Interocular Phase Difference Research Articles

Stereopsis for rapidly moving targets.

Stereoscopic depth perception is possible with luminance-defined target velocities at least as high as 600° s-1, up to the limit of 30 Hz imposed by the high-temporal frequency cut-off of the eye. The limitation for perceiving depth from stereo disparity of moving targets is not their velocity but the temporal frequency bandwidth of the eye, which is affected by adaption state. Stereoacuity for a depth shift in a horizontally moving grating depends not on spatial disparity between corresponding luminance points in spatial units of arc min, but on the spatial shift as a fixed proportion of the period of the grating, in other words, on the phase angle difference between the two eyes, as is also the case for obliquely orientated, stationary gratings. Phase differences explain not only the classic Pulfrich stereophenomenon but its equivalent with dynamic visual noise, and a new effect in which depth results from interocular phase differences in luminance modulation. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

Strong tuning for stereoscopic depth indicates orientation-specific recurrent circuitry in tree shrew V1

Neurons in the primary visual cortex (V1) are tuned to specific disparities between the two retinal images, which form the neural substrate for stereoscopic vision. We show that V1 neurons in tree shrews, but not in mice, display highly selective responses to narrow ranges of disparity in random-dot stereograms. Surprisingly, V1 neurons in both species show similarly strong tuning to gratings of varying interocular phase differences. This stimulus-dependent dissociation of disparity tuning can be explained by a network model that combines both feedforward and recurrent connections. The features of the model connections are supported by cortical organizations specific to each species. We validate this model by identifying putative inhibitory neurons and confirming their predicted disparity tuning in both species. Together, our studies establish a foundation for using tree shrews in studying binocular vision and raise an exciting possibility of how cortical columns could be uniquely important in computing stereoscopic depth.

Two Patterns of Interocular Delay Revealed by Spontaneous Motion-in-Depth Pulfrich Phenomenon in Amblyopes with Stereopsis.

PurposeTo assess interocular delays in amblyopes with stereopsis and to evaluate the relationship between interocular delays and the clinical characteristics.MethodsTwenty amblyopes with stereopsis (median, 400 arcseconds) and 20 controls with normal or corrected to normal visual acuity (≤0 logMAR) and normal stereopsis (≤60 arcseconds) participated. Using a rotating cylinder defined by horizontally moving Gabor patches, we produced a spontaneous Pulfrich phenomenon in order to determine the interocular delays, that is, the interocular phase difference at which ambiguous motion in plane was perceived. Two spatial frequencies—a low (0.95 cycles/degree [c/d]) and a medium (2.85 c/d) spatial frequency—were tested.ResultsThe absolute interocular delays of the amblyopic group was significantly longer than that of the controls at both low or medium spatial frequencies (P < 0.01). However, the interocular delays was not always in favor of the fellow eye: 35% of the amblyopes (7/20) showed a faster processing of the amblyopic eye than that of the fellow eye at 0.95 c/d and 29.5% (5/17) at 2.85 c/d. No significant correlation was found between interocular delays and the clinical characteristics (e.g., age, treatment history, stereoacuity, and magnitude of anisometropia) in this amblyopic cohort.ConclusionsThe interocular delays in amblyopes with stereopsis might result from either a faster or slower processing of the amblyopic eye relative to the fellow eye. This work provides important additional information for binocular processing of dynamic visual stimuli in amblyopia. However, the special role between this form of interocular delays and patients’ clinical characteristics remains unknown.

The Binocular Balance at High Spatial Frequencies as Revealed by the Binocular Orientation Combination Task.

How to precisely quantify the binocular eye balance (i.e., the contribution that each eye makes to the binocular percept) across a range of spatial frequencies using a binocular combination task, is an important issue in both clinical and basic research. In this study, we aimed to compare the precision of a binocular orientation combination paradigm with that of the standard binocular phase combination paradigm in measuring the binocular eye balance at low to high spatial frequencies. Nine normal adults (average age: 24.6 ± 2.0 years old) participated. Subjects viewed an LED screen dichoptically with polarized glasses in a dark room. The method of constant stimuli was used to quantitatively assess the point of subjective equality (PSE), i.e., the interocular contrast ratio when two eyes are balanced in binocular combination, for stimulus spatial frequencies from 0.5 to 8 cycles/degree. Precision was quantified by the variance [i.e., standard error (SE), obtained from 100 bootstrap estimates] associated to the PSE. Using stimuli whose interocular phase difference at the edge of the gratings was matched at 45°, we found that the orientation paradigm provides more precision than the standard binocular phase combination paradigm, especially at high frequencies (Experiment 1). Such differences remained when using stimuli that had three times larger interocular phase difference (Experiment 2) or displayed at four times higher stimuli resolution (Experiment 3). Our results indicate that a binocular combination tasked based on orientation rather than phase, provides a more precise estimate of binocular eye balance in human adults at high spatial frequencies, thus allowing a binocular balance to be assessed within the spatial region where amblyopes are most defective (i.e., high spatial frequencies).

Adaptation to interocular difference.

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we ask whether the mechanism for detecting interocular differences (IDs) is adaptable. Our stimuli were horizontally oriented multispatial-frequency grating patterns that could be subject to varying degrees of ID through the introduction of interocular phase differences in the grating components. Subjects adapted to patterns that were either correlated, uncorrelated, monocular (one eye only), or anticorrelated. Following adaptation, thresholds for detecting IDs were measured using a staircase procedure. It was found that ID thresholds were elevated following adaptation to uncorrelated, monocular, and anticorrelated but not correlated patterns. Threshold elevation was found to be maximal when the orientations of the adaptor and test gratings were the same, and when their spatial frequencies were similar. The results support the existence of a specialized mechanism for detecting IDs, the most likely candidate being the binocular differencing channel proposed in previous studies.

Latent binocular function in amblyopia

Recently, psychophysical studies have shown that humans with amblyopia do have binocular function that is not normally revealed due to dominant suppressive interactions under normal viewing conditions. Here we use magnetoencephalography (MEG) combined with dichoptic visual stimulation to investigate the underlying binocular function in humans with amblyopia for stimuli that, because of their temporal properties, would be expected to bypass suppressive effects and to reveal any underlying binocular function.We recorded contrast response functions in visual cortical area V1 of amblyopes and normal observers using a steady state visually evoked responses (SSVER) protocol. We used stimuli that were frequency-tagged at 4Hz and 6Hz that allowed identification of the responses from each eye and were of a sufficiently high temporal frequency (>3Hz) to bypass suppression. To characterize binocular function, we compared dichoptic masking between the two eyes in normal and amblyopic participants as well as interocular phase differences in the two groups.We observed that the primary visual cortex responds less to the stimulation of the amblyopic eye compared to the fellow eye. The pattern of interaction in the amblyopic visual system however was not significantly different between the amblyopic and fellow eyes. However, the amblyopic suppressive interactions were lower than those observed in the binocular system of our normal observers. Furthermore, we identified an interocular processing delay of approximately 20ms in our amblyopic group.To conclude, when suppression is greatly reduced, such as the case with our stimulation above 3Hz, the amblyopic visual system exhibits a lack of binocular interactions.

Delayed suppression shapes disparity selective responses in monkey V1

The stereo correspondence problem poses a challenge to visual neurons because localized receptive fields potentially cause false responses. Neurons in the primary visual cortex (V1) partially resolve this problem by combining excitatory and suppressive responses to encode binocular disparity. We explored the time course of this combination in awake, monkey V1 neurons using subspace mapping of receptive fields. The stimulus was a binocular noise pattern constructed from discrete spatial frequency components. We forward correlated the firing of the V1 neuron with the occurrence of binocular presentations of each spatial frequency component. The forward correlation yielded a complete set of response time courses to every combination of spatial frequency and interocular phase difference. Some combinations produced suppressive responses. Typically, if an interocular phase difference for a given spatial frequency produced strong excitation, we saw suppression in response to the opposite interocular phase difference at lower spatial frequencies. The suppression was delayed relative to the excitation, with a median difference in latency of 7 ms. We found that the suppressive mechanism explains a well-known mismatch of monocular and binocular signals. The suppressive components increased power at low spatial frequencies in disparity tuning, whereas they reduced the monocular response to low spatial frequencies. This long-recognized mismatch of binocular and monocular signals reflects a suppressive mechanism that helps reduce the response to false matches.

Binocular combination of phase and contrast explained by a gain-control and gain-enhancement model

We investigated suprathreshold binocular combination, measuring both the perceived phase and perceived contrast of a cyclopean sine wave. We used a paradigm adapted from Ding and Sperling (2006, 2007) to measure the perceived phase by indicating the apparent location (phase) of the dark trough in the horizontal cyclopean sine wave relative to a black horizontal reference line, and we used the same stimuli to measure perceived contrast by matching the binocular combined contrast to a standard contrast presented to one eye. We found that under normal viewing conditions (high contrast and long stimulus duration), perceived contrast is constant, independent of the interocular contrast ratio and the interocular phase difference, while the perceived phase shifts smoothly from one eye to the other eye depending on the contrast ratios. However, at low contrasts and short stimulus durations, binocular combination is more linear and contrast summation is phase-dependent. To account for phase-dependent contrast summation, we incorporated a fusion remapping mechanism into our model, using disparity energy to shift the monocular phases towards the cyclopean phase in order to align the two eyes' images through motor/sensory fusion. The Ding-Sperling model with motor/sensory fusion mechanism gives a reasonable account of the phase dependence of binocular contrast combination and can account for either the perceived phase or the perceived contrast of a cyclopean sine wave separately; however it requires different model parameters for the two. However, when fit to both phase and contrast data simultaneously, the Ding-Sperling model fails. Incorporating interocular gain enhancement into the model results in a significant improvement in fitting both phase and contrast data simultaneously, successfully accounting for both linear summation at low contrast energy and strong nonlinearity at high contrast energy.

The Effect of Interocular Phase Difference on Perceived Contrast

Binocular vision is traditionally treated as two processes: the fusion of similar images, and the interocular suppression of dissimilar images (e.g. binocular rivalry). Recent work has demonstrated that interocular suppression is phase-insensitive, whereas binocular summation occurs only when stimuli are in phase. But how do these processes affect our perception of binocular contrast? We measured perceived contrast using a matching paradigm for a wide range of interocular phase offsets (0–180°) and matching contrasts (2–32%). Our results revealed a complex interaction between contrast and interocular phase. At low contrasts, perceived contrast reduced monotonically with increasing phase offset, by up to a factor of 1.6. At higher contrasts the pattern was non-monotonic: perceived contrast was veridical for in-phase and antiphase conditions, and monocular presentation, but increased a little at intermediate phase angles. These findings challenge a recent model in which contrast perception is phase-invariant. The results were predicted by a binocular contrast gain control model. The model involves monocular gain controls with interocular suppression from positive and negative phase channels, followed by summation across eyes and then across space. Importantly, this model—applied to conditions with vertical disparity—has only a single (zero) disparity channel and embodies both fusion and suppression processes within a single framework.

Contrast and Phase Combination in Binocular Vision

BackgroundHow the visual system combines information from the two eyes to form a unitary binocular representation of the external world is a fundamental question in vision science that has been the focus of many psychophysical and physiological investigations. Ding & Sperling (2006) measured perceived phase of the cyclopean image, and developed a binocular combination model in which each eye exerts gain control on the other eye's signal and over the other eye's gain control. Critically, the relative phase of the monocular sine-waves plays a central role.Methodology/Principal FindingsWe used the Ding-Sperling paradigm but measured both the perceived contrast and phase of cyclopean images in three hundred and eighty combinations of base contrast, interocular contrast ratio, eye origin of the probe, and interocular phase difference. We found that the perceived contrast of the cyclopean image was independent of the relative phase of the two monocular gratings, although the perceived phase depended on the relative phase and contrast ratio of the monocular images. We developed a new multi-pathway contrast-gain control model (MCM) that elaborates the Ding-Sperling binocular combination model in two ways: (1) phase and contrast of the cyclopean images are computed in separate pathways, although with shared cross-eye contrast-gain control; and (2) phase-independent local energy from the two monocular images are used in binocular contrast combination. With three free parameters, the model yielded an excellent account of data from all the experimental conditions.Conclusions/SignificanceBinocular phase combination depends on the relative phase and contrast ratio of the monocular images but binocular contrast combination is phase-invariant. Our findings suggest the involvement of at least two separate pathways in binocular combination.

Spatial and temporal tuning of motion-in-depth perception

Purpose. We used the Pulfrich stereo-phenomenon to investigate motion-in-depth. Independent control over temporal and spatial frequency content in our stimuli revealed the spatio-temporal tuning characteristics of motion-in-depth perception. Methods. Stimuli were presented to the left and right eye on a calibrated CRT display with a refresh rate of 120 Hz using a split-screen Wheatstone configuration. On each trial Ss verged on a fixation cross flanked by nonius lines as two sinusoidally oscillating sine-wave gratings were presented stereoscopically. The gratings were displayed in a Gaussian spatial envelope for 1 sec. After each presentation Ss had to indicate whether direction of motion-in-depth was clockwise or counterclockwise from a bird's eye view. Interocular phase difference between left and right sine-wave gratings was randomised (−pi/4 to +pi/4) from trial to trial to determine a discrimination threshold. Spatial frequency of the gratings (1 cpd to 4 cpd) and temporal frequency of sinusoidal motions (0.25 Hz to 5 Hz) were systematically combined in different sessions. Results. Phase thresholds for the discrimination of direction of motion-in-depth showed relative broadband temporal frequency tuning with peak sensitivities between 1 and 4 Hz. Temporal frequency tuning appeared to be independent of spatial frequency content whereas velocity tuning curves shifted systematically with increasing spatial frequency. Conclusions. Physiological evidence indicates that binocular cells in V1 typically show temporal frequency tuning whereas cells in area MT are preferably tuned to velocity. The tuning characteristics from the present experiment support the idea that binocular disparity and motion are jointly encoded in V1 before image velocity is extracted at a later cortical stage, presumably area MT.

Mechanisms Underlying the Transformation of Disparity Signals from V1 to V2 in the Macaque

Stereo vision relies on cortical signals that encode binocular disparity. In V1, the disparity energy model explains many features of binocular interaction, but it overestimates the responses to anticorrelated images. Combining the outputs of two, or more, energy model-like subunits [two-subunit (2SU) model] can resolve this discrepancy and provides an alternative explanation for disparity signals previously thought to indicate phase disparity between the receptive fields (RFs) of each eye. The 2SU model naturally explains how "near/far" (odd-symmetric) tuning becomes dominant in extrastriate cortex. To compare the energy and the 2SU models, we used a broadband compound grating and applied a common interocular phase difference to all spatial frequency components (a stimulus phase disparity), combined with a common spatial displacement (a stimulus position disparity). This produces binocular images that never occur in natural viewing, for which the 2SU model and the energy model make distinctively different predictions. Responses of neurons recorded from both V1 and V2 of awake rhesus macaques systematically deviated from the predictions of the energy model, in accordance with the 2SU model. These deviations correlated with the symmetry of the tuning curve, indicating that the 2SU mechanism is exploited to produce odd symmetry. Nonetheless, individual subunits also contain RF phase disparity that contributes to odd symmetry. The results suggest that neurons in V2 probably inherit phase disparity signals from V1 neurons, but systematically combine input from V1 neurons with different position disparities, in a way that elaborates odd-symmetric tuning and extends the range of disparities encoded by single neurons.

Adaptation to Natural Binocular Disparities in Primate V1 Explained by a Generalized Energy Model

Sensory processing in the brain is thought to have evolved to encode naturally occurring stimuli efficiently. We report an adaptation in binocular cortical neurons that reflects the tight constraints imposed by the geometry of 3D vision. We show that the widely used binocular energy model predicts that neurons dedicate part of their dynamic range to impossible combinations of left and right images. Approximately 42% of the neurons we record from V1 of awake monkeys behave in this way (a powerful confirmation of the model), while about 58% deviate from the model in a manner that concentrates more of their dynamic range on stimuli that obey the constraints of binocular geometry. We propose a simple extension of the energy model, using multiple subunits, that explains the adaptation we observe, as well as other properties of binocular neurons that have been hard to account for, such as the response to anti-correlated stereograms.

Direction-of-Motion Detection and Motion VEP Asymmetries in Normal Children and Children with Infantile Esotropia

To investigate nasal-temporal asymmetries in the detection of horizontal motion and in cortical motion visual evoked potential (mVEP) responses in normal infants and children and in patients with infantile esotropia. Monocular motion-detection thresholds were obtained separately for nasalward- and temporalward-moving random-dot patterns in a forced-choice, preferential-looking paradigm. Monocular mVEP responses were obtained while subjects viewed a 6-Hz oscillating, 1 cyc/deg vertical sine-wave grating. Nasal-temporal mVEP asymmetry was investigated with two measures from each subject: asymmetric indices (AIs) and interocular phase differences. Performance was compared in 33 visits of 28 normal subjects and 73 visits of 54 patients with infantile esotropia, ranging in age from 2 months to 5 years. At 3 to 5 months of age, both normal infants and patients with infantile esotropia had robust nasal-temporal asymmetries in motion-detection and mVEP measures. By 2 years of age, measures in all normal subjects were symmetric, as they were in patients successfully treated with glasses or alignment surgery, whereas patients who had not yet undergone alignment surgery, regardless of surgery status, had highly asymmetric mVEP responses and motion-detection thresholds. Young normal and esotropic infants exhibited nasal-temporal asymmetries in both motion detection and mVEP. These asymmetries similarly disappeared over time in normal infants and in patients with esotropia who had received successful, timely correction of misalignment. Although the initial capacity for motion processing is normal in the youngest patients with untreated esotropia at 5 months, cumulative abnormal binocular experience in these patients may disrupt motion mechanisms.

Spatial and temporal tuning of motion in depth

We used the Pulfrich effect to investigate perception of motion in depth. Independent manipulation of spatial and temporal frequency content in stereoscopic motion stimuli revealed the tuning characteristics of motion-in-depth perception. Sensitivity to interocular phase difference between sinusoidally oscillating sine-wave gratings was measured in four observers who judged direction of motion in depth. Discrimination thresholds in terms of interocular phase difference were determined to investigate spatial and temporal tuning characteristics of a system that is based on interocular phase difference, interocular delay, binocular disparity and velocity difference. Temporal frequency tuning of interocular phase difference thresholds was band pass and relatively dependent on spatial frequency variation. These results together with evidence from two control experiments support the idea that sensitivity to direction of motion in depth is limited by a stereo-motion system that monitors binocular horizontal disparity and motion rather than interocular phase difference, interocular delay, or interocular velocity difference.

Coding of disparity information in extrastriate cortex of the cat.

We have used information theory to analyse the responses of neurons in area 21a of the cat to disparity stimuli. Visual stimuli consisted of drifting sinusoidal gratings presented simultaneously to each eye. The relative spatial phase of the gratings varied between stimulus periods in a pseudo-random sequence of 45 degrees increments that covered the full 360 degrees. The mean information content of the responses of all neurons across all phases was 0.72 bits (+/-0.10, SE, n=29). The information conveyed by each neuron was well correlated with the extent to which the interocular phase difference modulated the response of the cell. However, information content was not simply related to firing rate, as there was usually significant information content in the neuronal responses to phase differences that elicited the minimum firing rate. In general, burst responses (impulse intervals <4 ms) did not convey more information than that conveyed by the total response. The contribution to the cumulative information of the response in successive 100-ms segments decreased over the course of the 1-s stimulus. The ratio of information transmitted at 200 ms to that transmitted over the full second had a median of 0.30 while the ratio of 500 ms to 1 s was 0.68.

Motion-stereo mechanisms sensitive to inter-ocular phase

We measured depth from interocular delay (The Pulfrich effect) using a dynamic random-dot pattern, consisting of a spatially-random noise field, the individual elements of which were sinusoidally-modulated in luminance over time. When an interocular phase difference in the flicker was introduced the display appeared to rotate in depth around a vertical axis like a transparent textured cylinder. The threshold phase lag was in the region of 5–10° in different observers, which translated into a non-constant, decreasing interocular delay (ms) as the flicker frequency was increased. We conclude that phase, not delay, is the critical parameter in determining the detection of depth. Threshold signal/noise ratios were measured at different delays to determine the optimum phase difference, which was found to be in the region 60–90°. However, delays centred around 180° were less detectable than those around zero, ruling out a quadrature input to the stereo-motion mechanisms. We show that depth-from-phase is a natural consequence of paired monocularly motion-direction sensitive neurones. Complex energy-detecting neurones are not required to explain the findings.

Spatial scale interactions in stereo sensitivity and the neural representation of binocular disparity.

How are binocular disparities encoded and represented in the human visual system? An 'encoding cube' diagram is introduced to visualise differences between competing models. To distinguish the models experimentally, the depth-increment-detection function (discriminating disparity d from d +/- delta d) was measured as a function of standing disparity (d) with spatially filtered random-dot stereograms of different centre spatial frequencies. Stereothresholds degraded more quickly as standing disparity was increased with stimuli defined by high rather than low centre spatial frequency. This is consistent with a close correlation between the spatial scale of detection mechanisms and the disparities they process. It is shown that a simple model, where discrimination is limited by the noisy ratio of outputs of three disparity-selective mechanisms at each spatial scale, can account for the data. It is not necessary to invoke a population code for disparity to model the depth-increment-detection function. This type of encoding scheme implies insensitivity to large interocular phase differences. Might the system have developed a strategy to disambiguate or shift the matches made at fine scales with those made at the coarse scales at large standing disparities? In agreement with Rohaly and Wilson, no evidence was found that this is so. Such a scheme would predict that stereothresholds determined with targets composed of compounds of high and low frequency should be superior to those of either component alone. Although a small stereoacuity benefit was found at small disparities, the more striking result was that stereothresholds for compound-frequency targets were actually degraded at large standing disparities. The results argue against neural shifting of the matching range of fine scales by coarse-scale matches posited by certain stereo models.

The Detection of Interocular Phase Differences

When dynamic visual noise such as the ‘snow’ on a detuned television receiver is viewed with a temporal delay between the two eyes, the noise appears to rotate in depth around a vertical axis [Ross, 1974 Nature (London)248 363 – 364; Morgan and Tyler, 1995 Proceedings of the Royal Society of London, Series B262 371 – 376]. Random noise evidently contains a wide spread of spatiotemporal Fourier components, including those for horizontal motion, which may cause a Pulfrich effect when there is an interocular delay. To investigate the temporal-frequency components necessary for the effect, a display was designed containing only a single temporal frequency. Spatially broad-band grey-level noise was flickered, such that each pixel of the noise was sinusoidally modulated over time. An interocular delay was introduced as a phase lag of the flicker in one eye. This produced a rotating depth effect. The threshold for detecting the phase shift was about 5° of phase angle, irrespective of temporal frequency, except for frequencies greater than ∼20 Hz, when the depth effect was no longer seen. Tests of several models of the phase-lag detection are described, including the possibility that there are dichoptic motion detectors also tuned to stereodisparity.

Binocular interactions in accommodation control: effects of anisometropic stimuli

In binocular viewing of real targets, the accommodative demand in the two eyes is not in general identical, yet the accommodation response in the two eyes is equal. In order to investigate how the accommodative signals from the two eyes are combined, this study has examined the effects of several forms of dynamic anisometropic stimulation on the accommodation response in both man and the rhesus monkey (Macaca mulatta). All experiments were performed in a computer-controlled haploscopic apparatus to allow independent control of the accommodative stimuli to the two eyes and of the vergence stimulus. The vergence stimulus was held constant while the accommodation demand was modulated independently in each eye. Accommodation was monitored continuously with a dynamic infrared optometer. Four anisometropic conditions were used. In two of these conditions, accommodation demand was varied sinusoidally with time in both eyes, but with phases differing by 90 degrees or 180 degrees between the two eyes. In the two remaining conditions, accommodation demand in one eye varied sinusoidally, while the accommodation demand was constant in the other. In all cases, the form of the target pattern was identified in the two eyes. The accommodation responses observed with these stimulus conditions were similar in both man and the monkey. When presented with conflicting stimuli in the two eyes, the accommodation response appeared to be best described as a compromise between the inputs to the two eyes; there were no indications of a purely random alternation of eye dominance of the form seen in binocular contour rivalry. When the accommodation demand was modulated in only one eye, there was a modulated accommodation response of similar phase to the control condition (i.e., both eyes modulated in phase) but with a much smaller gain (mean, 39% of control gain). When the accommodation demand was modulated in both eyes with a phase difference of 180 degrees, no significant modulation was observed in the accommodation response at the stimulation frequency. When the interocular phase difference was 90 degrees, a modulated response was observed that showed a mean phase lag 41 degrees more than that observed in the control condition (both eyes modulated in phase) and an appreciably smaller gain (mean, 55% of control gain). The extent to which the results can be described by a linear vector average of the uniocular inputs is considered.