times at baseline, this test is well suited as an outcome measure for testing interventions. VV is less precise, but it is easy and rapid and shows no learning. It may be useful for testing interventions in patients who have no global stereopsis.

Translational Relevance

eRDS v6 is well suited as an outcome measure to evaluate treatments that improve adult stereodepth perception. VV can be considered for screening patient with compromised stereovision.

Differences in stereoacuity between crossed and uncrossed disparities reduce with practice

INTRODUCTION

Stereoacuity, like many forms of hyperacuity, improves with practice. We investigated the effects of repeated measurements over multiple visits on stereoacuity using two commonly utilised clinical stereotests, for both crossed and uncrossed disparity stimuli.

METHODS

Participants were adults with normal binocular vision (n = 17) aged between 18 and 50 years. Stereoacuity was measured using the Randot and TNO stereotests on five separate occasions over a six week period. We utilised both crossed and uncrossed stimuli to separately evaluate stereoacuity in both disparity directions. A subset of the subject group also completed a further five visits over an additional six week period. Threshold stereoacuity was determined by the lowest disparity level at which the subjects could correctly identify both the position and disparity direction (crossed or uncrossed) of the stimulus. Data were analysed by repeated measures analysis of variance.

RESULTS

Stereoacuity for crossed and uncrossed stimuli improved significantly across the first five visits (F1,21  = 4.24, p = 0.05). The main effect of disparity direction on stereoacuity was not significant (F1  = 0.02, p = 0.91). However, a significant interaction between disparity direction and stereotest was identified (F1  = 7.92, p = 0.01).

CONCLUSIONS

Stereoacuity measured with both the TNO and Randot stereotests improved significantly over the course of five repetitions. Although differences between crossed and uncrossed stereoacuity were evident, they depended on the stereotest used and reduced or disappeared after repeated measurements. A single measure of stereoacuity is inadequate for properly evaluating adult stereopsis clinically.

An updated review about perceptual learning as a treatment for amblyopia

The purpose of our work is to do an update of recent investigations about amblyopia treatment based on perceptual learning, dichoptic training and videogames. Therefore, we conducted a search of the studies published about this subject in the last six years. The review shows that the investigations during that period have used several kinds of treatments regarding their design (e.g., type of stimulus and context used, duration of the training), and in a wider range of age that also include adults. Most of the studies have found an improvement in some mono and binocular visual functions, such as visual acuity, contrast sensitivity and stereopsis, which for now, it seems advisable that these processes could be used, as an alternative or a complement of the traditional passive therapy. Nevertheless, it would be plausible to conduct additional, controlled and random, clinical trials in order to discover in a more deeply way which perceptive learning method of treatment is more effective for the improvement of visual functions and for how long the effects of the treatment could persist.

Transfer of Perceptual Learning From Local Stereopsis to Global Stereopsis in Adults With Amblyopia: A Preliminary Study

It has long been debated whether the analysis of global and local stereoscopic depth is performed by a single system or by separate systems. Global stereopsis requires the visual system to solve a complex binocular matching problem to obtain a coherent percept of depth. In contrast, local stereopsis requires only a simple matching of similar image features. In this preliminary study, we recruited five adults with amblyopia who lacked global stereopsis and trained them on a computerized local stereopsis depth task for an average of 12 h. Three out of five (60%) participants recovered fine global stereoscopic vision through training. Those who recovered global stereopsis reached a learning plateau more quickly on the local stereopsis task, and they tended to start the training with better initial local stereopsis performance, to improve more on local stereopsis with training, and to have less severe amblyopia. The transfer of learning from local stereopsis to global stereopsis is compatible with an interacting two-stage model.

Associations Between Binocular Depth Perception and Performance Gains in Laparoscopic Skill Acquisition

The ability to perceive differences in depth is important in many daily life situations. It is also of relevance in laparoscopic surgical procedures that require the extrapolation of three-dimensional visual information from two-dimensional planar images. Besides visual-motor coordination, laparoscopic skills and binocular depth perception are demanding visual tasks for which learning is important. This study explored potential relations between binocular depth perception and individual variations in performance gains during laparoscopic skill acquisition in medical students naïve of such procedures. Individual differences in perceptual learning of binocular depth discrimination when performing a random dot stereogram (RDS) task were measured as variations in the slope changes of the logistic disparity psychometric curves from the first to the last blocks of the experiment. The results showed that not only did the individuals differ in their depth discrimination; the extent with which this performance changed across blocks also differed substantially between individuals. Of note, individual differences in perceptual learning of depth discrimination are associated with performance gains from laparoscopic skill training, both with respect to movement speed and an efficiency score that considered both speed and precision. These results indicate that learning-related benefits for enhancing demanding visual processes are, in part, shared between these two tasks. Future studies that include a broader selection of task-varying monocular and binocular cues as well as visual-motor coordination are needed to further investigate potential mechanistic relations between depth perceptual learning and laparoscopic skill acquisition. A deeper understanding of these mechanisms would be important for applied research that aims at designing behavioral interventions for enhancing technology-assisted laparoscopic skills.

Binocular disparity-based learning is retinotopically specific and independent of sleep

Sleep supports the consolidation of recently encoded declarative and procedural memories. An important component of this effect is the repeated reactivation of neuronal ensemble activity elicited during memory encoding. For perceptual learning, however, sleep benefits have only been reported for specific tasks and it is not clear whether sleep targets low-level perceptual, higher-order temporal or attentional aspects of performance. Here, we employed a coarse binocular disparity discrimination task, known to rely on low-level stereoscopic vision. We show that human subjects improve over training and retain the same performance level across a 12-h retention period. Improvements do not generalize to other parts of the visual field and are unaffected by whether the retention period contains sleep or not. These results are compatible with the notion that behavioural improvements in binocular disparity discrimination do not additionally benefit from sleep when compared with the same time spent awake. We hypothesize that this might generalize to other strictly low-level perceptual tasks. This article is part of the Theo Murphy meeting issue 'Memory reactivation: replaying events past, present and future'.

A Random Dot Computer Video Game Improves Stereopsis

SIGNIFICANCE

Currently, treatments for amblyopia are occlusion or penalization of the stronger fellow eye. Fewer than 30% of patients improve stereoacuity using these treatments. In order to improve these outcomes, this group proposes a treatment to stimulate the stereoacuity through perceptual learning in a game format for use at home.

PURPOSE

The aim of this study was to determine whether perceptual learning with random dot stimuli (RDS) in the form of a computer video game improves stereopsis in patients with a history of amblyopia.

METHODS

Thirty-two stereo-deficient patients (7 to 14 years old) previously treated for amblyopia participated in a prospective, randomized, double-blind study. Participants followed a perceptual learning program at home using RDS software. In the experimental group, the demand of stereopsis was increased, until reaching the lowest detectable disparity. In the comparison group, the stimulation interval was a constant (840 to 750"). Stereoacuity was evaluated with the Randot Preschool Stereoacuity Test (RPST) and the Wirt Circles.

RESULTS

Median compliance was 100% (interquartile range [IQR] = 78.50 to 100). Log10 stereoacuity outcomes were significantly different between groups (RPST, P = .041; Wirt Circles Test, P = .009). Median stereoacuity improvement with RPST was 50% (IQR = 0.00 to 75%) and 0% (IQR = 0.00 to 7.5%), respectively, for experimental and comparison groups (P = .008). Wirt Circles improvement was 46.42% (IQR = 6.25 to 73.75%) and 0% (IQR = 0.00 to 57.50%), respectively, for experimental and comparison groups (P = .089). Stereoacuity improvement was not different between groups when success was considered a 70% gain in RPST (P = .113); it was statistically different when success was considered a gain of two levels on Wirt Circles and stereoacuity 140" or less (P = .023). Stereoacuity remained stable after 6 months when measured with RPST, whereas it worsened in two subjects when measured with Wirt Circles.

CONCLUSIONS

Direct stimulation of stereopsis at home using RDS in a game environment improves the stereoacuity in stereo-deficient subjects with a history of amblyopia.

Dressmakers show enhanced stereoscopic vision

The ability to estimate the distance of objects from one’s self and from each other is fundamental to a variety of behaviours from grasping objects to navigating. The main cue to distance, stereopsis, relies on the slight offsets between the images derived from our left and right eyes, also termed disparities. Here we ask whether the precision of stereopsis varies with professional experience with precise manual tasks. We measured stereo-acuities of dressmakers and non-dressmakers for both absolute and relative disparities. We used a stereoscope and a computerized test removing monocular cues. We also measured vergence noise and bias using the Nonius line technique. We demonstrate that dressmakers’ stereoscopic acuities are better than those of non-dressmakers, for both absolute and relative disparities. In contrast, vergence noise and bias were comparable in the two groups. Two non-exclusive mechanisms may be at the source of the group difference we document: (i) self-selection or the fact that stereo-vision is functionally important to become a dressmaker, and (ii) plasticity, or the fact that training on demanding stereovision tasks improves stereo-acuity.

Sharpening coarse-to-fine stereo vision by perceptual learning: asymmetric transfer across the spatial frequency spectrum

Neurons in the early visual cortex are finely tuned to different low-level visual features, forming a multi-channel system analysing the visual image formed on the retina in a parallel manner. However, little is known about the potential 'cross-talk' among these channels. Here, we systematically investigated whether stereoacuity, over a large range of target spatial frequencies, can be enhanced by perceptual learning. Using narrow-band visual stimuli, we found that practice with coarse (low spatial frequency) targets substantially improves performance, and that the improvement spreads from coarse to fine (high spatial frequency) three-dimensional perception, generalizing broadly across untrained spatial frequencies and orientations. Notably, we observed an asymmetric transfer of learning across the spatial frequency spectrum. The bandwidth of transfer was broader when training was at a high spatial frequency than at a low spatial frequency. Stereoacuity training is most beneficial when trained with fine targets. This broad transfer of stereoacuity learning contrasts with the highly specific learning reported for other basic visual functions. We also revealed strategies to boost learning outcomes 'beyond-the-plateau'. Our investigations contribute to understanding the functional properties of the network subserving stereovision. The ability to generalize may provide a key principle for restoring impaired binocular vision in clinical situations.

Variation of stereothreshold with random-dot stereogram density

PURPOSE

Reports of dissimilar stereothresholds for contour and random-dot (RD) targets may reflect differences in stimulus properties or differences between local and global stereoscopic processing mechanisms. In this study, we evaluated whether the stereothresholds obtained using low- and high-density RD stimuli are consistent with a distinction between local and global disparity processing.

METHODS

Stereothresholds were measured in eight normal subjects for a small disparate line segment superimposed on RD surrounds with densities that ranged between 0.07 and 28.3%.

RESULTS

Stereothresholds averaged 0.23 arc min for an RD density of 0.39% and approximately doubled for lower and higher densities. The increase in stereothresholds at low densities is likely because of the increased spacing between elements, which reduces their usefulness as a reference for relative disparity judgments. The increase in stereothresholds at high densities is attributed to a crowding effect.

CONCLUSIONS

Because the stereothresholds measured with RD stimuli of low and high density are limited by different constraints, they can be considered to be different types of stereotargets. However, because the stereothresholds measured for RD targets of varying densities are similar to those determined previously for a local, two-rod stereotarget, it is likely that all these stimuli are processed by a single disparity-processing mechanism.