A psychophysical measurement and analysis of motion perception in normal and binocularly deprived monkeys.

PURPOSE: To measure psychophysically the thresholds for motion detection in the nasal and temporal directions under monocular viewing conditions in monkeys reared under conditions of daily alternating monocular occlusion (AMO). The hypothesis was that motion perception would be asymmetric with more sensitivity for motion in the nasal direction. METHODS: Three monkeys subjected to AMO (AMO monkeys) and three normal monkeys were studied. All were trained with operant conditioning techniques to discriminate coherent from random motion in a random dot display. The percentage of dots in the display that moved either left or right was varied. Thresholds for motion detection of nasally directed and temporally directed stimuli were measured to determine whether the motion perception of AMO monkeys was asymmetric, as predicted. RESULTS: A two-factor analysis of variance revealed a statistically significant difference between treatment groups (normal versus AMO) and directions (nasal versus temporal) and a significant interaction. The interaction was due to a significant difference between nasal and temporal directions for the AMO group, but no significant difference for the normal group. Planned comparisons were performed based on each animal's best eye (eye most sensitive to nasal motion) and worst eye (eye least sensitive to temporal motion). No significant differences were found between the two groups in the best eyes' responses to the nasal direction, but the worst eyes' responses in the temporal direction were significantly poorer in the AMO group. A neural model that can account for these findings is based on a Hebbian teacher located in the nucleus of the optic tract that strengthens connections of a subpopulation of directionally selective cortical neurons. CONCLUSIONS: AMO rearing results in asymmetric motion perception. Thresholds for detecting nasally directed motion are normal, whereas thresholds for detecting temporally directed motion are deficient. These results demonstrate that motion-processing mechanisms in primates exhibit experience-dependent developmental neural plasticity. The locus of the neural plasticity could be a subpopulation of directionally selective neurons in the striate cortex (V1).

Continuous ambient lighting and eye growth in primates.

PURPOSE: To determine the effect of continuous light exposure on ocular growth and emmetropization in infant monkeys. METHODS: Nine infant rhesus monkeys were reared with the normal vivarium lights on continuously. The 24-hour light cycle was initiated between 1 and 4 weeks of age and maintained for 6 months. The ocular effects of continuous light were assessed by cycloplegic retinoscopy, keratometry, and A-scan ultrasonography. Longitudinal control data were obtained from 23 normal infants that were reared with an illumination cycle that included defined light and dark phases (either 12-hour light:12-hour dark or 8.5-hour light:15.5 hour dark). RESULTS: In contrast to previous studies involving light-reared chickens, no monkeys exhibited exaggerated ocular growth. There were no significant differences between treated and control monkeys in corneal radius, overall eye size, or the axial dimensions of individual ocular components. At the end of the treatment period, eight of the nine experimental monkeys also exhibited the moderate hyperopic errors (range, +1.5 to +3.4 D) that are typically found in normal animals. Aspects of emmetropization were, however, unusual for three monkeys. One monkey manifested a -0.50 D myopic error that was associated with an abnormally steep cornea but had normal axial lengths. Two additional monkeys developed persistent axial anisometropias. CONCLUSIONS: In infant primates constant light exposure does not promote the constellation of ocular changes (in particular corneal flattening, a decrease in anterior chamber depth, and an increase in vitreous chamber depth) that has been observed in light-reared chickens. The slight variations from the expected developmental sequence observed in three infants may reflect individual differences. However, it is also possible that aspects of the emmetropization process may not operate as effectively under constant light as they do under an ordinary light/dark cycle.

Visual outcome after contact lens and intraocular lens correction of neonatal monocular aphakia in monkeys.

PURPOSE: A monkey model was used to evaluate intraocular lenses (IOLs) and extended-wear contact lenses (EWCLs) for the optical treatment of infantile aphakia in humans. Specifically, the relative effectiveness of EWCLs used alone and IOLs used in combination with EWCLs in preventing amblyopia was assessed. METHODS: A total of 33 rhesus monkeys was studied in this project, 24 assigned to experimental treatment groups and 9 to normal controls. Contact lenses made from a diffusing material or dyed opaque were placed on one eye at birth to simulate an infantile cataract. A unilateral lensectomy was then performed on the same eye within 2.5 weeks after birth. In 15 monkeys this was combined with implantation of an IOL. The eyes were left aphakic in the remaining 9 animals. EWCLs were used to adjust the optical correction of both aphakic and pseudophakic eyes to a near point (3-5 D). Opaque lenses were used to maintain daily part-time (approximately 70%) occlusion of the fellow eye. The primary outcome measure was grating acuity assessed with behavioral methods. Some animals were also assessed for acuity with sweep visually evoked potentials (VEPs) and for optotype acuity (Landolt C) with behavioral methods. RESULTS: Two of the animals with IOLs developed complications in the eye that precluded completion of the behavioral assessment protocol. Only behavioral outcomes obtained before or in the absence of surgical complications are presented. There was a developmental delay in the maturation of grating acuity in both eyes of both treatment groups. Normal adult levels of grating acuity were eventually achieved in the group treated with IOLs combined with EWCLs. Grating acuity was significantly poorer than normal in aphakic eyes treated only with EWCLs. Comparison of the two treatment groups revealed that pseudophakic eyes treated with multifocal IOLs had significantly better gating acuity than aphakic eyes. Assessments of optotype acuity and sweep VEP acuity revealed amblyopic deficits in both pseudophakic and aphakic eyes. CONCLUSIONS: Given an absence of serious postoperative complications, neonatal correction of aphakia with IOLs combined with EWCLs can lead to normal grating acuity in a primate model. Correction with EWCLs alone was not sufficient to produce normal grating acuity. Multifocal IOL treatments combined with EWCL provided a significantly better outcome than EWCL methods alone. However, neither IOL nor EWCL methods were able to prevent amblyopia as evaluated using behavioral testing with optotypes or with sweep VEPs.

Asymmetric responses in cortical visually evoked potentials to motion are not derived from eye movements.

PURPOSE: Normal neonates and many adults after abnormal visual development have directional preferences for visual stimulus motions; i.e., they give better responses for optokinetic nystagmus (OKN) and visually evoked potentials (VEPs) in one direction than to those in the opposite direction. The authors tested whether the VEP responses were asymmetrical because of abnormal eye movements. METHODS: VEPs were recorded from the visual cortices of five macaque monkeys: one normal, one neonate, and three reared with alternating monocular occlusion (AMO). They were lightly anesthetized, followed by paralysis to prevent eye movements. They then had "jittered" vertical grating patterns presented in their visual fields. The steady state VEPs were analyzed with discrete Fourier transforms to obtain the amplitudes and phases of the asymmetries. RESULTS: The normal, control monkey had small, insignificant amplitudes of its asymmetrical Fourier component and random phases that were not 180 degrees out of phase across the left and right eyes. The neonatal monkey and the AMO monkeys all had large, significant asymmetries that were approximately 180 degrees out of phase between the left and right eyes. CONCLUSIONS: The neonate and abnormally reared monkeys continued to have asymmetrical responses even after their eyes were paralyzed. Therefore, eye movements cannot be the source of the asymmetrical amplitudes of the VEPs, and the visual cortex is at least one source responsible for asymmetries observed in neonates and adults reared under abnormal visual inputs.

Form deprivation myopia in adolescent monkeys.

BACKGROUND: Early in life, at ages corresponding to the rapid infantile phase of ocular growth in humans, visual feedback can modulate refractive development in monkeys and many other species. To determine if vision-dependent mechanisms can still influence refractive development in primates during the slow juvenile phase of ocular growth, the time period when myopia typically develops in human children, we examined the effects of form deprivation on adolescent monkeys. METHODS: Unilateral, form deprivation was produced in four rhesus monkeys by surgically fusing the eyelids of one eye. The onset of deprivation was between 3.7 and 5 years of age, which corresponds to onset ages between approximately 15 and 20 human years. The ocular effects of form deprivation were assessed by cycloplegic retinoscopy and A-scan ultrasonography. RESULTS: At the onset of form deprivation all four monkeys were isometropic and the axial dimensions in the two eyes were well matched. After 71 to 80 weeks of form deprivation, all of the deprived eyes had become relatively more myopic than their fellow non-treated eyes (mean anisometropia = -2.03 +/- 0.78 D) and they exhibited relative increases in vitreous chamber depth (mean = 0.55 +/- 0.31 mm) and axial length (mean = 0.49 +/- 0.35 mm). DISCUSSION: Our results demonstrate that vision-dependent mechanisms can influence ocular growth and refractive development in "teenage" monkeys. These results raise the possibility that visual experience may be involved in the genesis of school-age myopia in children.

The refractive development of untreated eyes of rhesus monkeys varies according to the treatment received by their fellow eyes.

To determine the extent to which the visual experience of one eye may influence the refractive development of its fellow eye, we analyzed the data of untreated (UT) eyes of monkeys that received different types of unilateral pattern deprivation. Subjects were 15 juvenile rhesus monkeys, with five monkeys in each of three treatment groups: aphakic eyes with optical correction (AC), aphakic eyes with no correction (ANC), and eyes that were occluded with an opaque contact lens (OC). Under general anaesthesia, refractive error (D) was determined by cycloplegic retinoscopy and axial length (mm) was determined with A-scan ultrasonography. For measurements of refractive error of the UT eyes, there was a significant main effect of groups according to the treatment of the fellow eyes, F(2, 12) = 6.6. While UT eyes paired with AC fellow eyes (mean = +4.2 D) were significantly more hyperopic than the eyes of age-matched normal monkeys (mean = +2.4 D), t(25), = 2.5, UT eyes paired with OC fellow eyes (mean = -0.5 D) were significantly more myopic than the eyes of normal monkeys, t(25) = -9. UT eyes paired with ANC fellow eyes (mean = +1.9 D) were not significantly different from normal eyes. For measurements of axial length there was also a significant main effect of groups, F(2, 12) = 6.9. While UT eyes paired with AC fellow eyes (mean = 16.9 mm) were significantly shorter than the eyes of age-matched normal monkeys (mean = 17.5 mm), t(25) = 2.3, UT eyes paired with OC fellow eyes (mean = 18.1 mm) were significantly longer than the eyes of normal monkeys, t(25) = 2.3. UT eyes paired with ANC fellow eyes (mean = 17.5 mm) were not significantly different from the eyes of normal monkeys. The measurements of axial length and of refractive error of the UT eyes were also significantly correlated with one another, probably indicating that the differences in refractive error were due to differences in axial length, r = -0.8. The present data reveal that despite normal visual experience, UT eyes can have their refractive development altered, systematically, simply as a function of the type of pattern deprivation received by their fellow eyes. These data add to the growing evidence that there is an interocular mechanism that is active during emmetropization. As a consequence, future models of eye growth will need to consider both: (1) the direct influence of visual input on the growing eye; as well as (2) the indirect influence coming from the fellow eye.

Emmetropization in the rhesus monkey (Macaca mulatta): birth to young adulthood.

PURPOSE: To provide baseline measurements on the postnatal changes in refractive error, corneal curvature, and axial elongation of the eyes of normal monkeys. Little is known about the course of normal eye growth from birth to adolescence, particularly how refractive parameters co-vary during development. In animal models of ametropia, usually one eye is manipulated and the fellow eye serves as a control. However, given individual differences, and without baseline data, it is impossible to determine whether either eye develops normally. METHODS: Measurements were obtained on 237 rhesus monkeys, whose ages ranged from birth to 5 years. Examinations included cycloplegic refraction by retinoscopy, keratometry measurements, and A-scan ultrasound measurements of axial length. The time course of development was evaluated using a growth curve analysis appropriate for a mixture of cross-sectional and longitudinal data. RESULTS: At birth, all three parameters were normally distributed and only weakly correlated. Monkeys had +7 D (SD=2.3 D) of hyperopia, corneal power of 58 D (SD=1 D), and axial length of 13.2 mm (SD=0.4 mm). Refractive error ranged from +0.5 D to +14.5 D, with a mean difference between the two eyes of 0.5 D. Corneal curvature ranged from 61 D to 54 D, with a mean difference between the two eyes of 0.8 D. Axial length ranged from 12.0 mm to 14.2 mm, with a mean difference between the two eyes of 0.1 mm. Although the degree of hyperopia achieved asymptote, of + 2 D, shortly after 1 year of age, corneal curvature and axial length did not achieve asymptote until nearly 5 years of age. By this time, refractive error had declined by 5 D, corneal curvature had declined by 7 D, and axial length had increased by 6 mm. CONCLUSIONS: The magnitude of the individual differences that can occur in a small sample of experimental subjects is large enough to necessitate reference to age norms derived from a large population. Our results provide a baseline for studies of normal and abnormal eye growth and ametropia in primates. Our results also led to the confirmation of a set of "rules" that have been offered as an explanation of how these three parameters interact during emmetropization.

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.

Maturation of binocular luminance interaction in normal young and adult rhesus monkeys.

PURPOSE: Our aim was to answer three questions 1) Do adult rhesus monkeys have binocular luminance interactions (BLIs) similar to those found in adult humans? 2) Is BLI in very young rhesus monkeys functionally mature? 3) If not, how does it change with age? METHODS: We recorded visually evoked potentials (VEPs) in response to sinusoidally modulated uniform fields. The fields were presented dichoptically by varying the relative temporal phase between the two eyes. Monkeys varied in age from 5.6 weeks to 5.25 years. RESULTS: VEPs were Fourier analyzed and the relative second harmonic amplitudes were taken as the response measure. The second harmonic amplitudes in adult monkeys had an asymmetrical 'V-shaped' function as interocular phase difference (IPD) varied from 0 degrees to 180 degrees, as had been observed previously in adult humans [1]. The youngest monkeys exhibited a symmetrical pattern which became more asymmetrical at older ages and was adult like by about 19 weeks. Asymmetry magnitude and log age correlated 0.97 (p < 0.05) in the monkeys younger than 19 weeks. CONCLUSIONS: The adult rhesus data are consistent with a model derived from humans which involves two types binocular luminance processing. One combines monocular responses nonlinearly (MNL) and a second combines monocular responses linearly followed by a binocular nonlinearity (MLBNL). These two processes have been associated with the parvocellular (P-) and magnocellular (M-) streams. Within this framework, the data from the youngest monkeys indicate that BLI in the P-stream is relatively less mature at birth than that in the M-stream and develops reaching functional maturity on these measures by around 19 weeks.

The development of stereoacuity in infant rhesus monkeys.

The emergence of stereopsis at 3-4 months postnatal in human infants is striking and has led to speculation that its rapid onset and subsequent development must be due to a dramatic reorganization of the brain. Stereopsis has never been measured in infant monkeys, but previous studies have demonstrated that many other visual functions develop four times faster in infant monkeys than in humans. We made longitudinal assessments of stereoacuity in 11 infant rhesus monkeys. A forced-choice preferential-looking technique was used to present random-dot stereograms during testing. By 8 weeks after birth, all of the monkeys were responding to at least coarse levels of disparity (1760" [seconds]), and by 13 weeks of age, all were responding to the relatively fine level of 88" disparity. Age of onset for stereopsis in monkeys was at about one-quarter the age when it occurs in humans, as expected. However, subsequent development proceeded at a similar absolute rate in monkeys and humans. The findings are discussed relative to the neural mechanisms which might be responsible for the differing rates of development.

Acuity and contrast sensitivity in monkeys after neonatal intraocular lens implantation with and without part-time occlusion of the fellow eye.

PURPOSE: The authors used a monkey model to evaluate intraocular lenses (IOLs) for the treatment of infantile cataract in humans. Specifically, they sought to assess the effectiveness of IOLs, with and without occlusion therapy, in preventing amblyopia. METHODS: A diffuser contact lens was placed on one eye each of 11 neonatal monkeys to simulate an infantile cataract. A unilateral lensectomy, combined with the implantation of an IOL, was performed on the same eye 1 to 2 weeks after birth. Clear contact lenses were used to adjust the optical correction of the pseudophakic eyes to a near point, and opaque lenses were used to maintain daily part-time (70%) occlusion of the fellow eyes of half the subjects. Behavioral methods were used to assess grating acuity, optotype acuity (Landolt C), and contrast sensitivity. RESULTS: In five of the animals, complications that developed in the eye with the implant were severe enough to interfere with visual function. The authors present only behavioral outcomes obtained before or in the absence of surgical complications. In monkeys that underwent daily 70% occlusion, grating acuity in the pseudophakic eyes eventually matured to normal adult levels. Grating acuity was significantly poorer in animals with no occlusion therapy. Even in animals with normal grating acuity, assessments of optotype acuity revealed amblyopic deficits; contrast sensitivity was impaired as well at middle and low spatial frequencies. CONCLUSIONS: The current study demonstrates that if there are no complications secondary to surgery, normal grating acuity can be obtained in neonatal monkey eyes that undergo IOL implantation, optical correction of the pseudophakic eye to a near point, and 70% occlusion of the fellow eye. However, these good outcomes for grating acuity cannot be attained without occlusion therapy. In addition, optotype acuity and sensitivity to contrast always are impaired.

Latent fixation nystagmus and nasotemporal asymmetries of motion visually evoked potentials in naturally strabismic primate.

PURPOSE: To determine whether macaque monkeys who had onset of natural, alternating esotropia in early infancy have the eye movement abnormalities and motion visually evoked potential (VEP) abnormalities observed in human infantile esotropes. METHODS: Recordings were obtained of horizontal pursuit eye movements and fixation nystagmus under conditions of monocular viewing. Motion VEPs in response to horizontal motion were recorded with the animals sedated to reduce the possibility of eye movement artifact. RESULTS: The strabismic monkeys had a naso-temporal asymmetry of horizontal pursuit favoring nasally directed motion when viewing with either eye. When fixating stationary targets, latent nystagmus was apparent; the eyes drifted conjugately and the slow phase of the nystagmus was always nasally directed with respect to the fixating eye. Motion VEPs were characterized by a strong horizontal asymmetry with the directional bias inverted approximately 180 deg in the two eyes. These eye movement and motion VEP asymmetries were not observed in a normal macaque. CONCLUSIONS: Macaque monkeys who have infantile esotropia possess asymmetries of horizontal pursuit and motion VEPs like those documented in strabismic humans. Macaques with infantile esotropia appear to be an appropriate animal model for study of neural mechanisms in strabismus.

Diffuser contact lenses retard axial elongation in infant rhesus monkeys.

In each of five monkeys, one eye was fitted with a diffuser lens at birth. This lens allowed pattern vision, but also reduced contrast by about 1 log unit. In four out of five monkeys, the treated eyes were shorter and more hyperopic than the untreated fellow eyes. At 25 weeks of age, interocular differences (OD -- OS) of the experimental group were significantly greater than interocular differences of age-matched normal monkeys for both axial length (P < 0.05) and refractive error (P < 0.02). In addition, while the treated eyes were significantly different from normal eyes for both axial length measurements (P < 0.01) and refractive error (P < 0.01), there were no significant differences between the untreated fellow eyes and normal eyes. In primates less severe pattern deprivation appears to produce an effect on eye growth that is opposite to that of severe pattern deprivation (little or no pattern vision), which typically results in axial myopia.

[Functional and structural abnormalities in the visual cortex in early childhood strabismus]. / Funktionelle und strukturelle Abnormitäten im visuellen Cortex bei frühkindlichem Strabismus.

BACKGROUND: To investigate the structural basis for functional deficits in infantile strabismus we used a neuroanatomic tracer and a histochemical label to examine the primary visual cortex (area V1) of adult esotropic macaque monkeys. ANIMALS AND METHODS: The animals had developed natural esotropia in the first months of life, alternated fixation, and exhibited the ocular motor signs that typify strabismus with onset in infancy. After behavioral and VEP testing, ocular dominance columns (ODCs) in V1 were injected with a neuronal traser and labeled for cytochrome oxidase activity. RESULTS: The strabismic monkeys showed striking deficits in binocular luminance visually-evoked potentials (VEPs) and characteristic directional asymmetries in motion VEPs. Binocular horizontal connections between ODCs were reduced an average of 50-60% in strabismic as compared to normal monkeys. ODCs also showed unequal metabolic activity with contralaterally (i.e. nasal retina) driven ODCs showing greater activity in each V1. DISCUSSION AND CONCLUSION: The maldevelopment of connections in upper V1 layers correlates with the abnormalities in binocular and motion VEPs. The unequal metabolic activity suggests interocular suppression. These results provide the first neuroanatomic evidence for cerebral cortex maldevelopments in natural infantile esotropia.

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.

Neonatal lensectomy and intraocular lens implantation: effects in rhesus monkeys.

PURPOSE: To compare the effects of a lensectomy with and without intraocular lens (IOL) implantation on a neonatal rhesus monkey eye. METHODS: A lensectomy and anterior vitrectomy was performed on 75 monkeys during the first 16 days of life; 21 of these monkeys also had an IOL implanted into the posterior chamber. The eyes were examined at regular intervals using biomicroscopy, applanation tonometry, and ophthalmoscopy. RESULTS: The pseudophakic monkeys were studied until they were 92.5 +/- 5.8 weeks of age and the aphakic monkeys until they were 80.4 +/- 5.7 weeks of age. Pupillary membranes (100% versus 55.5%; P < 0.01) and lens regeneration into the pupillary aperture (28.6% versus 5.6%; P = 0.02) occurred more often in the pseudophakic than the aphakic eyes. As a result, the pseudophakic eyes required more reoperations than the aphakic eyes to keep the visual axis clear (P < 0.01). There was not a significant difference in the incidence of ocular hypertension between the pseudophakic and aphakic eyes (9.5% versus 12.7%; P = 0.34). Pupillary capture of the IOL optic occurred in 52% and haptic breakage in 33% of the pseudophakic eyes. All of the eyes with broken haptics had a prominent Soemmerring's ring varying in maximum thickness from 0.6 to 2 mm. Nine of the haptics from the seven eyes with broken IOLs had eroded into the iris, two into the ciliary body, and one into the anterior chamber. CONCLUSIONS: Implanting an IOL into a neonatal monkey eye after a lensectomy and anterior vitrectomy increases the likelihood of a reoperation being necessary. Haptics frequently erode into the iris and ciliary body and may break because of stress placed on the optic-haptic junction by forward movement of the IOL.

Natural strabismus in monkeys: accommodative errors assessed by photorefraction and their relationship to convergence errors.

PURPOSE: To examine accommodation and accommodative convergence relationships in naturally strabismic monkeys. METHODS: Photorefraction was used to measure accommodative responses of each eye under monocular and binocular viewing conditions. These accommodative results were then compared to assessments of vergence state made under monocular viewing conditions using methods previously described. RESULTS: Accommodation was always accurate under monocular viewing conditions, with the exception of the inability of one myopic eye to focus distant targets. The accommodative response in the two eyes was always consensual. In animals with an anisometropia, the fixating eye was focused accurately on the target, and the fellow eye was in error by an amount predicted by the magnitude of the anisometropia. Some animals alternated fixation, and, under these conditions, control of accommodation and fixation switched in tandem. Accommodative convergence ratios (AC:A) were abnormally low in animals with strabismus syndromes, similar to human essential infantile esotropia, and were excessively high in animals with syndromes, similar to human early onset accommodative esotropia. CONCLUSIONS: There was no evidence that the strabismus seen in any of the monkeys was related to errors in accommodation. However, the crosslink gain between accommodation and vergence was abnormal in some of the animals. These abnormalities have the effect of driving vergence toward a misalignment that can only be overcome by other factors, such as fusional vergence. Thus, the authors speculate that these abnormalities in synkinesis between accommodation and convergence were present during early postnatal development when fusional vergence is weak, and that they acted as predisposing factors for the development of strabismus.

Multifocal versus monofocal correction of neonatal monocular aphakia.

We compared the efficacy of monofocal and multifocal intraocular lenses (IOLs) as a means of optically rehabilitating eyes with monocular congenital cataracts using a primate model. A congenital cataract was simulated in the right eye of 20 monkeys from birth to 2 weeks of age with an opalescent contact lens. A lensectomy was then performed and a monofocal IOL was implanted in 11 eyes and a multifocal IOL in 9 eyes. Any residual refractive error was then corrected with an extended-wear contact lens. The fellow eyes were then randomized to no treatment or 70% occlusion therapy. Grating acuity was assessed using forced-choice preferential looking with Teller acuity cards at regular intervals. At 32 weeks of age, the pseudophakic eyes were mildly amblyopic, but the grating acuities were not significantly different between the pseudophakic eyes with multifocal versus monofocal IOLs or between subjects who had received occlusion therapy versus no treatment of the fellow eye. We conclude that at least through 32 weeks of age, multifocal and monofocal IOLs are of similar efficacy and occlusion therapy may be less important to prevent amblyopia from developing in pseudophakic eyes compared to contact-lens corrected aphakic eyes.

Vision screening of individuals with severe or profound mental retardation.

A vision screening program was established at a regional center for individuals with severe or profound mental retardation. The visual acuity measure used was the acuity card procedure, a type of forced-choice preferential looking. Acuity values were evaluated in terms of test-retest reliability as well as inter- and intraobserver reliability. Acuity results compared favorably to those obtained in less compromised populations, and we conclude that the acuity card procedure is a useful screening tool for this population. Suggestions for its application in evaluating functional vision were also discussed.