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PubMed2001

The processing of stereoscopic information in human visual cortex: psychophysical and electrophysiological evidence.

Three-dimensional depth perception relies in part on the binocular fusion of horizontally disparate stimuli presented to the left and right eye. The mammalian visual system offers a unique possibility to study electrophysiologically cortical neuronal mechanisms: since the input of the two eyes remains separated up to the level of the visual cortex, evoked potential components that are generated exclusively by cortical structures may be explored when dynamic random-dot stereograms (dRDS) are presented. In a series of independent studies, we determined the scalp topography of dRDS evoked brain activity in different groups of healthy subjects, and we found consistent results. Major differences between stereoscopic and contrast evoked brain activity are seen in the strength of the potential fields as well as in their topography. Our findings suggest that there are fewer neurons in the human visual cortex that are responsive to horizontal disparity, and that higher visual areas like V2 are more engaged with stereoscopic processing than the primary visual cortex. On the other hand, component latencies of evoked brain activity show no effect signifying that the binocular information flow to the visual cortex has a similar time course for both the processing of contrast information and of dRDS stimuli. We could also verify that healthy subjects can learn to perceive 3D structure contained in dRDS. Changes in perceptual ability as measured with psychophysical tests are paralleled by systematic alterations in the topography of stereoscopically evoked potential fields. Stereoscopic VEP recordings may also be of clinical use: in patients with selectively disturbed depth perception but normal visual acuity there is a high correlation between clinical symptoms, perceptual deficiency, and altered VEP amplitudes and latencies.

Brain Mapping↗
PubMed2005

Integrating visual cues for motor control: a matter of time.

The visual system continuously integrates multiple sensory cues to help plan and control everyday motor tasks. We quantified how subjects integrated monocular cues (contour and texture) and binocular cues (disparity and vergence) about 3D surface orientation throughout an object placement task and found that binocular cues contributed more to online control than planning. A temporal analysis of corrective responses to stimulus perturbations revealed that the visuomotor system processes binocular cues faster than monocular cues. This suggests that binocular cues dominated online control because they were available sooner, thus affecting a larger proportion of the movement. This was consistent with our finding that the relative influence of binocular information was higher for short-duration movements than long-duration movements. A motor control model that optimally integrates cues with different delays accounts for our findings and shows that cue integration for motor control depends in part on the time course of cue processing.

Cues↗
PubMed1994

Vergence adaptation and senescence.

PURPOSE: The characteristics of prism adaptation have been evaluated by many previous studies but most have been performed on young subjects (typically 18 to 35 years of age). Because little information is available regarding the effect of age on vergence adaptation, we assessed prism adaptation on 25 visually normal subjects across a wide age range (19 to 85 years) for both convergent and divergent induced disparities. METHODS: A "flashed" (125 ms) Maddox rod procedure was used for distance fixation (5 m) over a 3.5 min adaptation period and a 2.5 min recovery period. Stimulus presentation and occlusion time were controlled with an Apple lle microcomputer through a ClL Microsystems interface. RESULTS: Prism adaptation was shown to be present in all age groups, with convergence showing a greater amplitude than divergence. However, the magnitude of adaptation declined significantly as a function of increasing age at a rate of approximately 0.6% per year. CONCLUSION: An adaptation mechanism is present in all subjects but it operates with reduced gain in older subjects. The finding may explain the clinical observation that older patients readily accept prismatic correction to control oculomotor imbalance.

Adaptation, Ocular↗
PubMed1988

The role of retinal correspondence in stereoscopic matching.

On brief viewing, stereo matching of a regularly-spaced horizontal row of points is determined by the disparity of the points at the edges. Stereo matches on corresponding retinal loci are initially overridden in favor of matches located on or near the disparity plane of the edge points. Edge-based matching is observed when the inter-point spacing is as large as 15-30 min arc for crossed disparities, and as large as 1 deg for uncrossed disparities. Edge points located at a lateral distance more than 2.5 deg away from the center of the row can still determine the initial stereo matching of the center. Given longer viewing time, vergence usually changes from the fixation plane towards the initially-perceived depth plane associated with the edges. However, if the eyes are held tightly converged in the fixation plane, the edge-based matches will gradually yield to matches in the fixation plane. This shift from edge-based matching to a match determined by retinal correspondence takes 1-4 sec if the inter-point spacing is large (10-30 min arc). For smaller inter-point spacings, the edge-based matches are very stable, and a shift in depth is seldom discernible.

Convergence, Ocular↗
PubMed1994

Interhemispheric depth judgement.

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemifield and the other to the right. Callosal agenesis subjects and neurologically-normal control subjects adjusted the plates so that they appeared to be at the same distance. Subjects viewed the plates monocularly or binocularly while keeping their head still, moving it side-to-side or moving it up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the results showed that the deviations from veridical settings were significantly smaller for the binocular than for the monocular viewing conditions. Moreover, there were no significant differences among the three binocular viewing conditions (horizontal, vertical or no head movement), indicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is available. These results further suggest that the precision of interhemispheric comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subjects judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that motion parallax is a useful depth cue when relative motion is extracted from different hemifields. Unlike the control subjects, the callosal agenesis subjects did not judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that interhemispheric comparison of depth using relative motion is not possible without the corpus callosum.

Adult↗
PubMed1999

The response to prism deviations in human infants.

Previous research has suggested that infants are unable to make a corrective eye movement in response to a small base-out prism placed in front of one eye before 14-16 weeks [1]. Three hypotheses have been proposed to explain this early inability, and each of these makes different predictions for the time of onset of a response to a larger prism. The first proposes that infants have a 'degraded sensory capacity' and so require a larger retinal disparity (difference in the position of the image on the retina of each eye) to stimulate disparity detectors [2]. This predicts that infants might respond at an earlier age than previously reported [1] when tested using a larger prism. The second hypothesis proposes that infants learn to respond to larger retinal disparities through practice with small disparities [3]. According to this theory, using a larger prism will not result in developmentally earlier responses, and may even delay the response. The third hypothesis proposes that the ability to respond to prismatic deviation depends on maturational factors indicated by the onset of stereopsis (the ability to detect depth in an image on the basis of retinal disparity cues only) [4] [5], predicting that the size of the prism is irrelevant. To differentiate between these hypotheses, we tested 192 infants ranging from 2 to 52 weeks of age using a larger prism. Results showed that 63% of infants of 5-8 weeks of age produced a corrective eye movement in response to placement of a prism in front of the eye when in the dark. Both the percentage of infants who produced a response, and the speed of the response, increased with age. These results suggest that infants can make corrective eye movements in response to large prismatic deviations before 14-16 weeks of age. This, in combination with other recent results [6], discounts previous hypotheses.

Adaptation, Physiological↗
PubMed1994

Stereo matching precedes dichoptic masking.

Stereo matching can intervene to prevent dichoptic masking. In a dichoptic masking paradigm we measured the contrast threshold for a bar target, presented to one eye, as a function of the contrast of an identical masking bar, presented at retinal correspondence in the other eye. Confirming previous studies of dichoptic masking with sinusoidal gratings, the test bar thresholds rose proportionally with increasing masking contrast. This threshold elevation was almost nullified when an extra bar was presented to the eye seeing the test stimulus. Release from masking occurred when the disparity between the masking bar and extra bar was < 20 min arc over a range of contrast levels (8-45%), and for bars containing either broad spatial frequency spectra or bars with only high spatial frequencies (peak = 12 c/deg). The latter result rules out an explanation for the release from masking based on contrast discrimination in low spatial frequency channels. The extra bar was effective in releasing the test bar from masking as long as the extra bar's contrast was greater than about one-fifth the contrast of the mask, a result that suggests that there is a contrast threshold for stereo matching. We interpret our findings to indicate that a stage of stereo matching occurs prior to the neural site limiting dichoptic contrast discrimination.

Contrast Sensitivity↗
PubMed1993

Fusional suppression in normal and stereoanomalous observers.

Observers with normal stereopsis suppress some of the monocular information contained in each stereo half-image, a phenomenon we call fusional suppression. We measured vernier acuity for an ordinary vertical vernier test target, presented to one eye, that was paired stereoscopically with a vernier target with a large fixed offset, presented to the other eye. The size of the fixed offset, and hence, the disparity of the upper target line, was varied parametrically. Vernier thresholds for the test target rose as a function of disparity, reaching a maximum at 20 min of disparity and then decreasing gradually as the disparity exceeded the limits of foveal fusion (40-60 min arc). In the two normal observers, fusional suppression was symmetrical; neither eye had good access to monocular information. In the stereoanomalous observers, fusional suppression was not symmetrical. When they viewed the vernier test target with the stronger of their two eyes, their vernier thresholds were barely affected by the stereo half-image in the other eye, and so were better than those of the normal observers measured in the same condition. When the stereoanomalous observers viewed the test target with their weaker eye, their fusional suppression was similar in range and magnitude to the suppression found in normal observers. Amblyopic suppression in mild amblyopes may be a residual effect of normal fusional suppression, operating to suppress monocular signals in the weaker eye, without conferring the benefits of normal stereopsis and fusion.

Amblyopia↗
PubMed1996

Adaptation of vertical eye alignment in relation to head tilt.

Binocular visual feedback is used to continually calibrate binocular eye alignment so that the retinal images of the two eyes remain in correspondence. Past experiments have shown that vertical eye alignment (measured as vertical phoria) can be altered by training to disparities that vary as a function of orbital eye position. The present experiments demonstrate that vertical eye alignment can also be trained to differ with head position when eye position (with respect to the orbit) is held constant. Changes in head position were about either an earth-vertical or earth-horizontal axis to distinguish otolith-ocular related adaptation from cervical-ocular related adaptation. Changes in head position were implemented by either by rotating the whole body (WB) or by rotating the head with the body stationary (HO). Following training, adaptation of eye alignment was observed in all cases of rotation about an earth-horizontal axis and for HO pitch rotations about an earth-vertical axis. The results illustrate the ability of the oculomotor system to compensate for imbalances in otolith-ocular pathways.

Adaptation, Physiological↗
PubMed2005

Divergence eye movements are dependent on initial stimulus position.

Previous studies on the speed and latency of convergence and divergence eye movements have produced varied, sometimes contradictory, results. Four subjects were studied and tracked 4 degrees disparity step changes for convergence and divergence at different initial target positions. Here we report that the dynamics of divergence movements not only differ from convergence movement, but depend on the initial vergence position. Velocities of divergence eye movements in response to targets that were initially near to the subject were approximately twice that of responses to initially distant targets and also exhibited shorter temporal properties. Hence, while convergence responses are fairly similar irrespective of the initial position, divergence dynamic and temporal properties are dependent on the initial stimulus position. It is speculated that the differences observed in divergence may be the result of nonlinear properties of the extraocular muscles or a difference in the underlying neural controller potentially a difference in the magnitude of the fusion initiating component of divergence.

Adolescent↗
PubMed2005

Depth aftereffects mediated by vertical disparities: evidence for vertical disparity driven calibration of extraretinal signals during stereopsis.

Perceptual adaptation often results in a repulsive aftereffect: stimuli are seen as biased away from the adaptation stimulus (). Here we report the absence of a repulsive aftereffect for a vertical gradient of vertical disparity (or vertical size ratio, VSR). We exposed observers to a binocular stimulus consisting of horizontal lines. This stimulus contains vertical, but not horizontal disparities. The visual system was able to measure the VSR of this stimulus: although the lines themselves always appeared unslanted, the VSR carried by the lines had a dramatic effect on the apparent slant of a horizontal row of dots, as predicted by recent accounts of Ogle's (1938) induced effect (e.g., Backus, Banks, van Ee, & Crowell, 1999). Yet we observed no repulsive aftereffect for the VSR signal: after adaptation to horizontal lines that were vertically larger in one eye, we found an attractive aftereffect, the magnitude of which was largest in stimuli that did not contain a VSR signal. We interpret these results as a case of recalibration: disagreement between extra-retinal eye position signals (EP) and VSR causes a recalibration in the use of EP as used in the stereoscopic perception of slant.

Adaptation, Physiological↗
PubMed2000

Human cortical areas underlying the perception of optic flow: brain imaging studies.

In summary, we have reviewed electrophysiological and brain imaging studies of motion and optic-flow processing. Single-unit studies indicate that MST (V5a) is a site of optic-flow extraction and that this information can be used to guide pursuit eye movements and to estimate heading. The EEG and MEG studies point to a localized electrical dipole in occipitotemporal cortex evoked by visual motion. We have also discussed the evidence from functional imaging studies for response specificity of the rCBF and BOLD effects in posterior cortex to visual motion and optic flow. Focal attention modulates the amplitude of the BOLD signal evoked by visual motion stimulation. Retinotopic mapping techniques have been used to locate region borders within the visual cortex. Our results indicate that striate (V1) and extrastriate areas (V2, V3/V3a) respond robustly to optic flow. However, with exception of a more pronounced response in V3/V3a to random walk, we found little evidence for response selectivity with respect to flow type and disparity in these early visual areas. In a similar fashion, the human V5/V5a complex responds well to optic flow, but these responses do not vary significantly with the type of flow field and do not seem to depend on disparity. In contrast, the kinetic occipital area (KO/V3b) responds well to optic-flow information, and it is the only area that produces more pronounced activation to the disparity in the flow fields. These initial results are promising because they suggest that the fMRI method can be sensitive to changes in stimulus parameters that define flow fields. More work will be required to explore the extent to which these responses reflect the neuronal processing of optic flow. Eye position tracking is now possible during fMRI experiments. We have demonstrated that the eye movements affect the BOLD responses in motion-sensitive areas (Kimming et al., 1999). Further experiments in our laboratory are aimed at understanding the effects of eye movements on the neuronal coding of complex optic-flow fields (Schira et al., 1999).

Animals↗
PubMed2004

Test-retest reliability of the Saladin card.

BACKGROUND: Test-retest reliability is a measure of the confidence that results will be identical when the same patient is measured with an instrument in the same manner on more than one occasion. METHOD: Using the Saladin Near Point Balance Card--an instrument designed to test various near visual findings, including visual acuity, phorias, AC/A ratios, dynamic retinoscopy, fixation disparity, associated phorias, fixation disparity curves, as well as accommodative and vergence facilities--28 first- and second-year optometry students were evaluated on two occasions by one clinician, with the tests separated by approximately two weeks. Subjects were required to demonstrate 20/20 distance visual acuity and at least 100 sec of arc stereopsis. A total of 38 findings were compared, which included near horizontal and vertical phorias, associated phoria, and fixation disparity curves. Twenty-two findings were performed through the subjects' habitual prescription and the remaining findings were taken through the lens indicated by MEM retinoscopy. PURPOSE: The purpose of this study was to investigate whether the Saladin Near Point Balance Card had acceptable test-retest reliability. If reliability can be demonstrated, this instrument could be used in clinical situations to diagnose visual departures from normal. RESULTS: All but two of the 38 tests performed demonstrated acceptable test-retest reliability. Coefficients of repeatability and 95% limits of agreement were calculated. The Saladin Near Point Balance Card demonstrated acceptable test-retest reliability. CONCLUSION: Since it is light-weight, portable, easily and quickly administered, and reliable, the Saladin card can be used by clinicians who are performing screenings or examinations in non-clinical situations, such as nursing homes or schools.

Adult↗
PubMed1999

Oculomotor function after virtual reality use differentiates symptomatic from asymptomatic individuals.

UNLABELLED: Some individuals who use virtual reality (VR) head-mounted displays (HMD) have adverse visual symptoms. PURPOSE: We measured oculomotor functions of symptomatic (n = 10) and asymptomatic (n = 10) individuals to determine if there were fundamental oculomotor performance differences. METHOD: Before and after 20 min of biocular VR-HMD use, we measured: phorias, fixation disparity, gradient accommodative convergence to accommodation ratio (AC/A), stereopsis, and nearpoint of convergence. RESULTS: We observed an exophoric shift in the nearpoint phoria of almost all subjects, whereas the farpoint phoria showed no trend. Interestingly, we observed that the phoric shift at far and near was highly correlated for the asymptomatic subjects but not for the symptomatic subjects. In addition, the (stimulus) AC/A ratio of symptomatic subjects was reduced after a period of VR-HMD use, whereas asymptomatic subjects' AC/A ratio was not reduced. CONCLUSION: The oculomotor changes among the symptomatic subjects (increased exophoria at near and reduced AC/A) appears consistent with a reduced accommodative response. In contrast, the asymptomatic subjects show changes (correlated change in phorias) which seem most consistent with adaptation in the tonic component of vergence and/or accommodation.

Accommodation, Ocular↗
PubMed1997

Convergence and divergence exhibit different response characteristics to symmetric stimuli.

The dynamic characteristics of horizontal convergence and divergence eye movement responses to symmetric stimuli were examined. Binocular eye movements were recorded in five, visually normal adult subjects using the infrared reflection technique for symmetric convergent and divergent blur-free, disparity-only, step stimuli of 2, 4, 8, 12, and 16 deg. The main sequence as well as other temporal parameters including latency, time-to-peak velocity, time constant, and total duration were analyzed. A number of fundamental differences in the response characteristics were found between convergence and divergence. First, the slope of the peak velocity vs amplitude curve was approximately twice as high for convergence than divergence. The results are consistent with neurophysiological findings in monkeys and most findings in humans. Second, the initial fast component for convergence exhibited a larger amplitude than for divergence. This may reflect differences in central neural gain for convergence and divergence. And, third, all temporally related components were shorter for convergence than divergence. These findings provide an overall framework for vergence control and suggest fundamental differences in neural processing delays and neural controller pathways for convergence and divergence.

Adult↗
PubMed1994

Variations in the monocular components of fixation disparity.

Measurements of the monocular components of fixation disparity through small amounts of lateral prism show changes in the proportional contribution of each eye with changing vergence stimuli. Prism does not usually alter the independence of the activity of each eye during binocular viewing. This independence allows the monocular shares to fluctuate from moment to moment. Responses to prism include fixating with one eye or the other, shifting both eyes laterally in the same direction, and distributing the deviations between the two eyes. Some of these strategies reduce the deviation and are therefore adaptive. Others appear to maintain a fairly stable state which frequently involves resorting to the equivalence of a monocular form of localization. Prism adaptation follows decidedly different time courses and occurs to differing extents for the different subjects and for the different conditions tested. It is not uncommon for many of these changes to occur unilaterally. Voluntary activity seems to be an important factor in the processes associated with adaptation to prism.

Fixation, Ocular↗
PubMed1999

The design of telepresence systems: the task-dependent use of binocular disparity and motion parallax.

The effect of different visual depth cues presented through a head-mounted display in a dark (no pictorial cue) environment was investigated. The relative effects of binocular disparity, motion parallax, and a combination of the 2, were assessed for 3 tasks at 2 viewing distances. These tasks (which varied in the minimum amount of information they required) were a nulling task, setting a triangle to be equilateral and matching the base-to-apex magnitude of 2 triangles at different distances. Performance within the tasks varied considerably but was most accurate for the nulling task. Differences between viewing conditions may be due to a failure in the assessment of absolute viewing distance. It is argued that these results are task specific. Although there was some variation between different cue types, they appear to be largely interchangeable within the tasks. These results have implications for system designers selecting an appropriate display device for a telepresence system.

Adult↗
PubMed1990

Selective nonconjugate binocular adaptation of vertical saccades and pursuits.

Hering's law describes the equal and symmetrical rotation of the two eyes. It is possible to calibrate the binocular yoking of the two eyes in response to disparate size and/or motion of the two ocular images. It is unclear if the ratio of movements by the two eyes is modified by selective adaptation of each versional system (i.e. saccades and pursuits) or if there is an apparent adaptation of Hering's law that results from a single underlying process. The latter could be accomplished by vergence (prism) adaptation, which could interact with all versional systems. In this investigation, binocularly stimulated saccades and pursuits were adapted separately for 2 hr to unequal vertical target displacements. Three adaptation paradigms were used; each included a 10% binocular gradient disparity. The adapting stimulus for the pursuit system was 0.25 Hz vertical triangular motion of 20 deg, peak to peak. Two saccade adaptation paradigms included one which emphasized correcting vertical disparity during the pulse component of the saccade, the other minimized the influence of disparity prior to, during and immediately after saccades (vergence paradigm). Yoking ratios (YRs) for vertical pursuits and saccades were compared before and after adaptation. The pursuit paradigm produced marked adaptation of the pursuit YRs while it had negligible effect on saccade YRs. The pulse saccade paradigm adapted the saccade YRs twice as much as the pursuit YRs whereas the vergence paradigm resulted in little adaptation of YRs for either saccades or pursuits. Pursuits adapted to the first paradigm in 15-30 min whereas saccades adapted to the second paradigm in 1.5-2 hr. These results indicate that there is not a single common nonconjugate adaptation mechanism for vertical pursuits and saccades. Results of the vergence paradigm demonstrate that feedback during or immediately after eye movements is necessary in order to stimulate the binocular versional adaptation mechanism. Versional adaptation may be considered as a calibration of Hering's law.

Adaptation, Ocular↗