Biases of motion perception revealed by reversing gratings in humans who had infantile-onset strabismus

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: sensory and motor outcomes

Infantile esotropia is a common ophthalmic disorder in childhood. It is often accompanied by profound maldevelopment of stereopsis, motion processing, and eye movements, despite successful surgical realignment of the eyes. The proper timing of surgery has been debated for decades. There is growing evidence from clinical and animal studies that surgery during the early critical periods enhances sensory and ocular motor development. The Congenital Esotropia Observational Study has defined a clinical profile of infants who will benefit most from early surgery, and several other studies have shown that early surgery does not lead to adverse long-term effects. Clinicians now should consider offering early surgery to patients with large-angle, constant infantile esotropia at or before 10 months of age.

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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Motion detection in normal infants and young patients with infantile esotropia

The purpose of this study was to investigate asymmetries in detection of horizontal motion in normal infants and children and in patients with infantile esotropia. Motion detection thresholds (% motion signal) were measured in 75 normal infants and in 36 eyes of 27 infants with infantile esotropia (ET), using a forced-choice preferential looking paradigm with random-dot patterns. Absolute motion detection sensitivity and asymmetries in sensitivity for nasalward (N) vs. temporalward (T) directions of motion were compared in normal and patient populations, ranging in age from 1 month to 5 years. In normal infants, N and T thresholds were equivalent under 2.5 months of age, whereas a superiority for monocular detection of N motion was observed between 3.5 and 6.5 months of age. The nasalward advantage gradually diminished to symmetrical T:N performance by 8 months of age, matching that of adults. No asymmetry was observed in 15 normal infants who performed the task binocularly, hence, the asymmetry was not a leftward/rightward bias. In the youngest infantile ET patients tested, at 5 months of age, a nasalward superiority in motion detection was observed and was equivalent to that of same-age normal infants. However, unlike normals, this asymmetry persists in older patients. This greater asymmetry in infantile ET represents worse detection of T than N motion. This is the first report of an asymmetry in motion detection in normal infants across a wide age range. Initially, motion detection is normal in infants with infantile esotropia. Cumulative abnormal binocular experience in these patients may disrupt motion mechanisms.

Paucity of horizontal connections for binocular vision in V1 of naturally strabismic macaques: Cytochrome oxidase compartment specificity

To describe the structural basis for lack of binocular fusion in strabismic primates, we investigated intrinsic horizontal connections within striate cortex (area V1) of normal and strabismic, adult macaque monkeys. The strabismic animals had early-onset natural esotropia (the visual axes deviated nasally), normal visual acuity in each eye, and the constellation of ocular motor deficits that typify human infantile strabismus. Horizontal patchy connections and synaptic boutons were labeled by injections of the neuronal tracer biotinylated dextran amine. Ocular dominance columns (ODCs), and blob vs. interblob compartments, were revealed by using cytochrome oxidase (CO). In layers 2/3 and 4B of the strabismic monkeys, patchy projections and boutons terminated much more frequently in same-eye (73%) as opposed to opposite-eye (27%) ODCs (normal monkeys 58% and 42%, respectively). The deficiency of binocular connections in the strabismic cortex was evident qualitatively as a "skip" pattern, in which every other row of ODCs had labeled patches. Analysis of V1 in normal monkeys revealed that the deficits in strabismic V1 were due mainly to a loss of binocular connections between neurons in CO-interblob compartments. In both normal and strabismic monkeys: (1) CO-blob compartment neurons showed a more pronounced bias for monocular connectivity, and (2) commitment of connections to the same CO-compartment as the injection site (blob-to-blob, or interblob-to-interblob) was moderately strong (64%) but far from absolute. These findings help elucidate the relative roles of visual experience vs. innate mechanisms in the development of axonal connections between ocular dominance domains and compartments within macaque V1. They also provide the first detailed description of the V1 maldevelopments associated with unrepaired natural, infantile-onset strabismus in primates.

Apparent motion processing in strabismic observers with varying levels of stereo vision

Deficits of motion processing have been reported in individuals with infantile strabismus. Here we examined motion perception in early and late onset strabismic individuals with varying levels of stereo acuity using the Fine Grain Motion Illusion (FGMI). The FGMI is an illusion of motion, where under some conditions, observers report exaggerated motion. It has been speculated that this illusion is due to the spatial range of cortical cells stimulated. We used this illusion to compare this motion exaggeration in the central vision of both the deviating and non-deviating eye of strabismic observers with control eyes. While individuals with early onset strabismus showed some mild exaggeration of motion, those with low stereo acuity, regardless of age at onset, showed significantly larger exaggeration of this motion illusion in both the deviating and non-deviating eyes as compared with control observers. These results might indicate that this illusion and stereopsis involve common mechanisms.

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.

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.