Abnormal motion processing and binocularity: infantile esotropia as a model system for effects of early interruptions of binocularity

Motion-Defined Form Perception in Deprivation Amblyopia

Purpose

The purpose of this study was to assess motion-defined form perception, including the association with clinical and sensory factors that may drive performance, in each eye of children with deprivation amblyopia due to unilateral cataract.

Methods

Coherence thresholds for orientation discrimination of motion-defined form were measured using a staircase procedure in 30 children with deprivation amblyopia and 59 age-matched controls. Visual acuity, stereoacuity, fusion, and interocular suppression were also measured. Fixation stability and fellow-eye global motion thresholds were measured in a subset of children.

Results

Motion-defined form coherence thresholds were elevated in 90% of children with deprivation amblyopia when viewing with the amblyopic eye and in 40% when viewing with the fellow eye. The deficit was similar in children with a cataract that had been visually significant at birth (congenital) and in children for whom the cataract appeared later in infancy or childhood (developmental). Poorer motion-defined form perception in amblyopic eyes was associated with poorer visual acuity, poorer binocular function, greater interocular suppression, and the presence of nystagmus. Fellow-eye deficits were not associated with any of these factors, but a temporo-nasal asymmetry for global motion perception in favor of nasalward motion suggested a general disruption in motion perception.

Conclusions

Deficits in motion-defined form perception are common in children with deprivation amblyopia and may reflect a problem in motion processing that relies on binocular mechanisms.

Motion Processing Deficits in Children With Cerebral Visual Impairment and Good Visual Acuity

Purpose

We sought to characterize neural motion processing deficits in children with cerebral visual impairment (CVI) who have good visual acuity using an objective, quantifiable method (steady-state visual evoked potentials [SSVEPs]).

Methods

We recorded SSVEPs in response to three types of visual motion – absolute motion and more complex relative and rotary motion, comparing them to form-related vernier and contour responses. We studied a group of 31 children with CVI diagnosed via detailed clinical examinations and 28 age-matched healthy controls.

Results

Using measurements made at the appropriate response harmonics of the stimulation frequency, we found significant deficits in cerebral processing of relative and rotary motion but not of absolute motion in children with CVI compared with healthy controls. Vernier acuity, in keeping with good recognition acuity in both groups, was not different, nor were contour-related form responses.

Conclusions

Deficits for complex motion but relative sparing of elementary motion and form-related signals suggests preferential damage to extra-striate visual motion areas in children with CVI. The fact that these preferential losses occur in the absence of significant acuity loss indicates that they are not secondary to reduced visual acuity, but rather are an independent vulnerability in CVI. These results corroborate parental and caregivers’ reports of difficulties with tasks that involve motion perception in children with CVI.

Vertical Optokinetic Stimulation Induces Diagonal Eye Movements in Patients with Idiopathic Infantile Nystagmus

Purpose

In patients with early ocular misalignment and nystagmus, vertical optokinetic stimulation reportedly increases the horizontal component of the nystagmus present during fixation, resulting in diagonal eye movements. We tested patients with infantile nystagmus syndrome but normal ocular alignment to determine if this crosstalk depends on strabismus.

Methods

Eye movements were recorded in seven patients with infantile nystagmus. All but one patient had normal ocular alignment with high-grade stereopsis. Nystagmus during interleaved trials of right, left, up, and down optokinetic stimulation was compared with waveforms recorded during fixation. Six patients with strabismus but no nystagmus were also tested.

Results

In infantile nystagmus syndrome, horizontal motion evoked a mostly jerk nystagmus with virtually no vertical component. A vertical optokinetic pattern produced nystagmus with a diagonal trajectory. It was not simply a combination of a vertical component from optokinetic stimulation and a horizontal component from the subject's congenital nystagmus, rather in six of seven patients, the slow-phase velocity of the horizontal component during vertical optokinetic stimulation differed from that recorded during fixation. In the six strabismus patients without nystagmus, responses to vertical optokinetic stimulation were normal.

Conclusions

In patients with congenital motor nystagmus, a vertical noise pattern drives a diagonal nystagmus. This appears to arise because of crosstalk between the vertical and horizontal components of the optokinetic system. This abnormal response to vertical stimulation is not caused by strabismus because it occurs in patients with infantile nystagmus without strabismus. Moreover, it is absent in patients with strabismus and no spontaneous nystagmus.

Convergence and divergence to radial optic flow in infancy

Research finds a relationship between the development of depth perception and ocular motion functions including smooth pursuit and ocular following response. Infants' reactions to looming stimuli also suggest sensitivity to optic flow information that specifies relative distance. With radial optic flow, an expanding flow field elicits involuntary convergent eye movements while a contracting one elicits involuntary divergent eye movements. This response suggests the visual system is interpreting the radial flow as a change in relative depth. We measured the oculomotor response to radial optic flow in infants aged two to five months. The stimulus comprised a radial optic flow pattern that expanded or contracted across eight 400 ms trials while eye position was monitored with a Tobii X120 eye tracker. A subset of infants also viewed trials of a static version of the stimulus. On average, most infants in each age group demonstrated convergence to the expanding pattern and divergence to the contracting one. Moreover, the difference in gain between the convergence and divergence eye movements was significant. The presence of correct-direction vergence eye movements in response to expansion and contraction provides further evidence that infants are sensitive to information that specifies relative motion in depth.

Is infantile esotropia subcortical in origin?

Essential infantile esotropia (EIE) is often attributed to a primary disturbance within the visual cortex based upon the findings of monocular horizontal optokinetic asymmetry and correlative horizontal motion detection asymmetry. However, these physiologic aberrations conform to what would be observed if the visual cortex secondarily reconfigured itself to the preexisting subcortical optokinetic motion template. This analysis examines the perspective that the measured cortical aberrations can be explained by prolonged subcortical neuroplasticity, leading to a secondary rewiring of cortical motion pathways. Evolutionary evidence indicates that EIE is generated by subcortical ocular motor centers that subserve nasalward optokinesis. These phylogenetically older subcortical visuo-vestibular pathways include the nucleus of the optic tract, accessory optic system, inferior olive, cerebellar flocculus, and vestibular nucleus. In normal humans, the subcortical visual system becomes inactivated after the first few months of infancy. Mutations or other perturbations that prolong subcortical neuroplasticity may create a persistent simultaneous nasalward optokinetic bias in both eyes to generate infantile esotropia.

Disappearance of Biased Visual Attention in Infants: Remediated Tonic Neck Reflex or Maturating Visual Asymmetry?

Typically, infants younger than four months fail to attend to the left side of their spatial field, most likely due to an innate asymmetrical tonic neck reflex (ATNR). In a critical transition, by four months of age, infants begin to reach and develop depth perception; and, by five months, they tend to monitor the entire spatial field. However, this developmental transition can be delayed. Moreover, there is always a residual right-sided spatial bias under cognitive load, a phenomenon that may also occur among adult stroke patients. While causative factors of biased visual attention in both infants and brain-injured adults may vary, mechanisms of remediation may be similar. This literature review addresses whether the infant's emergence of attention toward a full visual spatial field and the associated shift from monocular to binocular vision occurs because of (a) increased left side reaching, loosening the rarely mentioned high muscle tension ATNR or (b) maturational resolution of visual asymmetry in motion perception. More research is needed to investigate the origins of the infants' visual control system and factors involved in its development, especially because Alzheimer and dementia patients may also show primitive two-dimensional vision and deficits in perceiving objects-in-motion that seem to mirror infant visual perception.

Strabismic amblyopia disrupts the hemispheric asymmetry for spatial stimuli in cortical visual processing

Hemispheric asymmetry in processing visual stimuli was assessed in anisometropic and strabismic amblyopia and control subjects. Measurements of contrast sensitivity for low and high spatial frequencies were performed psychophysically and tested under functional magnetic resonance imaging (fMRI) using a stimulus configuration that generates measurements for each temporal and nasal hemifield. The fMRI and the psychophysics results showed a marked hemispheric asymmetry in processing spatial frequencies for normal and anisometropic adults, in which low spatial frequencies were mainly processed in the left visual field – right hemisphere (LVF-RH: 0.3 cycles per degree [cpd]; F = 12.548; p = .002) and the high spatial frequencies were predominating processed in the right visual field – left hemisphere (RVF-LH: 2.0 cpd; F = 4.582; p = .021 and 8.3 cpd; F = 8.561; p = .001). No asymmetry was present in the amblyopic and the fellow eye of the strabismic amblyopia subjects. We conclude that the developmental organization of visual cortex in strabismic amblyopia is impaired differently from what happens in the anisometropic amblyopia and support the impairment of high-level visual-related functions observed in strabismic children.

The steady-state visual evoked potential in vision research: A review

Periodic visual stimulation and analysis of the resulting steady-state visual evoked potentials were first introduced over 80 years ago as a means to study visual sensation and perception. From the first single-channel recording of responses to modulated light to the present use of sophisticated digital displays composed of complex visual stimuli and high-density recording arrays, steady-state methods have been applied in a broad range of scientific and applied settings.The purpose of this article is to describe the fundamental stimulation paradigms for steady-state visual evoked potentials and to illustrate these principles through research findings across a range of applications in vision science.

Development of sampling efficiency and internal noise in motion detection and discrimination in school-aged children

The aim of this study was to use an equivalent noise paradigm to investigate the development and maturation of motion perception, and how the underlying limitations of sampling efficiency and internal noise effect motion detection and direction discrimination in school-aged children (5-14 years) and adults. Contrast energy thresholds of a 2c/deg sinusoidal grating drifting at 1.0 or 6.0 Hz were measured as a function of added dynamic noise in three tasks: detection of a drifting grating; detection of the sum of two oppositely drifting gratings and direction discrimination of oppositely drifting gratings. Compared to the ideal observer, in both children and adults, the performance for all tasks was limited by reduced sampling efficiency and internal noise. However, the thresholds for discrimination of motion direction and detection of moving gratings show very different developmental profiles. Motion direction discrimination continues to improve after the age of 14 years due to an increase in sampling efficiency that differs with speed. Motion detection and summation were already mature at the age of 5 years, and internal noise was the same for all tasks. These findings were confirmed in a 1-year follow-up study on a group of children from the initial study. The results support suggestions that the detection of a moving pattern and discriminating motion direction are processed by different systems that may develop at different rates.

The role of eye movements in depth from motion parallax during infancy

Motion parallax is a motion-based, monocular depth cue that uses an object's relative motion and velocity as a cue to relative depth. In adults, and in monkeys, a smooth pursuit eye movement signal is used to disambiguate the depth-sign provided by these relative motion cues. The current study investigates infants' perception of depth from motion parallax and the development of two oculomotor functions, smooth pursuit and the ocular following response (OFR) eye movements. Infants 8 to 20 weeks of age were presented with three tasks in a single session: depth from motion parallax, smooth pursuit tracking, and OFR to translation. The development of smooth pursuit was significantly related to age, as was sensitivity to motion parallax. OFR eye movements also corresponded to both age and smooth pursuit gain, with groups of infants demonstrating asymmetric function in both types of eye movements. These results suggest that the development of the eye movement system may play a crucial role in the sensitivity to depth from motion parallax in infancy. Moreover, describing the development of these oculomotor functions in relation to depth perception may aid in the understanding of certain visual dysfunctions.

Why do only some hyperopes become strabismic?

Children with hyperopia greater than +3.5 diopters (D) are at increased risk for developing refractive esotropia. However, only approximately 20% of these hyperopes develop strabismus. This review provides a systematic theoretical analysis of the accommodation and vergence oculomotor systems with a view to understanding factors that could either protect a hyperopic individual or precipitate a strabismus. The goal is to consider factors that may predict refractive esotropia in an individual and therefore help identify the subset of hyperopes who are at the highest risk for this strabismus, warranting the most consideration in a preventive effort.

The neural mechanism for Latent (fusion maldevelopment) nystagmus

Latent nystagmus (LN) is the by-product of fusion maldevelopment in infancy. Because fusion maldevelopment--in the form of strabismus and amblyopia--is common, LN is a prevalent form of pathologic nystagmus encountered in clinical practice. It originates as an afferent visual pathway disorder. To unravel the mechanism for LN, we studied patients and nonhuman primates with maldeveloped fusion. These experiments have revealed that loss of binocular connections within striate cortex (area V1) in the first months of life is the necessary and sufficient cause of LN. The severity of LN increases systematically with longer durations of binocular decorrelation and greater losses of V1 connections. Decorrelation durations that exceed the equivalent of 2-3 months in human development result in an LN prevalence of 100%. No manipulation of brain stem motor pathways is required. The binocular maldevelopment originating in area V1 is passed on to downstream extrastriate regions of cerebral cortex that drive conjugate gaze, notably MSTd. Conjugate gaze is stable when MSTd neurons of the right and left cerebral hemispheres have balanced binocular activity. Fusion maldevelopment in infancy causes unbalanced monocular activity. If input from one eye dominates and the other is suppressed, MSTd in one hemisphere becomes more active. Acting through downstream projections to the ipsilateral nucleus of the optic tract, the eyes are driven conjugately to that side. The unbalanced MSTd drive is evident as the nasalward gaze-holding bias of LN when viewing with either eye.

Timing of surgery for infantile esotropia in humans: effects on cortical motion visual evoked responses

PURPOSE

Infantile esotropia is associated with maldevelopment of cortical visual motion processing, manifested as directional asymmetry of motion visual evoked potentials (mVEPs). The purpose of this study was to determine whether early surgery at or before age 11 months could promote the development of cortical visual motion processing in human infants, compared with standard surgery at age 11 to 18 months.

METHODS

Sixteen children with a constant, infantile esotropia >or=30 prism diopters and onset before age 6 months were recruited prospectively. Eight of them underwent early surgery at

RESULTS

The mean asymmetry index and interocular phase difference in the early surgery group were comparable to that in age-matched control subjects, and they were significantly lower than those in the standard surgery group.

CONCLUSIONS

Early surgery for infantile esotropia promotes the development of cortical visual motion processing, whereas standard surgery is associated with abnormal mVEPs. The results provide additional evidence that early strabismus repair is beneficial for cortical development in human infants.

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Key Words Collapse

MESH Headings

• Evoked Potentials, Visual/physiology
• Eye Movements/physiology
• Female
• Humans
• Infant
• Male
• Motion Perception/physiology
• Nystagmus, Congenital/physiopathology
• Nystagmus, Congenital/surgery
• Oculomotor Muscles/surgery
• Prospective Studies
• Tendons/surgery
• Time Factors
• Visual Cortex/physiology

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Grants

• 67104 CIHR

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Affiliation(s)

Christina Gerth Department of Ophthalmology and Vision Science, The Hospital for Sick Children and University of Toronto, Toronto, Ontario, Canada
Giuseppe Mirabella
Xiaoqing Li
Thomas Wright
Carol Westall
Linda Colpa
Agnes M F Wong

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Variables associated with the incidence of infantile esotropia

PURPOSE

Infantile esotropia (manifesting from birth to 6 months) is a common type of strabismus, accounting for 28% to 54% of all esotropias and with an incidence of 1% of the general population. The purpose of this cohort study was to evaluate risk factors for infantile esotropia. Such information may aid in early intervention to prevent manifestation of infantile esotropia.

METHODS

A retrospective chart review of 5,347 records (October 1, 1993, to September 30, 2003) of birth mothers and infants at the W.W. Hastings Indian Health Science Hospital in Tahlequah, Oklahoma, was performed to identify children with varying degrees of Native American blood who had infantile esotropia. A nonstrabismic birth cohort control group was also identified. Twenty-three medical records indicating a diagnosis of infantile esotropia that were complete enough to be used in analysis were identified. Normal infants were compared with infants with esotropia.

RESULTS

Infantile esotropia in this preliminary study was associated with 24 factors including prematurity, family ocular history, cardiovascular disease, systemic disease, pregnancy-associated hypertension and low birth weight (<2,500 g) among others.

CONCLUSIONS

Prematurity, family history or secondary ocular history, perinatal or gestational complications, systemic disorders, use of supplemental oxygen as a neonate, use of systemic medications, and male sex were found to be significant risk factors for infantile esotropia. Our results provide additional evidence that might help facilitate early detection and intervention in cases in which these risk factors are identified.

A primer on motion visual evoked potentials

Motion visual evoked potentials (motion VEPs) have been used since the late 1960s to investigate the properties of human visual motion processing, and continue to be a popular tool with a possible future in clinical diagnosis. This review first provides a synopsis of the characteristics of motion VEPs and then summarizes important methodological aspects. A subsequent overview illustrates how motion VEPs have been applied to study basic functions of human motion processing and shows perspectives for their use as a diagnostic tool.

Effect of infantile strabismus on visuomotor development in the squirrel monkey (Saimiri sciureus): optokinetic nystagmus, motion VEP and spatial sweep VEP

PURPOSE

To determine whether squirrel monkeys made artificially strabismic in infancy had ocular fixation abnormalities, directional asymmetries of horizontal optokinetic nystagmus (OKN) and asymmetries of motion visually evoked potentials (MVEPs) similar to those of humans with infantile strabismus.

METHODS

Esotropia was produced in a newborn squirrel monkey by surgical tenotomy of both lateral rectus muscles. The alignment and eye rotations of the monkey were examined longitudinally and VEP testing was performed at the age of one year. Visual acuity was measured using spatial frequency sweep VEPs (SSVEP) in response to grating stimulation. OKN was tested under conditions of monocular viewing using full-visual-field, vertically oriented, moving stripes. MVEPs in response to horizontal motion were recorded with the animal sedated to reduce the possibility of eye movement artifact.

RESULTS

The artificially strabismic squirrel monkey displayed a constant, committant esotropic strabismus accompanied by latent nystagmus. Monocular SSVEP acuity was subnormal in one eye, consistent with mild monocular strabismic amblyopia. The monkey demonstrated asymmetric OKN favoring nasally-directed stimulus motion when viewing with either eye. Monocular MVEPs were also characterized by a horizontal asymmetry with a directional bias inverted 180 degrees between the right and the left eyes. The eye movements and MVEP asymmetries were similar to those observed in strabismic macaque monkeys and humans with early-onset strabismus. Neither the OKN asymmetry nor the MVEP asymmetry was evident in a normal squirrel or normal macaque monkey.

CONCLUSION

The artificially strabismic squirrel monkey is an appropriate eye movement and VEP model for the study of neural mechanisms in human infantile strabismus.

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.

Asymmetric motion visually evoked potentials in infantile strabismus are not an artifact of latent nystagmus

BACKGROUND

Patients who have infantile strabismus exhibit a directional asymmetry of motion visually evoked potentials (MVEPs) recorded under conditions of monocular viewing. The majority of these patients also have latent nystagmus, raising the possibility that the MVEP asymmetry is an artifact of the nystagmus. To explore this issue, we correlated MVEPs and eye movements under conditions that eliminated or increased latent nystagmus.

METHODS

MVEPs and eye movements were recorded under conditions of monocular viewing in three adults who had combined infantile-onset strabismus and latent nystagmus. The subjects viewed vertically oriented grating stimuli that oscillated horizontally at temporal frequencies of 6.6 to 11.0 Hz by use of spatial frequencies of 1 to 3 cycles/degree. Quantitative eye movement recordings of latent nystagmus and horizontal pursuit/optokinetic nystagmus were also obtained.

RESULTS

Eye movement recordings showed that the latent nystagmus was absent or markedly diminished when the viewing eye was in a 45-degree adducted position, whereas nystagmus velocity increased 10 to 40 times (to 2.2 to 4.5 degrees/second) when the viewing eye was in an abducted position (p < 0.05). MVEPs were abnormal (asymmetry indices > 0.40) when the viewing eye was in an adducted or abducted position of gaze. No correlation was found between the MVEP asymmetry index and the velocity of latent nystagmus.

CONCLUSIONS

MVEP asymmetries in infantile strabismus remain robust under conditions that eliminate or greatly reduce the oscillations of latent nystagmus. MVEP asymmetries and ocular motor abnormalities both characterize infantile strabismus, but the ocular motor defects do not cause the MVEP asymmetries. The nasotemporal asymmetry of MVEPs and the nasotemporal asymmetry of pursuit and latent nystagmus are likely caused by deficits in related but separate binocular visual cortical circuits.

Development of directional motion symmetry in the monocular visually evoked potential of infant monkeys

Motion processing in humans and monkeys exhibit a directional asymmetry during infancy which is not present in adults except following abnormal visual rearing conditions. To characterize the time course for maturation of a symmetric response, we measured the monocular visually evoked potential (MVEP) response to 0.26 c/deg gratings oscillating horizontally at 6 Hz in 13 infant rhesus monkeys between 1 and 52 weeks of age. An asymmetric (F1) and a symmetric (F2) frequency component were extracted from the MVEP using Fourier analysis. At early ages the asymmetric F1 component measured from the two eyes exhibited a 180 deg interocular phase shift, demonstrating that there was a directional bias in opposite directions between the left and right eyes. Although our methods could not determine whether the bias was in the nasal or temporal direction, our results would be consistent with a nasal bias, as has been observed in previous motion studies. Magnitude of the asymmetry was quantified in the form of an asymmetry index, F1/(F1 + F2). Based on developmental changes in the asymmetry index, and phase and amplitudes of F1 and F2, we conclude that the MVEP loses its directional asymmetry at 6 weeks of age. The development of directional motion symmetry observed in monkeys over the first 6 weeks is similar to that observed in humans over the first 5 months.

The neural processing of 3-D visual information: evidence from eye movements

Primates have several reflexes that generate eye movements to compensate for bodily movements that would otherwise disturb their gaze and undermine their ability to process visual information. Two vestibulo-ocular reflexes compensate selectively for rotational and translational disturbances of the head, and each has visual backups that operate as negative feedback tracking mechanisms to deal with any residual disturbances of gaze. Of particular interest here are three recently discovered visual tracking mechanisms that specifically address translational disturbances and operate in machine-like fashion with ultra-short latencies (< 60 ms in monkeys, < 85 ms in humans). These visual reflexes deal with motions in all three dimensions and operate as automatic servos, using preattentive parallel processing to provide signals that initiate eye movements before the observer is even aware that there has been a disturbance. This processing is accomplished by visual filters each tuned to a different feature of the binocular images located in the immediate vicinity of the plane of fixation. Two of the reflexes use binocular stereo cues and the third is tuned to particular patterns of optic flow associated with the observer's forward motion. Some stereoanomalous subjects show tracking deficits that can be attributed to a lack of just one subtype of cortical cell encoding motion in one particular direction in a narrow depth plane centred on fixation. Despite their rapid, reflex nature, all three mechanisms rely on cortical processing and evidence from monkeys supports the hypothesis that all are mediated by the medial superior temporal (MST) area of cortex. Remarkably, MST seems to represent the first stage in cortical motion processing at which the visual error signals driving each of the three reflexes are fully elaborated at the level of individual cells.

What happens to binocularity in primate strabismus?

Normal humans for whom the positions and movements of the two eyes are constrained to be yoked together are able to extract rich binocular sensory information from the environment. Humans with strabismus are deficient in extracting some of this information. Studies of strabismus in non-human primates can augment what has been learned from humans about relationships between strabismus and sensory binocular function. For example, speculation about the role of binocular vision in primate evolution can help us understand why it is that the advantages of sensory binocular function outweigh the disadvantages of having the positions of the two eyes yoked together. Physiological optics assessments of fixation patterns and accommodative responses in monkeys provide information about how the brain accomplishes and coordinates motor and sensory binocular functions, and sets the stage for determining underlying neural mechanisms responsible for this coordination. Finally, a developmental perspective, concerned with events that occur during an early sensitive period in the life span of an infant primate, can help us understand how nature and nurture interact to set up this complex neural system in normal individuals, and how this process is disrupted in conditions such as strabismus.