Mechanisms of perceptual learning of depth discrimination in random dot stereograms

Association between stereoacuity and simulated clinical performance among dental students: an exploratory investigation

INTRODUCTION

Dentistry is a profession that requires a high level of the hand-eye coordination. Although considerable attention has been put on dental students' manual dexterity, little emphasis was made on students' visual competency as a variable influencing their competency. In this study, we aimed to assess the association between dental students' stereopsis and their pre-clinical performance.

METHODS

Dental students (n = 49) were required to perform the porcelain-fused-to-crown preparation for a lower first molar and an upper central incisor plastic teeth mounted on phantom heads. Performance was assessed by two faculty members based on two rubrics, which collectively included 11 assessment criteria. Afterward, students undertook a stereoacuity test using Randot stereotest. Stereopsis values greater than 40 arc/s were considered abnormal.

RESULTS

A significant negative correlation was seen between stereopsis scores and students' grades for the upper incisor (p < 0.01), but not for the lower molar. An association was also seen between abnormal stereopsis and students' poor performance in preparing the finish line of the incisor tooth (p = 0.024).

CONCLUSION

Abnormal stereopsis can impact dental students' performance, particularly when preparing teeth indirectly using a dental mirror. Dental educators should give attention to students' stereoacuity, in a manner similar to that given to students' manual dexterity.

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.

Abnormal effective connectivity in visual cortices underlies stereopsis defects in amblyopia

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Abnormal effective connectivity inherent stereopsis defects in amblyopia was studied.

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A weakened connection from V2v to LO2 relates to stereopsis defects in amblyopia.

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Higher-order visual cortices may serve as key nodes to the stereopsis defects.

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An independent longitudinal dataset was used to validate the obtained results.

The neural basis underlying stereopsis defects in patients with amblyopia remains unclear, which hinders the development of clinical therapy. This study aimed to investigate visual network abnormalities in patients with amblyopia and their associations with stereopsis function. Spectral dynamic causal modeling methods were employed for resting-state functional magnetic resonance imaging data to investigate the effective connectivity (EC) among 14 predefined regions of interest in the dorsal and ventral visual pathways. We adopted two independent datasets, including a cross-sectional and a longitudinal dataset. In the cross-sectional dataset, we compared group differences in EC between 31 patients with amblyopia (mean age: 26.39 years old) and 31 healthy controls (mean age: 25.71 years old) and investigated the association between EC and stereoacuity. In addition, we explored EC changes after perceptual learning in a novel longitudinal dataset including 9 patients with amblyopia (mean age: 15.78 years old). We found consistent evidence from the two datasets indicating that the aberrant EC from V2v to LO2 is crucial for the stereoscopic deficits in the patients with amblyopia: it was weaker in the patients than in the controls, showed a positive linear relationship with the stereoscopic function, and increased after perceptual learning in the patients. In addition, higher-level dorsal (V3d, V3A, and V3B) and ventral areas (LO1 and LO2) were important nodes in the network of abnormal ECs associated with stereoscopic deficits in the patients with amblyopia. Our research provides insights into the neural mechanism underlying stereopsis deficits in patients with amblyopia and provides candidate targets for focused stimulus interventions to enhance the efficacy of clinical treatment for the improvement of stereopsis deficiency.

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.

An Evaluation of the Agreement Between a Computerized Stereoscopic Game Test and the TNO Stereoacuity Test

PURPOSE

Stereo-anomaly is commonly associated with amblyopia. An investigation was conducted to determine whether the measurements of stereoacuity obtained with the stereoacuity reference test (TNO Test) show an agreement with a computer stereoscope video game.

METHODS

Thirty-two subjects (mean age 9.37±2.00 years) with an amblyopia history were selected for a blind and randomized study of stereoacuity improvement through a new random dot game. A masked examiner measured the stereoacuity three times per subject using the TNO test (at the beginning, at the end and after 6 months of the treatment). A second masked examiner measured stereoacuity using the new computerized game after the TNO masked evaluation.

RESULTS

The Pearson's correlation coefficient one test against the other was r² = 0.767 and the Bland-Altman plot was r²= 0.069 (mean difference -0.03 log sec). Using three categories: poor (840-300 seconds of arc), coarse (480-210 seconds of arc) and moderate-fine stereoacuity (210-30 seconds of arc). Positive predictive values were 89.5% for moderate-fine; 72.7% for coarse; and 90.0% for poor stereoacuity. In addition, the agreement was evaluated using the Kappa coefficient (K= 0.743) with a 0.95 confidence interval and lower and upper Kappa limits were (0.628 and 0.858), respectively. Kappa coefficient and limits were still good when analyzing data before (K =0.663, 0.420 and 0.906) and after the treatment (K= 0.765, 0.632 and 0.899).

CONCLUSION

The Computerized Stereoscopic Game test allows the measure of stereoacuity. It can be used for both the purpose of detecting stereo vision deficits or tracking stereo vision development.

General and Specific Effects of Stereo Learning

Technological advancements in virtual reality challenge the human vision, especially stereopsis, a function, which characterizes how two eyes coordinate to form a unified three-dimensional (3D) representation of the external world and is found to be deficient in 30% of the normal population. Although a few previous studies have consistently found that the perceptual learning of stereopsis significantly improved stereoacuity, an underlying mechanism of stereo learning remains heavily debated. Here, we trained subjects with normal stereo vision (assessed with the FLY Stereo Acuity Test) to judge stereopsis containing three types of binocular disparity orders (i.e., zero-, first-, and second-order), aiming to systematically examine the characteristics and plasticity of stereo learning. Thirty subjects were randomly assigned to the three training groups (each for the zero-, first-, or second-order disparity separately). The disparity thresholds were measured before and after training. The disparity threshold was measured in 10 additional control subjects only at the pre- and post-training phase. Stereoscopic images were displayed through a shutter goggle, which is synchronized to a monitor. We found that the training significantly improved the zero-, first-, and second-order disparity threshold by 52.42, 36.28, and 14.9% in the zero-order training condition; 30.44, 63.74, and 21.07% in the first-order training condition; and 30.77, 25.19, and 75.12% in the second-order training condition, respectively. There was no significant improvement in the control group. Interestingly, the greatest improvements in the first- and second-order disparity threshold were found in the corresponding disparity training group; on the contrary, the improvements in the zero-order disparity threshold were comparable across all the three disparity training groups. Our findings demonstrated both general (related to the zero-order disparity) and specific improvements (related to the first- and second-order disparity) in stereo learning, suggesting that stereo training occurs at different visual processing stages and its effects might depend on the specific training sites.

Detection of Static and Dynamic Stereopsis after Femtosecond Laser Small Incision Lenticule Extraction for High Myopia

Purpose

The purpose of this study is to test binocular visual function after femtosecond laser small incision lenticule extraction (SMILE) for high myopia. The traditional Titmus stereotest and dynamic stereotest based on the visual perception biological model were used for comparative analysis.

Methods

A total of 43 patients were enrolled in this prospective study. At Week 1, Month 1, and Month 3 after surgery, the Titmus stereotest and dynamic stereotest generated by MATLAB were conducted. Dynamic stereopsis consists of randomly flickering Gabor spots and is divided into two models of high energy and low energy according to flicker frequency.

Results

The preoperative manifest refraction spherical equivalent was −7.21 ± 0.70 D. The preoperative anisometropia was 0.52 ± 0.54D. The quartiles of static stereoacuity in preoperation and 3 follow-ups were as follows: 50.00 (25.00, 100.00) in preoperation, 63.00 (40.00, 63.00) at Week 1, 40.00 (32.00, 63.00) at Month 1, and 40.00 (25.00, 50.00) at Month 3. Static stereopsis improved at Month 1 and Month 3 compared with preoperation and Week 1 (P < 0.05). There were statistically significant differences in high energy dynamic stereopsis at Week 1 and Month 1 compared to preoperation (P < 0.05). In addition, significant differences in low energy dynamic stereopsis were detected between Month 1 and preoperation and also at Month 3 compared to Month 1 (P < 0.05).

Conclusion

Most high myopia patients have a dynamic stereopsis deficiency before refractive correction. SMILE surgery can improve both static and dynamic stereopsis early in the postoperation period. However, in the long term, there is no significant difference or even a decrease in dynamic stereopsis.

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.

Willingness-to-Pay-Associated Right Prefrontal Activation During a Single, Real Use of Cosmetics as Revealed by Functional Near-Infrared Spectroscopy

Use of applied neuroscience to complement traditional methods of consumer research is increasing. Previously, fMRI has shown that prefrontal activity contains information relating to willingness-to-pay (WTP). The aim of the present study was to determine if functional near infrared spectroscopy (fNIRS) can record WTP-related brain activation in the dorsolateral prefrontal cortex (DLPFC) during a single, real use of cosmetic products. Thirty female participants, were divided into two groups (one low frequency users of foundation and one high frequency users of foundation), asked to apply different foundations to their face and then record how much money they were willing to pay. The oxyhemoglobin time series was analyzed with the GLM and the correlation between the beta scores for the foundations and their respective WTP values conducted for each participant. These subject level correlations were then converted to z scores and averaged for each group. The results revealed a significant mean correlation for the high but not low frequency group. In other words, the brain activity in right hemisphere dorsolateral PFC (RH-DLPFC) during single, real use of foundations correlated with their respective WTP values for the high frequency but not low frequency group. The difference between groups may reflect the importance of learning and automation on activity in RH-DLPFC. Our research provides further evidence supporting the use of fNIRS to complement traditional consumer research in a commercial setting and to extend neuroscience research into more naturalistic environments.

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.

Perceptual and motor learning underlies human stick-balancing skill

We investigated the acquisition of skill in balancing a stick (52 cm, 34 g) on the fingertip in nine participants using three-dimensional motion analysis. After 3.5 h of practice over 6 wk, the participants could more consistently balance the stick for longer durations with greatly reduced magnitude and speed of stick and finger movements. Irrespective of level of skill, the balanced stick behaved like a normal noninverted pendulum oscillating under greater-than-gravity torque with simple harmonic motion about a virtual pivot located at the radius of gyration above the center of mass. The control input parameter was the magnitude ratio between the torque applied on the stick by the participant and the torque due to gravity. The participants utilized only a narrow range of this parameter, which did not change with practice, to rotate the stick like a linear mass-spring system. With increased skill, the stick therefore maintained the same period of oscillation but showed marked reductions in magnitude of both oscillation and horizontal translation. Better balancing was associated with 1) more accurate visual localization of the stick and proprioceptive localization of the finger and 2) reduced cross-coupling errors between finger and stick movements in orthogonal directions; i.e., finger movements in the anteroposterior plane became less coupled with stick tip movements in the mediolateral plane, and vice versa. Development of this fine motor skill therefore depended on perceptual and motor learning to provide improved estimation of sensorimotor state and precision of motor commands to an unchanging internal model of the rotational dynamics.

Screening and sampling in studies of binocular vision

Binocular deficits are relatively common within a typical sample of observers. This has implications for research on binocular vision, as a variety of stereo deficits can affect performance. Despite this, there is no agreed standard for testing stereo capabilities in observers and many studies do not report visual abilities at all. Within the stereo literature, failure to report screening and sampling has the potential to undermine the results of otherwise strictly controlled research. We reviewed research articles on binocular vision published in three journals between 2000 and 2008 to illustrate how screening for binocular deficits and sampling of participants is approached. Our results reveal that 44% of the studies do not mention screening for stereo deficits and 91% do not report selection of participants. The percentage of participants excluded from studies that report stereo screening amounts to 3.9% and 0.7% for studies that do not report stereo screening. These low numbers contrast with the exclusion of 17.6% of participants in studies that report screening for binocular deficits as well as selection of participants. We discuss various options for stereo testing and the need for stereo-motion testing with reference to recent research on binocular perception.

Recovery of stereopsis through perceptual learning in human adults with abnormal binocular vision

Stereopsis, the perception of depth based on the disparity of the images projected to the retinas of the two eyes, is an important process in our three-dimensional world; however, 3-5% of the population is stereoblind or has seriously impaired stereovision. Here we provide evidence for the recovery of stereopsis through perceptual learning, the repetitive practice of a demanding visual task, in human adults long deprived of normal binocular vision. We used a training paradigm that combines monocular cues that were correlated perfectly with the disparity cues. Following perceptual learning (thousands of trials) with stereoscopic gratings, five adults who initially were stereoblind or stereoanomalous showed substantial recovery of stereopsis, both on psychophysical tests with stimuli that contained no monocular cues and on clinical testing. They reported that depth "popped out" in daily life, and enjoyed 3D movies for the first time. After training, stereo tests with dynamic random-dot stereograms and band-pass noise revealed the properties of the recovered stereopsis: It has reduced resolution and precision, although it is based on perceiving depth by detecting binocular disparity. We conclude that some human adults deprived of normal binocular vision can recover stereopsis at least partially.

Effectively reducing sensory eye dominance with a push-pull perceptual learning protocol

Much knowledge of sensory cortical plasticity is gleaned from perceptual learning studies that improve visual performance [1-7]. Although the improvements are likely caused by modifications of excitatory and inhibitory neural networks, most studies were not primarily designed to differentiate their relative contributions. Here we designed a novel push-pull training protocol to reduce sensory eye dominance (SED), a condition that is mainly caused by unbalanced interocular inhibition [8-10]. During the training, an attention cue presented to the weak eye precedes the binocular competitive stimulation. The cue stimulates the weak eye (push) while causing interocular inhibition of the strong eye (pull). We found that this push-pull protocol reduces SED (shifts the balance toward the weak eye) and improves stereopsis more so than the push-only protocol, which solely stimulates the weak eye without inhibiting the strong eye. The stronger learning effect with the push-pull training than the push-only training underscores the crucial involvement of a putative inhibitory mechanism in sensory plasticity. The design principle of the push-pull protocol can potentially lend itself as an effective, noninvasive treatment of amblyopia.

The precision of binocular and monocular depth judgments in natural settings

We measured binocular and monocular depth thresholds for objects presented in a real environment. Observers judged the depth separating a pair of metal rods presented either in relative isolation, or surrounded by other objects, including a textured surface. In the isolated setting, binocular thresholds were greatly superior to the monocular thresholds by as much as a factor of 18. The presence of adjacent objects and textures improved the monocular thresholds somewhat, but the superiority of binocular viewing remained substantial (roughly a factor of 10). To determine whether motion parallax would improve monocular sensitivity for the textured setting, we asked observers to move their heads laterally, so that the viewing eye was displaced by 8-10 cm; this motion produced little improvement in the monocular thresholds. We also compared disparity thresholds measured with the real rods to thresholds measured with virtual images in a standard mirror stereoscope. Surprisingly, for the two naive observers, the stereoscope thresholds were far worse than the thresholds for the real rods-a finding that indicates that stereoscope measurements for unpracticed observers should be treated with caution. With practice, the stereoscope thresholds for one observer improved to almost the precision of the thresholds for the real rods.

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

Several previous studies reported differences when stereothresholds are assessed with local-contour stereograms vs. complex random-dot stereograms (RDSs). Dissimilar thresholds may be due to differences in the properties of the stereograms (e.g. spatial frequency content, contrast, inter-element separation, area) or to different underlying processing mechanisms. This study examined the transfer of perceptual learning of depth discrimination between local and global RDSs with similar properties, and vice versa. If global and local stereograms are processed by separate neural mechanisms, then the magnitude and rate of training for the two types of stimuli are likely to differ, and the transfer of training from one stimulus type to the other should be minimal. Based on previous results, we chose RDSs with element densities of 0.17% and 28.3% to serve as the local and global stereograms, respectively. Fourteen inexperienced subjects with normal binocular vision were randomly assigned to either a local- or global- RDS training group. Stereothresholds for both stimulus types were measured before and after 7700 training trials distributed over 10 sessions. Stereothresholds for the trained condition improve for approximately 3000 trials, by an average of 0.36+/-0.08 for local and 0.29+/-0.10 for global RDSs, and level off thereafter. Neither the rate nor the magnitude of improvement differ statistically between the local- and global-training groups. Further, no significant difference exists in the amount of improvement on the trained vs. the untrained targets for either training group. These results are consistent with the operation of a single mechanism to process both local and global stereograms.

Relationship between threshold and suprathreshold perception of position and stereoscopic depth

We seek to determine the relationship between threshold and suprathreshold perception for position offset and stereoscopic depth perception under conditions that elevate their respective thresholds. Two threshold-elevating conditions were used: (1) increasing the interline gap and (2) dioptric blur. Although increasing the interline gap increases position (Vernier) offset and stereoscopic disparity thresholds substantially, the perception of suprathreshold position offset and stereoscopic depth remains unchanged. Perception of suprathreshold position offset also remains unchanged when the Vernier threshold is elevated by dioptric blur. We show that such normalization of suprathreshold position offset can be attributed to the topographical-map-based encoding of position. On the other hand, dioptric blur increases the stereoscopic disparity thresholds and reduces the perceived suprathreshold stereoscopic depth, which can be accounted for by a disparity-computation model in which the activities of absolute disparity encoders are multiplied by a Gaussian weighting function that is centered on the horopter. Overall, the statement "equal suprathreshold perception occurs in threshold-elevated and unelevated conditions when the stimuli are equally above their corresponding thresholds" describes the results better than the statement "suprathreshold stimuli are perceived as equal when they are equal multiples of their respective threshold values."