Temporal integration for stereoscopic vision

Optimal Stereoacuity Reveals More Than Critical Time in Patients With Intermittent Exotropia

Synopsis

Both optimal stereoacuity and integration time to achieve that are impaired in patients with intermittent exotropia. The deterioration of stereoacuity is more revealing since it correlates well with exotropia control score.

Background

Despite the periodic misalignment of two eyes, some intermittent exotropia (IXT) patients exhibit normal stereoacuity, particularly when evaluated with static tests. It is not clear if the temporal integration process of stereopsis is altered in IXT patients, thus warranting further research.

Methods

IXT patients (n = 29) and age-matched normal controls (n = 36) were recruited. Static stereopsis was measured with the Titmus stereoacuity test. In computer-generated random dots tests, stereoacuity was measured with a stimuli presentation duration varying from 100 to 1,200 ms. And the relationship between stereoacuity and stimuli duration was fitted into a quadratic model. Optimal stereoacuity was achieved when fitted curve flattened and the critical integration time was the duration needed to achieve optimal stereoacuity.

Results

IXT patients were not found to differ significantly from control subjects under the Titmus test, while the Random Dots stereotest showed significantly worse optimal stereoacuity and significantly longer critical integration time. Multiple regression analysis showed that age (R = −4.83; P = 0.04) had statistically significant negative correlation on the critical integration time, age (R = −6.45; P = 0.047) and exotropia control scores (R = 60.71; P = 0.007) had statistically significant effects on optimal stereoacuity.

Conclusion

The temporal integration for stereopsis is impaired in IXT patients, requiring longer critical integration time to achieve elevated optimal stereoacuity.

Impact of intraocular scatter on stereopsis

To investigate the effect of intraocular scatter on stereopsis, we measured stereoacuity with scatter either remaining at the natural eye-induced level or further enhanced by a set of three different scatter filters. Stereo thresholds at different viewing durations were obtained using a psychophysical method of constant stimuli. The results indicate that stereoacuity was degraded with a binocular increase in scatter levels for all the subjects. Measurements were also performed in the presence of interocular differences in scatter levels. In this case, stereoacuity was found to be even a little worse than that obtained with both eyes at a higher scatter level. This suggests that stereoacuity in the presence of interocular differences in scatter levels is mainly dependent on the higher scatter level.

Short-term visual memory for location in depth: A U-shaped function of time

Short-term visual memory was studied by displaying arrays of four or five numerals, each numeral in its own depth plane, followed after various delays by an arrow cue shown in one of the depth planes. Subjects reported the numeral at the depth cued by the arrow. Accuracy fell with increasing cue delay for the first 500 ms or so, and then recovered almost fully. This dipping pattern contrasts with the usual iconic decay observed for memory traces. The dip occurred with or without a verbal or color-shape retention load on working memory. In contrast, accuracy did not change with delay when a tonal cue replaced the arrow cue. We hypothesized that information concerning the depths of the numerals decays over time in sensory memory, but that cued recall is aided later on by transfer to a visual memory specialized for depth. This transfer is sufficiently rapid with a tonal cue to compensate for the sensory decay, but it is slowed by the need to tag the arrow cue's depth relative to the depths of the numerals, exposing a dip when sensation has decayed and transfer is not yet complete. A model with a fixed rate of sensory decay and varied transfer rates across individuals captures the dip as well as the cue modality effect.

Simple reaction times to cyclopean stimuli reveal that the binocular system is tuned to react faster to near than to far objects

Binocular depth perception is an important mechanism to segregate the visual scene for mapping relevant objects in our environment. Convergent evidence from psychophysical and neurophysiological studies have revealed asymmetries between the processing of near (crossed) and far (uncrossed) binocular disparities. The aim of the present study was to test if near or far objects are processed faster and with higher contrast sensitivity in the visual system. We therefore measured the relationship between binocular disparity and simple reaction time (RT) as well as contrast gain based on the contrast-RT function in young healthy adults. RTs were measured to suddenly appearing cyclopean target stimuli, which were checkerboard patterns encoded by depth in dynamic random dot stereograms (DRDS). The DRDS technique allowed us to selectively study the stereoscopic processing system by eliminating all monocular cues. The results showed that disparity and contrast had significant effects on RTs. RTs as a function of disparity followed a U-shaped tuning curve indicating an optimum at around 15 arc min, where RTs were minimal. Surprisingly, the disparity tuning of RT was much less pronounced for far disparities. At the optimal disparity, we measured advantages of about 80 ms and 30 ms for near disparities at low (10%) and high (90%) contrasts, respectively. High contrast always reduced RTs as well as the disparity dependent differences. Furthermore, RT-based contrast gains were higher for near disparities in the range of disparities where RTs were the shortest. These results show that the sensitivity of the human visual system is biased for near versus far disparities and near stimuli can result in faster motor responses, probably because they bear higher biological relevance.

Evidence for the effect of depth on visual working memory

Visual working memory (VWM) is a cognitive memory buffer for temporarily holding, processing, and manipulating visual information. Previous studies have demonstrated mixed results of the effect of depth perception on VWM, with some showing a beneficial effect while others not. In this study, we employed an adapted change detection paradigm to investigate the effects of two depth cues, binocular disparity and relative size. The memory array consisted of a set of pseudo-randomly positioned colored items, and the task was to judge whether the test item was changed compared to the memory item after a retention interval. We found that presenting the items in stereoscopic depth alone hardly affected VWM performance. When combining the two coherent depth cues, a significant larger VWM capacity of the perceptually closer-in-depth items was observed than that of the farther items, but the capacity for the two-depth-planes condition was not significantly different from that for the one-plane condition. Conflicting the two depth cues resulted in cancelling the beneficial effect of presenting items at a closer depth plane. The results indicate that depth perception could affect VWM, and the visual system may have an advantage in maintaining closer-in-depth objects in working memory.

Temporal integration property of stereopsis after higher-order aberration correction

Based on a binocular adaptive optics visual simulator, we investigated the effect of higher-order aberration correction on the temporal integration property of stereopsis. Stereo threshold for line stimuli, viewed in 550nm monochromatic light, was measured as a function of exposure duration, with higher-order aberrations uncorrected, binocularly corrected or monocularly corrected. Under all optical conditions, stereo threshold decreased with increasing exposure duration until a steady-state threshold was reached. The critical duration was determined by a quadratic summation model and the high goodness of fit suggested this model was reasonable. For normal subjects, the slope for stereo threshold versus exposure duration was about -0.5 on logarithmic coordinates, and the critical duration was about 200 ms. Both the slope and the critical duration were independent of the optical condition of the eye, showing no significant effect of higher-order aberration correction on the temporal integration property of stereopsis.

The Effect of Cataract on Early Stage Glaucoma Detection Using Spatial and Temporal Contrast Sensitivity Tests

BACKGROUND

To investigate the effect of cataract on the ability of spatial and temporal contrast sensitivity tests used to detect early glaucoma.

METHODS

Twenty-seven glaucoma subjects with early cataract (mean age 60 ± 10.2 years) which constituted the test group were recruited together with twenty-seven controls (cataract only) matched for age and cataract type from a primary eye care setting. Contrast sensitivity to flickering gratings at 20 Hz and stationary gratings with and without glare, were measured for 0.5, 1.5 and 3 cycles per degree (cpd) in central vision. Perimetry and structural measurements with the Heidelberg Retinal Tomograph (HRT) were also performed.

RESULTS

After considering the effect of cataract, contrast sensitivity to stationary gratings was reduced in the test group compared with controls with a statistically significant mean difference of 0.2 log units independent of spatial frequency. The flicker test showed a significant difference between test and control group at 1.5 and 3 cpd (p = 0.019 and p = 0.011 respectively). The percentage of glaucoma patients who could not see the temporal modulation was much higher compared with their cataract only counterparts. A significant correlation was found between the reduction of contrast sensitivity caused by glare and the Glaucoma Probability Score (GPS) as measured with the HRT (p<0.005).

CONCLUSIONS

These findings indicate that both spatial and temporal contrast sensitivity tests are suitable for distinguishing between vision loss as a consequence of glaucoma and vision loss caused by cataract only. The correlation between glare factor and GPS suggests that there may be an increase in intraocular stray light in glaucoma.

Self-motion perception in the elderly

Self-motion through space generates a visual pattern called optic flow. It can be used to determine one's direction of self-motion (heading). Previous studies have already shown that this perceptual ability, which is of critical importance during everyday life, changes with age. In most of these studies subjects were asked to judge whether they appeared to be heading to the left or right of a target. Thresholds were found to increase continuously with age. In our current study, we were interested in absolute rather than relative heading judgments and in the question about a potential neural correlate of an age-related deterioration of heading perception. Two groups, older test subjects and younger controls, were shown optic flow stimuli in a virtual-reality setup. Visual stimuli simulated self-motion through a 3-D cloud of dots and subjects had to indicate their perceived heading direction after each trial. In different subsets of experiments we varied individually relevant stimulus parameters: presentation time, number of dots in the display, stereoscopic vs. non-stereoscopic stimulation, and motion coherence. We found decrements in heading performance with age for each stimulus parameter. In a final step we aimed to determine a putative neural basis of this behavioral decline. To this end we modified a neural network model which previously has proven to be capable of reproduce and predict certain aspects of heading perception. We show that the observed data can be modeled by implementing an age related neuronal cell loss in this neural network. We conclude that a continuous decline of certain aspects of motion perception, among them heading, might be based on an age-related progressive loss of groups of neurons being activated by visual motion.

Vergence eye movements are not essential for stereoscopic depth

The brain receives disparate retinal input owing to the separation of the eyes, yet we usually perceive a single fused world. This is because of complex interactions between sensory and oculomotor processes that quickly act to reduce excessive retinal disparity. This implies a strong link between depth perception and fusion, but it is well established that stereoscopic depth percepts are also obtained from stimuli that produce double images. Surprisingly, the nature of depth percepts from such diplopic stimuli remains poorly understood. Specifically, despite long-standing debate it is unclear whether depth under diplopia is owing to the retinal disparity (directly), or whether the brain interprets signals from fusional vergence responses to large disparities (indirectly). Here, we addressed this question using stereoscopic afterimages, for which fusional vergence cannot provide retinal feedback about depth. We showed that observers could reliably recover depth sign and magnitude from diplopic afterimages. In addition, measuring vergence responses to large disparity stimuli revealed that that the sign and magnitude of vergence responses are not systematically related to the target disparity, thus ruling out an indirect explanation of our results. Taken together, our research provides the first conclusive evidence that stereopsis is a direct process, even for diplopic targets.

Is visual short-term memory depthful?

Does visual short-term memory (VSTM) depend on depth, as it might be if information was stored in more than one depth layer? Depth is critical in natural viewing and might be expected to affect retention, but whether this is so is currently unknown. Cued partial reports of letter arrays (Sperling, 1960) were measured up to 700 ms after display termination. Adding stereoscopic depth hardly affected VSTM capacity or decay inferred from total errors. The pattern of transposition errors (letters reported from an uncued row) was almost independent of depth and cue delay. We conclude that VSTM is effectively two-dimensional.

Angular declination and the dynamic perception of egocentric distance

The extraction of the distance between an object and an observer is fast when angular declination is informative, as it is with targets placed on the ground. To what extent does angular declination drive performance when viewing time is limited? Participants judged target distances in a real-world environment with viewing durations ranging from 36-220 ms. An important role for angular declination was supported by experiments showing that the cue provides information about egocentric distance even on the very first glimpse, and that it supports a sensitive response to distance in the absence of other useful cues. Performance was better at 220-ms viewing durations than for briefer glimpses, suggesting that the perception of distance is dynamic even within the time frame of a typical eye fixation. Critically, performance in limited viewing trials was better when preceded by a 15-s preview of the room without a designated target. The results indicate that the perception of distance is powerfully shaped by memory from prior visual experience with the scene. A theoretical framework for the dynamic perception of distance is presented.

The role of binocular disparity in rapid scene and pattern recognition

We investigated the contribution of binocular disparity to the rapid recognition of scenes and simpler spatial patterns using a paradigm combining backward masked stimulus presentation and short-term match-to-sample recognition. First, we showed that binocular disparity did not contribute significantly to the recognition of briefly presented natural and artificial scenes, even when the availability of monocular cues was reduced. Subsequently, using dense random dot stereograms as stimuli, we showed that observers were in principle able to extract spatial patterns defined only by disparity under brief, masked presentations. Comparing our results with the predictions from a cue-summation model, we showed that combining disparity with luminance did not per se disrupt the processing of disparity. Our results suggest that the rapid recognition of scenes is mediated mostly by a monocular comparison of the images, although we can rely on stereo in fast pattern recognition.

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.

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."

Mechanisms of perceptual learning of depth discrimination in random dot stereograms

Perceptual learning is a training induced improvement in performance. Mechanisms underlying the perceptual learning of depth discrimination in dynamic random dot stereograms were examined by assessing stereothresholds as a function of decorrelation. The inflection point of the decorrelation function was defined as the level of decorrelation corresponding to 1.4 times the threshold when decorrelation is 0%. In general, stereothresholds increased with increasing decorrelation. Following training, stereothresholds and standard errors of measurement decreased systematically for all tested decorrelation values. Post training decorrelation functions were reduced by a multiplicative constant (approximately 5), exhibiting changes in stereothresholds without changes in the inflection points. Disparity energy model simulations indicate that a post-training reduction in neuronal noise can sufficiently account for the perceptual learning effects. In two subjects, learning effects were retained over a period of six months, which may have application for training stereo deficient subjects.

Seeing at a glance, smelling in a whiff: rapid forms of perceptual decision making

Intuitively, decisions should always improve with more time for the accumulation of evidence, yet psychophysical data show a limit of 200-300 ms for many perceptual tasks. Here, we consider mechanisms that favour such rapid information processing in vision and olfaction. We suggest that the brain limits some types of perceptual processing to short, discrete chunks (for example, eye fixations and sniffs) in order to facilitate the construction of global sensory images.

Stereo dynamics are not scale-dependent

The experiments reported here focus on the temporal dynamics of stereopsis in an effort to shed light on how low level mechanisms might contribute to the execution of coarse-to-fine processing in the human stereo system. Because previous studies have used a variety of stimuli and configurations, we assess the effect of exposure duration on stereo thresholds using band-limited Gabor patches for a range of stimulus configurations. In preliminary studies, we found that the best stereo sensitivity-spatial frequency relationship was obtained when using configurations in which the size and target-reference spacing were consistent with spatially scaled stimuli. Sub-optimal stereo sensitivity as a function of spatial frequency was observed when the size and separation were fixed. Further, we found that the temporal properties of stereopsis were consistently sustained in nature irrespective of the stimulus spatial frequency content. This latter finding suggests that if coarse-to-fine stereo processing does occur it does not follow as a consequence of the dynamics of low-level disparity transduction.

Dynamic properties of orientation discrimination assessed by using classification images

Recent physiological studies indicate that the tuning properties of neurons under acute preparation in primary visual cortex can change over time. We used a psychophysical reverse correlation paradigm to examine the potential repercussions of this neuronal property for human observers' ability to discriminate the orientation of targets over time. Observers were required to identify the orientation of a Gabor target presented within dynamic white noise. Frames from the noise movies were pooled to compute dynamic classification images (CIs) associated with the observers' discrimination performance, which then were fit with a weighted difference-of-Gabor function. Best-fitting templates were temporally bandpass, tuned to more oblique orientations than the stimulus but, crucially, did not change over time. The results suggest that the template for orientation discrimination is selected within the first 50 ms of stimulus onset and that, unlike the response of single cells, there is no measurable dynamic component to either orientation or spatial frequency tuning of human orientation discrimination.

Quantitative perceived depth from sequential monocular decamouflage

We present a novel binocular stimulus without conventional disparity cues whose presence and depth are revealed by sequential monocular stimulation (delay > or = 80 ms). Vertical white lines were occluded as they passed behind an otherwise camouflaged black rectangular target. The location (and instant) of the occlusion event, decamouflaging the target's edges, differed in the two eyes. Probe settings to match the depth of the black rectangular target showed a monotonic increase with simulated depth. Control tests discounted the possibility of subjects integrating retinal disparities over an extended temporal window or using temporal disparity. Sequential monocular decamouflage was found to be as precise and accurate as conventional simultaneous stereopsis with equivalent depths and exposure durations.

Spatial and temporal properties of stereoscopic surface interpolation

It is well established that under a wide range of conditions when a sparse collection of texture elements varies smoothly in depth, the spaces between the elements are assigned depth values. This disparity interpolation process has been studied in an effort to define some of its fundamental spatial and temporal constraints. To assess disparity interpolation we employed two tasks: a novel task that relies on the bisection of illusory boundaries created when subjective stereoscopic surfaces intersect, and one that relies on a 3-D shape discrimination. The results of both experiments show that there is no improvement in performance when texture density is increased from near 0.20 to 0.85 or when exposure duration is increased from 50-100 to 1000 ms. This lack of dependence on the addition of features that define the interpolated surface, along with the abrupt decline in performance below a critical value, is consistent with the view that surface interpolation is an important function of human stereoscopic vision.

Stereoscopic depth magnitude estimation: effects of stimulus spatial frequency and eccentricity

To determine the effects of stimulus spatial frequency and retinal eccentricity on the perception of depth magnitude derived from disparity cues alone, subjects were asked to estimate the magnitude of depth of a stereoscopically viewed Gabor patch presented to the central or peripheral field with either crossed or uncrossed absolute disparity. Disparity vergence responses to the same Gabor stimuli were separately estimated subjectively by determining the offset required for dichoptic nonius alignment following presentation of the stimulus. The normalized stereoscopic magnitude estimation data generally showed that crossed disparities were perceived with greater depth than uncrossed disparities of the same magnitude, whether presented to the central or peripheral field. Asymmetries in magnitude of depth perception ranged from mild differences between depth directions to complete lack of depth perception for one direction. Disparity vergence response functions varied from (1) appropriate initiation of vergence to both directions of disparity, (2) initiation of vergence to only one direction of disparity, or (3) an attenuated initiation of vergence response to either direction of disparity. Within subjects, their asymmetries in magnitude of depth perception did not correlate with their asymmetries in vergence initiation. The similarity of the asymmetric depth magnitude estimation for a given individual at both stimulus locations tested suggests that common neural mechanisms are responsible for central and peripheral depth magnitude estimation. The lack of correlation between the perceptual and motor responses to the same stimuli suggests that the neural pathways for these responses diverge shortly after the detection of disparity in primary visual cortex.

Stereothresholds for moving line stimuli for a range of velocities

This study examined the influence of lateral target motion on the stereothresholds for bright vertical lines at a range of velocities. Stimuli were presented for 200 ms with horizontal velocities from 0 to 12 deg/s. Observers' horizontal eye movements were recorded on additional trials, and confirmed that the velocity of retinal image motion closely matched the velocity of the stimulus. In three auxiliary experiments, stereothresholds were measured (1) after equating the detectability of targets that moved at different velocities, (2) for moving and stationary stimuli with durations between 20 and 200 ms, and (3) for stationary stimuli presented at eccentricities of 0.6 and 1.2 deg. The results indicate that stereothresholds are unaffected by velocities up to approximately 2 deg/s, but worsen in proportion to the velocity at higher speeds. The results of our auxiliary experiments demonstrate that the increase in stereothresholds during image motion cannot be attributed primarily to a reduction in the detectability of the stimulus, a decrease in the effective exposure duration, or non-foveal viewing. We conclude that the elevation of stereo thresholds during lateral motion is consistent with a shift in the sensitivity of the visual system toward lower spatial frequencies as a result of motion blur.