Behavioral studies of local stereopsis and disparity vergence in monkeys

Optics and neural adaptation jointly limit human stereovision

Stereovision is the ability to perceive fine depth variations from small differences in the two eyes' images. Using adaptive optics, we show that even minute optical aberrations that are not clinically correctable, and go unnoticed in everyday vision, can affect stereo acuity. Hence, the human binocular system is capable of using fine details that are not experienced in everyday vision. Interestingly, stereo acuity varied considerably across individuals even when they were provided identical perfect optics. We also found that individuals' stereo acuity is better when viewing with their habitual optics rather than someone else's (better) optics. Together, these findings suggest that the visual system compensates for habitual optical aberrations through neural adaptation and thereby optimizes stereovision uniquely for each individual. Thus, stereovision is limited by small optical aberrations and by neural adaptation to one's own optics.

Similar masking effects of natural backgrounds on detection performances in humans, macaques, and macaque-V1 population responses

Visual systems evolve to process the stimuli that arise in the organism's natural environment, and hence, to fully understand the neural computations in the visual system, it is important to measure behavioral and neural responses to natural visual stimuli. Here, we measured psychometric and neurometric functions in the macaque monkey for detection of a windowed sine-wave target in uniform backgrounds and in natural backgrounds of various contrasts. The neurometric functions were obtained by near-optimal decoding of voltage-sensitive-dye-imaging (VSDI) responses at the retinotopic scale in primary visual cortex (V1). The results were compared with previous human psychophysical measurements made under the same conditions. We found that human and macaque behavioral thresholds followed the generalized Weber's law as function of contrast, and that both the slopes and the intercepts of the threshold as a function of background contrast match each other up to a single scale factor. We also found that the neurometric thresholds followed the generalized Weber's law with slopes and intercepts matching the behavioral slopes and intercepts up to a single scale factor. We conclude that human and macaque ability to detect targets in natural backgrounds are affected in the same way by background contrast, that these effects are consistent with population decoding at the retinotopic scale by down-stream circuits, and that the macaque monkey is an appropriate animal model for gaining an understanding of the neural mechanisms in humans for detecting targets in natural backgrounds. Finally, we discuss limitations of the current study and potential next steps.NEW & NOTEWORTHY We measured macaque detection performance in natural images and compared their performance to the detection sensitivity of neurophysiological responses recorded in the primary visual cortex (V1), and to the performance of human subjects. We found that 1) human and macaque behavioral performances are in quantitative agreement and 2) are consistent with near-optimal decoding of V1 population responses.

Multiple Short Daily Periods of Normal Binocular Vision Preserve Stereopsis in Strabismus

Purpose

Infantile strabismus impedes the development of stereopsis. In optically strabismic monkeys, 2 continuous hours of normal binocular vision per day has been shown to preserve near-normal stereopsis. In this study, we investigated whether, as in learning, multiple shorter periods of intervention would further boost performance.

Methods

To simulate infantile esotropia, infant monkeys were reared with 30 prism diopters base-in starting at 4 weeks of age. Daily periods of normal binocular vision were provided by replacing prisms with plano lenses. Altogether, 14 monkeys were prism reared: 2 with continuous prism, 2 with 2 continuous hours of normal binocular vision per day, 6 with 2 noncontinuous hours, and 4 with 1 noncontinuous hour of binocular vision each day. Seven normally reared monkeys provided control data. Behavioral methods were employed to measure spatial contrast sensitivity, eye alignment, and stereopsis.

Results

One monkey reared with continuous prism had poor stereopsis, and the other had no stereopsis. Ten of the 12 monkeys reared with periods of normal binocular vision had stereopsis, and those with longer and more continuous periods of binocular vision had stereopsis approaching that of normally reared monkeys.

Conclusions

During early development, multiple short periods of binocular vision were effective in preserving clinically significant stereopsis in monkeys. These results suggest that by providing relatively short multiple daily intervention periods, stereopsis may be preserved in strabismic human children.

Recovery of visual functions in amblyopic animals following brief exposure to total darkness

KEY POINTS

Occlusion of one eye of kittens (monocular deprivation) results in a severe and permanent loss of visual acuity in that eye, which parallels closely the vision loss characteristic of human amblyopia. We extended earlier work to demonstrate that amblyopic vision loss can be either blocked or erased very fast by a 10 day period of total darkness following a period of monocular deprivation that begins near birth and extends to at least 8 weeks of age. The parameters of darkness were strict because no visual recovery was observed after 5 days of darkness. In addition, short periods of light introduced each day during an otherwise 10 day period of darkness obliterated the benefits. Despite recovery of normal visual acuity, only one-quarter of the animals showed evidence of having attained normal stereoscopic vision. A period of total darkness may catalyse and improve treatment outcomes in amblyopic children. A 10 day period of total darkness has been shown to either block or erase the severe effects on vision of a prior short period of monocular deprivation (MD) in kittens depending on whether darkness is contiguous or is delayed with respect to the period of MD. We have extended these earlier findings from kittens for which the period of MD began at 1 month and lasted for 1 week to more clinically relevant situations where MD began near birth and lasted for ≥ 6 weeks. Despite the far longer MD and the absence of prior binocular vision, all animals recovered normal visual acuity in the previously deprived eye. As before, when the period of darkness followed immediately after MD, the vision of both eyes was initially very poor but, subsequently, the acuity of each eye increased gradually and equally to attain normal levels in ∼ 7 weeks. By contrast, when darkness was introduced 8 weeks after MD, the visual acuity of the deprived eye recovered quickly to normal levels in just 1 week without any change in the vision of the fellow (non-deprived) eye. Short (15 or 30 min) periods of illumination each day during an otherwise 10 day period of darkness obliterated all the benefits for vision, and a 5 day period of darkness was also completely ineffective. Measurements of depth perception indicated that, despite possessing normal visual acuity in both eyes, only about one-quarter of the animals showed evidence of having attained normal stereoscopic vision.

Early monocular defocus disrupts the normal development of receptive-field structure in V2 neurons of macaque monkeys

Experiencing different quality images in the two eyes soon after birth can cause amblyopia, a developmental vision disorder. Amblyopic humans show the reduced capacity for judging the relative position of a visual target in reference to nearby stimulus elements (position uncertainty) and often experience visual image distortion. Although abnormal pooling of local stimulus information by neurons beyond striate cortex (V1) is often suggested as a neural basis of these deficits, extrastriate neurons in the amblyopic brain have rarely been studied using microelectrode recording methods. The receptive field (RF) of neurons in visual area V2 in normal monkeys is made up of multiple subfields that are thought to reflect V1 inputs and are capable of encoding the spatial relationship between local stimulus features. We created primate models of anisometropic amblyopia and analyzed the RF subfield maps for multiple nearby V2 neurons of anesthetized monkeys by using dynamic two-dimensional noise stimuli and reverse correlation methods. Unlike in normal monkeys, the subfield maps of V2 neurons in amblyopic monkeys were severely disorganized: subfield maps showed higher heterogeneity within each neuron as well as across nearby neurons. Amblyopic V2 neurons exhibited robust binocular suppression and the strength of the suppression was positively correlated with the degree of hereogeneity and the severity of amblyopia in individual monkeys. Our results suggest that the disorganized subfield maps and robust binocular suppression of amblyopic V2 neurons are likely to adversely affect the higher stages of cortical processing resulting in position uncertainty and image distortion.

Postnatal development of disparity sensitivity in visual area 2 (v2) of macaque monkeys

Macaque monkeys do not reliably discriminate binocular depth cues until about 8 wk of age. The neural factors that limit the development of fine depth perception in primates are not known. In adults, binocular depth perception critically depends on detection of relative binocular disparities and the earliest site in the primate visual brain where a substantial proportion of neurons are capable of discriminating relative disparity is visual area 2 (V2). We examined the disparity sensitivity of V2 neurons during the first 8 wk of life in infant monkeys and compared the responses of V2 neurons to those of V1 neurons. We found that the magnitude of response modulation in V2 and V1 neurons as a function of interocular spatial phase disparity was adult-like as early as 2 wk of age. However, the optimal spatial frequency and binocular response rate of these disparity sensitive neurons were more than an octave lower in 2- and 4-wk-old infants than in adults. Consequently, despite the lower variability of neuronal firing in V2 and V1 neurons of infant monkeys, the ability of these neurons to discriminate fine disparity differences was significantly reduced compared with adults. This reduction in disparity sensitivity of V2 and V1 neurons is likely to limit binocular depth perception during the first several weeks of a monkey's life.

Effects of perceptual learning on local stereopsis and neuronal responses of V1 and V2 in prism-reared monkeys

Visual performance improves with practice (perceptual learning). In this study, we sought to determine whether or not adult monkeys reared with early abnormal visual experience improve their stereoacuity by extensive psychophysical training and testing, and if so, whether alterations of neuronal responses in the primary visual cortex (V1) and/or visual area 2 (V2) are involved in such improvement. Strabismus was optically simulated in five macaque monkeys using a prism-rearing procedure between 4 and 14 wk of age. Around 2 yr of age, three of the prism-reared monkeys ("trained" monkeys) were tested for their spatial contrast sensitivity and stereoacuity. Two other prism-reared monkeys received no training or testing ("untrained" monkeys). Microelectrode experiments were conducted around 4 yr of age. All three prism-reared trained monkeys showed improvement in stereoacuity by a factor of 7 or better. However, final stereothresholds were still approximately 10-20 times worse than those in normal monkeys. In V1, disparity sensitivity was drastically reduced in both the trained and untrained prism-reared monkeys and behavioral training had no obvious effect. In V2, the disparity sensitivity in the trained monkeys was better by a factor of approximately 2.0 compared with that in the untrained monkeys. These data suggest that the observed improvement in stereoacuity of the trained prism-reared monkeys may have resulted from better retention of disparity sensitivity in V2 and/or from "learning" by upstream neurons to more efficiently attend to residual local disparity information in V1 and V2.

Vergence target selection in rhesus monkeys: behavior and modeling

Previous studies have shown that a LATER (Linear Approach to Threshold with Ergodic Rate) race model can be used to explain saccadic target selection and latencies. The goal of the present study was to determine whether a comparable model could be applied to the underlying decision-making processes involved in target selection for transient vergence eye movements in rhesus monkeys. Luminance contrast of near and far Gabor pair stimuli were manipulated in a forced-choice paradigm to investigate their influence on vergence target selection. The distributions of responses and their latencies were evaluated by cumulative recinormal and reciprobit plots. With all targets set to 20% luminance contrast, animals showed a bias for the divergent target. Increasing luminance contrast of the near Gabor pair, while holding the far Gabor at the base contrast, resulted in increasing selection of the convergent target. This change in bias from divergent to convergent target selection correlated with decreases in convergent latency and increases in divergent latency. Monte Carlo simulations were used to estimate the internal rates of the divergent and convergent decision-making processes which, given a fixed threshold, would result in the observed distributions of vergence responses and their latencies. Statistical tests show that the LATER race model can predict observed values, and strongly suggests that competition between internal convergent and divergent target selection processes determines relative frequencies and latencies of these movements.

Binocular Deficits Associated With Early Alternating Monocular Defocus. I. Behavioral Observations

To study the binocular vision deficits associated with anisometropia, monkeys were reared with alternating monocular defocus, which allowed monocular mechanisms to develop normally while binocular mechanisms were selectively compromised. A defocusing contact lens of –1.5 D, –3 D, or –6 D was worn on alternate eyes on successive days ( n = 3 per lens power) from 3 wk to 9 mo of age. The control subjects were two normally reared monkeys and two human observers. Functional binocular vision was assessed through behavioral measurements of stereoscopic depth discrimination thresholds as a function of spatial frequency. To characterize the extent of the deficits in disparity processing at a given spatial frequency, the contrast required to support stereopsis was determined for a range of disparities that exceeded the subjects' measured stereoacuity. The lens-reared monkeys showed spatial-frequency-selective deficits in stereopsis that depended on the magnitude of the simulated anisometropia experienced during the rearing period. For a given spatial frequency, the treated monkeys generally required higher than normal contrasts to support stereopsis even for large disparities. Moreover, a given increase in contrast produced smaller than normal improvements in stereo discrimination in our treated subjects, which suggests that in addition to deficits in contrast sensitivity, disparity-sensitive mechanisms exhibited low contrast gains. The spatial-frequency selective nature of the binocular deficits produced by the imposed anisometropia indicate that disparity processing mechanisms are normally spatial-frequency selective and that mechanisms tuned to different spatial frequencies can be differentially affected by abnormal binocular visual experience.

Do visual cues contribute to the neural estimate of viewing distance used by the oculomotor system?

Perceived shape and depth judgments that require knowledge of viewing distance are strongly influenced by both vergence angle and the pattern of vertical disparities across large visual fields. On the basis of this established contribution of visual cues to the neural estimate of viewing distance, we hypothesized that the oculomotor system would also make use of high-level visual cues to distance. To address this hypothesis, we investigated how compensatory eye movements during whole-body translation scale with viewing distance. Monkeys viewed large-field (85 x 68 degrees ) random-dot stereograms that were rear projected onto a fixed screen and simulated either a textured wall or pyramid at different viewing distances. In these stereograms, we independently manipulated vergence angle, horizontal and vertical disparity gradients, relative horizontal disparities, and textural cues to viewing distance. For comparison, random-dot patterns were also projected onto a moveable screen placed at different physical distances from the animal. Several cycles of left-right sinusoidal motion of the monkey at 5 Hz were interleaved with several cycles of motion in darkness, and the relationship between eye movement responses and viewing distance was quantified. As expected from previous work, the amplitude of compensatory eye movements depended strongly on vergence angle. Although visual cues to distance had a statistically significant effect on eye movements, these effects were approximately 20-fold weaker than the effect of vergence angle. We conclude that sensory and motor systems do not share a common neural estimate of viewing distance and that the oculomotor system relies far less on visual cues than the perceptual system.

Temporal integration for stereoscopic vision

With normal binocular vision, maximal stereoacuity requires an extended viewing duration, but the relationship between the critical viewing duration for stereopsis and other variables affecting stereoacuity is unknown. The purposes of the study were to investigate the properties of normal temporal integration for stereoscopic vision with respect to the effects of contrast and spatial frequency of the stimuli and to determine whether the temporal summation of disparity is affected in deficient stereopsis caused by abnormal binocular vision during infancy. Psychophysical methods were used to measure stereothresholds in human and monkey subjects with either normal binocular vision or abnormal binocular vision. The results showed that the critical viewing duration for stereoscopic depth discrimination was independent of variations in basic stimulus parameters and/or the subject's stereoacuity. A critical duration of approximately 100 ms was found for both local (narrowband Gabor and broadband line targets) and global (dynamic random dots) stimuli. Although stereothresholds increased with decreasing stimulus contrast, the properties of temporal integration did not. Stereothresholds were substantially elevated for monkeys and humans with abnormal binocular vision, but the critical durations for these subjects were not significantly different from those of subjects with normal binocular vision. Overall, the results demonstrate that the general properties of temporal integration for stereopsis are similar to other detection and discrimination tasks that do not require binocular processing. In addition, increased integration time does not account for the elevated stereothresholds of subjects with abnormal binocular vision.

Quantitative analysis of the responses of V1 neurons to horizontal disparity in dynamic random-dot stereograms

Horizontal disparity tuning for dynamic random-dot stereograms was investigated for a large population of neurons (n = 787) in V1 of the awake macaque. Disparity sensitivity was quantified using a measure of the discriminability of the maximum and minimum points on the disparity tuning curve. This measure and others revealed a continuum of selectivity rather than separate populations of disparity- and nondisparity-sensitive neurons. Although disparity sensitivity was correlated with the degree of direction tuning, it was not correlated with other significant neuronal properties, including preferred orientation and ocular dominance. In accordance with the Gabor energy model, tuning curves for horizontal disparity were adequately described by Gabor functions when the neuron's orientation preference was near vertical. For neurons with orientation preferences near to horizontal, a Gaussian function was more frequently sufficient. The spatial frequency of the Gabor function that described the disparity tuning was weakly correlated with measurements of the spatial frequency and orientation preference of the neuron for drifting sinusoidal gratings. Energy models make several predictions about the relationship between the response rates to monocular and binocular dot patterns. Few of the predictions were fulfilled exactly, although the observations can be reconciled with the energy model by simple modifications. These same modifications also provide an account of the observed continuum in strength of disparity selectivity. A weak correlation between the disparity sensitivity of simultaneously recorded single- and multiunit data were revealed as well as a weak tendency to show similar disparity preferences. This is compatible with a degree of local clustering for disparity sensitivity in V1, although this is much weaker than that reported in area MT.

Clinical suppression in monkeys reared with abnormal binocular visual experience

To determine if monkeys exhibit clinical suppression in response to early abnormal binocular vision, we compared dichoptic to monocular luminance increment thresholds in monkeys reared with alternating monocular defocus or optically induced strabismus. In the absence of amblyopia, clinical suppression was associated with strabismus and with as little as 1.50 diopters of anisometropia. The severity of suppression was roughly correlated with the magnitude of anisometropia. The demonstration of clinical suppression in monkeys provides a model for future investigations of factors that may influence the development of suppression, but which are not possible to accurately document or manipulate in human subjects.

Three-dimensional shape coding in inferior temporal cortex

Neurons in the rostral lower bank of the superior temporal sulcus (TEs), part of the inferior temporal cortex, respond selectively to three-dimensional (3D) shapes. We have investigated how these neurons represent disparity-defined 3D structure. Most neurons were selective for either first-order (disparity gradients) or second-order (disparity curvature) disparities. The latter selectivity proved remarkably vulnerable to disparity discontinuities, such as sharp edges or steps in disparity. The majority of the neurons remained selective for small disparity variations within the stimulus. 3D shape selectivity was preserved when the frontoparallel position or the stimulus size was altered. Thus, in TEs, 3D shape is coded by first- and second-order disparity-selective neurons, which are highly sensitive to spatial variations of disparity.

The precision of single neuron responses in cortical area V1 during stereoscopic depth judgments

The performance of single neurons in cortical area V1 of alert macaque monkeys was compared against the animals' psychophysical performance during a binocular disparity discrimination task. Performance was assessed with stimuli that consisted of a patch of dynamic random dots, whose disparity varied from trial to trial, surrounded by an annulus of similar dots at a fixed disparity. On each trial, the animals indicated whether the depth of the central patch was in front of or behind the annulus. For each disparity of the center patch, neural performance was assessed by calculating the probability that the response of the neuron was greater or less than the response when the center disparity was the same as that of the annulus. Initially the animals performed the task simultaneously with the neural recording. However, the range of disparities used, which was appropriate for the neuronal recording, may have affected performance, because the thresholds were substantially lower (2.6x) when the psychophysical measurements were repeated later. Average neuronal thresholds were approximately 4x poorer than these behavioral thresholds, although the best neurons were marginally better than the animals' behavior. Thus, the well known precision of relative depth judgments can be supported with signals from a small number of V1 neurons. Interference with the relative depth information in the stimulus profoundly affected behavioral thresholds, which were approximately 10x poorer when the surround was absent or contained binocularly uncorrelated dots. In this case, single V1 neurons consistently outperform the observer: presumably here, psychophysical thresholds are limited by other factors (such as uncertainty about vergence eye position).

Assessment of stereopsis in rhesus monkeys using visual evoked potentials

Rhesus monkeys can have deficiencies in stereo vision, making it necessary to screen monkey subjects intended for single cell studies of stereo-based depth processing. We measured VEPs in two monkeys using a dynamic random-dot display in which a stereo-defined checkerboard reversed in depth. Monkeys fixated upon a small dot during stimulus presentation. One monkey showed clear evoked potentials in response to changes in disparity that were similar to those obtained in human subjects, using an identical stimulus paradigm. Controls with presentations of the monocular stimulus sequences (in which no depth reversal can be perceived) yielded no or much weaker VEPs. In the other animal, however, there was no difference in evoked potential between the two conditions. These electrophysiological findings closely match the performance of these same two subjects in a disparity discrimination task in which they were previously trained. We conclude that VEPs using this type of stimulus display can be used to screen monkeys for single cell or behavioral studies of stereopsis.

Fixation disparity and nonius bias

Fixation disparity, i.e. the vergence error within Panum's area, can be measured psychophysically with two nonius (vernier) lines that are presented dichoptically, i.e. one to each eye. The observer adjusts these nonius lines to subjective alignment; the resulting physical nonius offset indicates the amount of fixation disparity. The present experiments investigate the relation between fixation disparity and the nonius bias, which is the physical offset of the nonius lines that is adjusted by the observer in order to perceive them as aligned when both nonius lines are presented to both eyes (binocular nonius bias) or both to the left or both to the right eye (monocular nonius bias). It was found that (1) the fixation disparity is correlated with the binocular nonius bias in the horizontal and vertical meridian and (2) the binocular nonius bias can be predicted from the average of the right eye and left eye monocular nonius bias. To remove the influence of the nonius bias on measured fixation disparity it is possible to calculate the fixation disparity relative to the individual binocular nonius bias, rather than to the physical coincidence of the nonius lines. This procedure tends to increase the correlation between fixation disparity and the tonic resting position of vergence. We discuss the clinical relevance of the dichoptic nonius method for measuring fixation disparity and its limitations as compared to physical recordings of eye position.

Neural mechanisms underlying stereoscopic vision

The progressive frontalization of both eyes in mammals causes overlap of the left and right visual fields, having as a consequence a region of binocular field with single vision and stereopsis. The horizontal separation of the eyes makes the retinal images of the objects lying in this binocular field have slight horizontal and vertical differences, termed disparities. Horizontal disparities are the main cue for stereopsis. In the past decades numerous physiological studies made on monkeys, which have in many aspects a similar visual system to humans, showed that a population of visual cells are capable of encoding the amplitude and sign of horizontal disparity. Such disparity detectors were found in cortical visual areas V1, V2, V3, V3A, VP, MT (V5) and MST of monkeys and in the superior colliculus of the cat and opossum. According to their disparity tuning function, these cells were first grouped into tuned excitatory, tuned inhibitory, near and far sub-groups. Subsequent studies added two more categories, tuned near and tuned far cells. Asymmetries between left and right receptive field position, on and off regions, and intra-receptive field wiring are believed to be the neural mechanisms of disparity detection. Because horizontal disparity alone is insufficient to compute reliable stereopsis, additional information about fixation distance and angle of gaze is required. Thus, while there is unequivocal evidence of cells capable of detecting horizontal disparities, it is not known how horizontal disparity is calibrated. Sensitivity to vertical disparity and information about the vergence angle or eye position may be the source of this additional information.

Stereopsis and disparity vergence in monkeys with subnormal binocular vision

The surgical treatment for strabismus in infants generally results in microtropia or subnormal binocular vision. Although the clinical characteristics of these conditions are well established, there are important questions about the mechanisms of binocular vision in these patients that can best be investigated in an appropriate animal model. In the present psychophysical investigations, spatial frequency response functions for disparity-induced fusional vergence and for local stereopsis were studied in macaque monkeys, who demonstrated many of the major visual characteristics of patients whose eyes were surgically aligned during infancy. In six rhesus monkeys, unilateral esotropia was surgically induced at various ages (30-184 days of age). However, over the next 12 months, all of the monkeys recovered normal eye alignment. Behavioral measurements at 4-6 years of age showed that the monkeys' prism-induced fusional vergence responses were indistinguishable from those of control monkeys or humans with normal binocular vision. Investigations of stereo-depth discrimination demonstrated that each of the experimental monkeys also had stereoscopic vision, but their stereoacuities varied from being essentially normal to severely stereo-deficient. The degree of stereo-deficiency was not related to the age at which surgical esotropia was induced, or to the presence or absence of amblyopia, and was not dependent on the spatial frequency of the test stimulus. Altogether, these experiments demonstrate that a temporary, early esotropia can affect the binocular disparity responses of motor and sensory components of binocular vision differently, probably because of different sensitive periods of development for the two components.

Contextual modulation in primary visual cortex

We studied extra-receptive field contextual modulation in area V1 of awake, behaving macaque monkeys. Contextual modulation was studied using texture displays in which texture covering the receptive field (RF) was the same in all trials, but the perceptual context of this texture could vary depending on the configuration of extra-RF texture elements. We found robust contextual modulation when disparity, color, luminance, and orientation cues variously defined a textured figure centered on the RF of V1 neurons. We found contextual modulation to have a spatial extent of approximately 8 to 10 degrees diameter parafoveally. Contextual modulation correlated with perceptual experience of both binocularly rivalrous texture displays and of displays with a simple example of surface occlusion. We found contextual modulation in V1 to have a characteristic latency of 80-100 msec after stimulus onset, potentially allowing feedback from extrastriate areas to underlie to this effect.

Judgments by monkeys of apparent depth in dynamic random-dot stereograms

Young macaques discriminated apparent depths of targets embedded in dynamic random dot stereograms; a test of stereopsis. In a 'same/different' paradigm, the discrimination took longer if the pair of stimuli appeared to be in same depth plane, than when they appeared to be located in a different depth plane. The decision time was an inverse function of the disparity difference. Apparent depth discrimination performance decreased as a function of disparity, with no differences in judgments regarding crossed or uncrossed disparities.

Motor and sensory fusion in monkeys: psychophysical measurements

Motor and sensory fusion, the basic processes of binocularity, must be present for bifoveal fixation with true fusion and stereopsis during ordinary viewing. The characteristics of motor and sensory fusion have been established for patients with normal and subnormal binocular vision; the present report describes our psychophysical studies of these processes in the macaque monkey. Three recent investigations of motor and sensory fusion in monkeys are described. The studies involved: (1) the comparability of motor and sensory fusion in monkeys and humans with normal binocular vision, (2) the effects of an early period of abnormal binocular vision on motor and sensory fusion in monkeys, and (3) the contrast sensitivity for binocular disparity in monkeys with stereo-deficiencies. The results of these studies demonstrated an excellent homology between the normal binocular vision of monkeys and humans. We also found that a period of esotropia during infancy caused deficiencies in sensory fusion, but not motor fusion. In some monkeys, the sensory deficiency persisted over the entire range of binocular disparities that were compatible with stereopsis, while other subjects demonstrated normal stereo-sensitivity for the largest fusible binocular disparities. The stereo-deficiencies of these monkeys, along with other visual attributes, suggest that their binocular vision is a viable model for the binocularity of patients with subnormal binocular vision or the monofixation syndrome.