Biomedical subjects

C M Schor

Publications and source records attributed to C M Schor.

At least 19 recordsLinked to original sources

PubMed2001

Envelope size-tuning for transient disparity vergence.

Our prior studies have demonstrated that the transient-vergence system responds preferably to dichoptic stimulus pairs that contain the highest combined energy, regardless of dichoptic differences in spatial frequency, contrast, orientation, or luminance polarity (Edwards, M., Pope, D. R., & Schor, C. M. (1998), Vision Research 38, 705; Pope, D. R., Edwards, M., & Schor, C. M. (1999) Vision Research 39, 575). This broadband tuning for spatial frequency, orientation and contrast is indicative of a second order (non-linear) extraction system. The current study examined the potential size-tuning of binocular channels to the contrast envelope that is extracted by a non-linear process. Stimuli were size-scaled Gabor patches with parallel and orthogonal carrier orientations that subtended a large (3.8 degrees ) disparity. Results indicate that the transient-vergence system exhibits broad band-pass tuning to overall size of dichoptic targets, independent of interocular differences in carrier orientation, spatial frequency or contrast. Equal sizes elicited a higher proportion of vergence responses than unequal sizes, however responses to unequal size still occurred over a 2-octave range, illustrating broad band-pass tuning. Size tuning was found to be broader for small than large envelope sizes. The broad tuning for envelope size is likely to result from the overlapping extracted low-pass frequency spectra of the contrast envelopes. However, the transient-vergence system also responds to monocular, hemi-retinal stimuli over a larger (3-octave) range. Thus some of the observed "binocular tuning" may be due to these monocular responses.

Adult↗

PubMed2001

Plasticity of convergence-dependent variations of cyclovergence with vertical gaze.

Binocular alignment of foveal images is facilitated by cross-couplings of vergence eye movements with distance and direction of gaze. These couplings reduce horizontal, vertical and cyclodisparities at the fovea without using feedback from retinal image disparity. Horizontal vergence is coupled with accommodation. Vertical vergence that aligns tertiary targets in asymmetric convergence is thought to be coupled with convergence and horizontal gaze. Cyclovergence aligns the horizontal retinal meridians during gaze elevation in symmetrical convergence and is coupled with convergence and vertical gaze. The latter vergence-dependent changes of cyclovergence have been described in terms of the orientation of Listing's plane and have been referred to as the binocular extension of Listing's law. Can these couplings be modified? Plasticity has been demonstrated previously for two of the three dimensions of vergence (horizontal and vertical). The current study demonstrates that convergence-dependent changes of the orientation of Listing's plane can be adapted to either exaggerate or to reduce the cyclovergence that normally facilitates alignment of the horizontal meridians of the retinas with one another during gaze elevation in symmetrical convergence. The adaptability of cyclovergence demonstrates a neural mechanism that, in conjunction with the passive forces determined by biomechanical properties of the orbit, could play an active role in implementing Listing's extended law and provide a means for calibrating binocular eye alignment in three dimensions.

Accommodation, Ocular↗

PubMed2001

Envelope size tuning for stereo-depth perception of small and large disparities.

Stereopsis is the sense of depth derived from binocular disparities that are formed between targets that are matched between the two retinal images. Binocular matches for sustained stereopsis are based on similarity of orientation, spatial frequency and contrast of the two retinal images whereas matches for transient stereopsis depend on these parameters to a very limited extent. In this investigation we have tested the possibility that transient stereopsis forms matches between objects of similar overall size. The tuning of sustained and transient stereopsis to contrast-envelope size was investigated by presenting narrow-band Gabor targets of unequal size to the two eyes. Bandwidth for envelope-size tuning was estimated from the range of dichoptic size-differences over which stereo performance remained above chance level. An equal bandwidth of 2 octaves was found for the sustained and transient stereo systems when stimulated with parallel orientation Gabors that subtended a small disparity. Sustained-stereo performance with orthogonal carriers was reduced with large envelope sizes. Bandwidth of the transient stereo system increased to 3 octaves when tested with a larger disparity stimulus and it was independent of carrier orientation. Reducing the contrast of the larger-size Gabor improved transient-stereo performance from near chance (48-58%) to 85-95%. Thus the bandwidth for envelope-size tuning is much broader than indicated with equal physical contrast stimuli. The observed tuning to envelope size, while broad, is tighter than that observed for carrier spatial-frequency [Vis. Res. 38 (1998) 3057], carrier orientation [Vis. Res. 39 (1999) 2717] and contrast polarity [Vis. Res. 39 (1999) 4010] of the stimulus. Thus it would appear that envelope size and, to a greater extent, temporal synchrony of the dichoptic stimuli [Perception 24 (1995) 33] are the primary means for selecting matched binocular inputs for transient stereopsis.

Contrast Sensitivity↗

PubMed2001

Spatial interactions minimize relative disparity between adjacent surfaces.

Computational models of stereopsis employ a number of algorithms that constrain stereo matches to produce the smallest absolute disparity and to minimize the relative disparity between nearby features. In some natural scenes, such as large slanted textured surfaces, these two constraints lead to different matching solutions. The current study utilized a stimulus in which there was a large discrepancy in both the magnitude and direction of matches that solved for minimum absolute and minimum relative disparity. This discrepancy revealed a dominance for the minimum relative disparity over the minimum absolute disparity matching solution that increased with spatial proximity, spatial frequency and width of adjacent features. The likelihood of a minimum-relative-disparity matching solution also increased when the difference between the amplitudes of the alternative relative disparities was large. When alternative relative disparity matching solutions had similar amplitudes but opposite signs (crossed vs. uncrossed), an idiosyncratic depth bias served as a tie-breaker. The present results show that absolute disparity matches are constrained to minimize relative disparity between adjacent features.

Depth Perception↗

PubMed2001

Adaptation of torsional eye alignment in relation to smooth pursuit and saccades.

The long-term fusion of vertical or horizontal disparities by vergence eye movements is known to evoke persistent changes in vertical and horizontal eye alignment. Adaptive changes in response to torsional disparities have not been well studied. Torsional eye position was measured binocularly with a video system before and after 90 min training periods in which subjects attempted to fuse cyclodisparities. Subjects trained with either a single cyclodisparity presented at a single vertical eye position or with cyclodisparities that varied smoothly from an incyclodisparity to an excyclodisparity as a function of either vertical or horizontal eye position. All five subjects showed persistent changes in binocular torsional eye alignment following both types of training. Incyclodisparities were more easily fused during training and the training aftereffect was greater in that direction. The training aftereffect was observed in relation to both saccades and smooth pursuit under both open-loop and closed-loop viewing conditions. During saccades, the dynamics of the cyclovergence training aftereffect more closely resembled the dynamics of cyclofusional movements than the dynamics of the saccades with which they were associated.

Adaptation, Physiological↗

PubMed2001

Fatigue reduces tonic accommodation.

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.

Accommodation, Ocular↗

PubMed2001

Age changes in the interactions between the accommodation and vergence systems.

Experiments are described in which static and dynamic accommodation (Ar), accommodative convergence (AC), vergence (C) and convergence accommodation (CA) responses to small stimuli were measured dynamically in 13 subjects with ages in the range 16 to 48 years. Analysis showed that the amplitudes of both blur and disparity-driven accommodation declined significantly with age, whereas the two types of vergence response did not. As a result, the AC/Ar ratio rose significantly with age, whereas the CA/C ratio fell. No significant change with age was found in response latencies and durations.

Accommodation, Ocular↗

PubMed2000

First- and second-order processing in transient stereopsis.

Large-field stimuli were used to investigate the interaction of first- and second-order pathways in transient-stereo processing. Stimuli consisted of sinewave modulations in either the mean luminance (first-order stimulus) or the contrast (second-order stimulus) of a dynamic-random-dot field. The main results of the present study are that: (1) Depth could be extracted with both the first-order and second-order stimuli; (2) Depth could be extracted from dichoptically mixed first- and second-order stimuli, however, the same stimuli, when presented as a motion sequence, did not result in a motion percept. Based upon these findings we conclude that the transient-stereo system processes both first- and second-order signals, and that these two signals are pooled prior to the extraction of transient depth. This finding of interaction between first- and second-order stereoscopic processing is different from the independence that has been found with the motion system.

Depth Perception↗

PubMed2000

Unconstrained stereoscopic matching of lines.

The computation of horizontal binocular disparities used in stereoscopic depth perception depends upon the identification of corresponding features in the two retinal images. In principle, binocular matching is a two-dimensional problem that considers matches in all possible meridians. Normally, constraints such as end points or crossing points limit the direction and magnitude of matches. If matching is unconstrained, such as is the case with long lines, it is completely ambiguous. Under these conditions the default match will be determined by the operating range, or upper disparity limit, of matchable vertical and horizontal disparities. We computed the operating range of vertical matches for stereoscopic depth as a function of line orientation. Our results suggest that the two-dimensional operating range is anisotropic for vertical and horizontal disparity and that unconstrained matches are not based upon either epipolar geometry or nearest neighbor constraints, but rather the mean of disparity estimates within the operating range for binocular matches. This operating range can be extended vertically when matches are constrained by image primitives.

Depth Perception↗

PubMed1999

Orientation and luminance polarity tuning of the transient-vergence system.

Previously, Edwards, Pope & Schoor, Vision Research, 38, 705-717, demonstrated that transient disparity vergence appears to be mediated by a system that employs a single low-pass sensitive spatial channel whose performance is not reduced by dichoptic mixed contrasts (no contrast paradox) or dichoptic mixed spatial frequencies. This broadband tuning to both contrast and spatial frequency may be indicative of a second-order or non-linear envelope extraction system. The current study tests for lack of tuning to orientation and luminance polarity which are typically taken as evidence of a second-order system. We found that when the transient vergence system was simultaneously presented with both convergent and divergent disparities, there was a small but distinct bias in favor of responding in the direction defined by matched orientations or luminance polarities over unmatched pairs. Although less frequent, responses to orthogonal carriers or opposite luminance polarities were possible. The vergence system could match a horizontal with a vertical carrier, or a light gaussian with a dark gaussian. The degree of orientation or luminance polarity tuning varied inversely with the disparity magnitude over the range of 2.5-5 degrees, and the orientation tuning peaked at a spatial frequency about 2 cpd. At all disparities tested, however, the tuning was very broad, and other candidate features for mediating transient-vergence need to be investigated.

Contrast Sensitivity↗

PubMed1999

Orientation tuning of the transient-stereopsis system.

Stereo-perception appears to be mediated by at least two systems: a transient system that processes stimuli presented briefly and a sustained one that processes stimuli presented for longer durations. In this paper we investigated the tuning of the transient-stereopsis system to stimulus orientation. Narrowband-gabor targets with a constant envelope size (Gaussian standard deviation of 1 degree) were presented for brief (140 ms) durations at large (from 4 to 8 degrees) disparities. The results were as follows: (1) while observers could extract depth from orthogonally-oriented gabors at above chance levels, their performance was worse than that with gabors of matched orientation; (2) varying the relative contrasts of the two orthogonally oriented gabors of the same spatial frequency resulted in a reduction in performance; (3) varying the relative spatial frequencies of the orthogonally-oriented gabors impaired performance, relative to that for matched frequencies; and (4) varying the relative contrasts of orthogonal gabors that were at different spatial frequencies could improve performance. These results indicate that transient stereo-performance in the orthogonal condition was not mediated by the channels that extracted depth in either the horizontal- or vertically-matched gabor conditions. This apparent lack of orientation tuning is indicative of a second-order pathway. That this performance was mediated by a binocular, as opposed to a monocular channel, is supported by the finding that performance decreased as the contrast of one of the gabors was reduced. The finding that performance with orthogonal gabors of unmatched spatial frequency (0.5 and 4 cpd) could be improved by varying their relative contrasts suggests that the binocular spatial-frequency tuning exhibited by this channel is broadband in nature. Finally, the observation that lowering the contrast of either the high or low spatial-frequency gabor improved performance suggests the presence of at least two broadband channels: one with its peak sensitivity at a low and the other at a high spatial-frequency.

Contrast Sensitivity↗

PubMed1999

Saccades reduce latency and increase velocity of ocular accommodation.

Horizontal vergence can be stimulated binocularly with disparity (disparity vergence) or monocularly with accommodation (accommodative vergence). The latter results from a neural cross-coupling that causes both horizontal vergence and accommodation to respond when either one is stimulated [Alpern, M., & Ellen, P. (1956). American Journal of Ophthalmology, 42, 289-303]. The velocity of disparity and accommodative vergence is enhanced when accompanied by saccades [Enright, J. T. (1984). Journal of Physiology (London) 350, 9-31; Enright, J. T. (1986). Journal of Physiology (London) 371, 69-89]. Based upon the coupling between accommodation and vergence, we predicted that accommodation should also be facilitated by saccades. An SRI Dual Purkinje Eyetracker was used to measure left and right eye position, and the accommodation of the left eye, in response to stimulation. Horizontal saccades were stimulated by targets separated by 2-6 degrees and accommodation was stimulated monocularly over a range of +/- 2 diopters (D). When saccades occurred within 0-400 ms following a monocular step stimulus to accommodation, latency of accommodation decreased and the associated accommodative-vergence response was synchronized with the saccade. Saccades also enhanced the velocity of accommodation and accommodative-vergence, and this facilitation increased with saccade amplitude. Transient vergence responses that are normally associated with saccades [Erkelens, C. J., Steinman, R. M., & Collewijn, H. (1989). Proceedings of the Royal Society of London B. Biological Sciences, 236, 441-465; Maxwell, J. S., & King, W. M. (1992). Journal of Neurophysiology, 68 (4), 1248-1260] did not affect accommodation when it was not stimulated by defocus. Because saccades and accommodation utilize separate plants and final common pathways, the synchronization of saccades and accommodation and the enhanced velocity of accommodation and accommodative-vergence must occur at more central sites. Possibilities include the superior colliculus, which represents both accommodation and saccades [Nagasaka, Y., & Ohtsuka, K., (1998). Investigative Ophthalmology AVRO supplement], vestibular nuclei which project to regions near the oculomotor nuclei [Lang, W., Buttner-Ennever, J. A., & Buttner, U. (1979). Brain Research, 177, 3-17], and interactions between omni pause neurons and near response cells of the mesencephalic reticular formation (MRF) [Mays, L. E., & Gamlin, P. D. R. (1995a). Current Opinions in Neurobiology, 5, 763-768; Mays, L. E., & Gamlin, P. D. R. (1995b). Eye movement research: Mechanisms, processes and applications. New York: Elsevier] which represent both accommodation and vergence [Judge, S. J., & Cumming, B. G. (1986). Journal of Neurophysiology, 55, 915-930; Zhang, Y., Mays, L. E., & Gamli, P. D. R. (1992). Journal of Neurophysiology, 67, 944-960].

Accommodation, Ocular↗

PubMed1999

Adaptation of torsional eye alignment in relation to head roll.

The coordination of head tilt, ocular counter-roll and vertical vergence is maintained by adaptive mechanisms; the desired outcome being clear single vision. A disruption or imbalance in otolith-ocular pathways may result in diplopia which stimulates these adaptive processes. In the present experiment, dove prisms were used to create cyclodisparities that varied with head tilt about a naso-occipital axis (roll). A stimulus for incyclovergence was presented with the head rolled 45 degrees to one side and a stimulus for an excyclovergence was presented with the head rolled 45 degrees to the other side. At the end of 1 h of training, all subjects demonstrated a change in open-loop cyclovergence that would help to correct for the cyclodisparities experienced during the closed-loop training period. The change appeared to be a simple gain change in the ocular counter-roll of one or both eyes.

Adaptation, Physiological↗

PubMed1999

Depth aliasing by the transient-stereopsis system.

A fundamental problem in stereo-processing is determining which images in the two eyes correspond to the same object. This problem is particularly pronounced with periodic stimuli where it is theoretically possible to binocularly match a given feature in one eye with any of the identical features in the other eye. One way to minimise the likelihood of the occurrence of such aliasing is to restrict the upper-disparity limit that a particular binocular cell can process to one-half of the spatial period to which the cell is sensitive. While such a restriction would not be a major problem for the sustained stereo-system (which processes small disparities) it would be for the transient system (which is capable of processing disparities as large as 10 degrees). Large-field sinewave variations in luminance were used to compare the propensity of the sustained and transient systems to exhibit depth aliasing--that is to signal a depth sign that corresponds to a binocular match that is greater than the nearest-neighbour pairing. Results were that: depth aliasing was exhibited at short, but not at long durations; decreasing the disparity of the stimulus reduced the likelihood of depth-aliasing; and the critical disparity for this reduction in depth aliasing was dependent upon the spatial frequency of the stimulus, i.e. it was phase, not absolute disparity dependent. Based upon these results, we conclude that while the sustained system implements the half-cycle disparity-processing limit, the transient system does not.

Depth Perception↗

PubMed1999

The effect of target proximity on the aniso-accommodative response.

Aniso-accommodation (unequal binocular accommodation) to lens-induced anisometropia has been demonstrated by subjective and objective measurement techniques (Marran and Schor, Vision Res. 38(22), 3601-3619). The gain of the response was significantly reduced for some subjects when aniso-accommodation was stimulated by a target at 1 m compared to a target at 20 cm, even when the targets viewed were matched in retinal image size, convergence levels and aniso-accommodative stimuli. The two conditions did differ in both the accommodative level and proximity of the target. Thus the higher gain of the response in the 20 cm condition could have arisen from either high proximity, high accommodative level or a combination of both. In this investigation, target proximity and accommodative level were manipulated independently while extra-retinal cues such as absolute disparity and image size were held constant. The results show that high target proximity alone rather than accommodative level or a combination of the two was responsible for the distance dependent effect demonstrated by subjects. Furthermore, accommodative level did not affect the response when target proximity was held constant. Subjects who demonstrated invariant aniso-accommodation with changes in viewing distance also demonstrated invariant aniso-accommodation with experimental manipulation of target proximity at the target distances tested, as would be expected. These results suggest that high level processing, rather than a reflex blur response, is involved in aniso-accommodation. This conclusion is strengthened by the previous finding of long reaction and response times, 11 and 15 s respectively, to step aniso-accommodative stimuli (Marran and Schor, Vision Res. 38(22), 3601-3619). Since the experimental target provided subjects with visual feedback of the relative blur of the dichoptically viewed letters, subjects had access to perceptual blur information cues. Some subjects seemed to have required both aniso-blur and proximity to exercise this volitional response. Those subjects who showed a distance invariant aniso-accommodative response may have been able to use perceived aniso-blur alone and their ability to disregard proximal cues may have resulted from greater experience with the aniso-accommodative stimuli. Alternatively, these subjects may have had a lower threshold to proximal stimuli and experienced target proximity at the more distant (1 m) viewing condition.

Accommodation, Ocular↗

PubMed1998

Luminance contrast and spatial-frequency tuning of the transient-vergence system.

Vergence has transient components that are stimulated by brief presentations of stimuli at large disparities (up to several degrees). The question that we have addressed is what stimulus features are encoded by this system. A competition paradigm [Jones & Kerr, (1972)]. Vision Research, 12, 1425-1430) was used in which three gabors were presented. A single Gabor was presented to the fovea of one eye and two gabors, 2.5 deg to either side of the fovea, to the other; one of which, when paired with the single Gabor defined a convergent direction, the other a divergent direction. First we determined if increasing the luminance contrast of the Gabor pair whose disparity was opposite to the observer's response-bias direction (variable-contrast pair) relative to the remaining Gabor (reference) could alter the observer's response direction. Secondly, we determined if the contrast required for such a change in response was affected by the relative spatial frequency of the convergent and divergent Gabors. The reference Gabor was held at 2 cpd and the variable Gabor pair was varied between 5.6 and 0 (a gaussian) cpd. Results demonstrated that increasing the luminance contrast of the variable pair relative to the reference Gabor could alter the observer's response direction, even when the contrast of only one of the variable-pair Gabors was increased. The luminance contrast required for this change to occur was directly related to the spatial frequency of the variable pair over the entire frequency range tested. Vergence responses were preferentially made to lower spatial frequencies, even when a low spatial frequency was pared with a high one. We conclude that transient-vergence responses are not reduced by mixed contrasts (i.e. no contrast-paradox effect) and appear to be mediated by a system that employs a single lowpass sensitive channel.

Contrast Sensitivity↗

PubMed1998

OKN, perceptual and VEP direction biases in strabismus.

The present study quantified nasalward/temporalward biases in monocular optokinetic nystagmus (MOKN) and perceived velocity in patients with either early onset esotropia, late onset esotropia and in normals. MOKN was measured with low spatial frequency, small-field gratings drifting at 9.4 degrees/s. MOKN bias was quantified as the ratio of nasalward slow-phase velocity divided by the sum of temporalward and nasalward slow-phase velocities (N/(N + T)). Observers also rated the perceived velocity of gratings moving in nasalward and temporalward directions (3 or 9.4 degrees/s) using a two interval forced choice task. MOKN and perceived velocity biases were correlated negatively in both early onset and late onset groups in the perceptual task--nasalward moving targets were rated as slower than temporalward targets, but in the MOKN task, slow-phase gain was higher for nasalward than for temporalward targets. Oscillatory-motion, visual evoked potentials (VEPs), were recorded in response to 1 c/deg gratings undergoing apparent motion at 10 Hz in a subset of the observers. VEP direction biases were quantified by calculating the ratio of first harmonic response amplitudes to the sum of first and second harmonic amplitudes. Significant correlations were found between the direction biases obtained on all three measures. Perceived velocity and MOKN bias measures were also correlated negatively. Patients with early onset esotropia (infantile esotropia) had larger biases than late onset esotropes or normals on each measure and the biases were more frequently bilateral in the early onset patients. The pattern of result is consistent with early critical periods for the mechanism(s) underlying MOKN, perceived velocity and cortical responsiveness. A single site model for all three asymmetries is unlikely, at least in simple form, because of the negative correlation between MOKN and perceived velocity biases and because of the differences in relative magnitude between the perceptual and MOKN biases.

Age of Onset↗

PubMed1998

Spatial-frequency and contrast tuning of the transient-stereopsis system.

The tuning of the transient-stereopsis system to luminance contrast and spatial-frequency (SF) was investigated with narrow-band gabor targets with a constant sigma of 1 degree. They were presented for brief (140 ms) durations and subtended a large (6 degrees) disparity. When dichoptic gabor stimuli were matched in SF (0-5 cpd), transient stereo performance was either uniform across SF or greater at frequencies below 1 cpd. When dichoptic stimuli had unmatched SF (0.5 + 0-5 cpd) and matched contrast (100%), stereo performance was impaired below that of the matched SF condition. Stereo performance with matched SF at 0.5 cpd was impaired when contrast of one eye's image was reduced, demonstrating a contrast-paradox effect (i.e. contrast tuning) for transient stereopsis. Performance with three dichoptic unmatched SF conditions (0.5 and 1.0 cpd; 0.5 and 5.0 cpd; 1.5 and 3.5 cpd) was improved when the contrasts of the low SF gabor was reduced while holding the contrast of the high SF gabor constant at 100%. However stereo performance was not improved by reducing the contrast of a high SF gabor (3.5 cpd) while holding the contrast of the lower SF gabor (1.5 cpd) constant at 100%. We interpret these findings as indicating that transient-stereopsis performance is mediated by a single spatial-channel that has low-pass spatial-frequency sensitivity and which compares the ocular based signals prior to binocular combination so that signals that are not balanced in terms of their strength lead to a weaker binocular signal, as per the model proposed by Kontsevich and Tyler (Vis Res 1994; 3417:2317-2329) for sustained stereopsis.

Contrast Sensitivity↗