Keeping an eye on the brain: the role of visual experience in monkeys and children

tDCS recovers depth perception in adult amblyopic rats and reorganizes visual cortex activity

Amblyopia or lazy eye is a neurodevelopmental disorder that arises during the infancy and is caused by the interruption of binocular sensory activity before maturation of the nervous system. This impairment causes long-term deterioration of visual skills, particularly visual acuity and depth perception. Although visual function recovery has been supposed to be decreased with age as consequence of reduced neuronal plasticity, recent studies have shown that it is possible to promote plasticity and neurorestoration in the adult brain. Thus, transcranial direct current stimulation (tDCS) has been shown effective to treat amblyopia in the adulthood. In the present work we used postnatal monocular deprivation in Long Evans rats as an experimental model of amblyopia and the cliff test task to assess depth perception. Functional brain imaging PET was used to assess the effect of tDCS on cortical and subcortical activity. Visually deprived animals ability to perceive depth in the cliff test was significantly reduced in comparison to their controls. However, after 8 sessions of tDCS applied through 8 consecutive days, depth perception of amblyopic treated animals improved reaching control level. PET data showed 18F-FDG uptake asymmetries in the visual cortex of amblyopic animals, which disappeared after tDCS treatment. The possibility of cortical reorganization and stereoscopy recovery following brain stimulation points at tDCS as a useful strategy for treating amblyopia in adulthood. Furthermore, monocular deprivation in Long Evans rats is a valuable research model to study visual cortex mechanisms involved in depth perception and neural restoration after brain stimulation.

An Assessment of Stereovision Acquired in Adulthood

SIGNIFICANCE

Increasing evidence indicates that childhood binocular vision disorders that lead to stereodeficiency may be treated in adulthood. Reports of patients who gain stereopsis as adults indicate that this achievement provides for a qualitatively different and dramatically improved sense of space and depth.

PURPOSE

Increasing evidence suggests that stereopsis can be achieved in adult patients despite long-standing binocular disorders. We polled individuals who gained stereopsis as adults to ascertain their initial binocular disorders, the length of time they were stereodeficient, effective treatments, and the nature of their recovered stereovision.

METHODS

A questionnaire was posted online and announced in a brief article in the journal Vision Development and Rehabilitation.

RESULTS

Of the 63 responders, 56 (89%) reported strabismus and/or amblyopia, and 55 (87%) indicated that they had been stereodeficient for as long as they could remember. All but seven participants (89%) achieved stereovision through vision training or a combination of surgery and vision training, and many reported vivid visual changes.

CONCLUSIONS

Despite childhood binocular disorders, patients may be able to achieve stereopsis following interventions in adulthood. This achievement provides for a qualitatively different and dramatically improved sense of space and depth.

Altered Balance of Receptive Field Excitation and Suppression in Visual Cortex of Amblyopic Macaque Monkeys

In amblyopia, a visual disorder caused by abnormal visual experience during development, the amblyopic eye (AE) loses visual sensitivity whereas the fellow eye (FE) is largely unaffected. Binocular vision in amblyopes is often disrupted by interocular suppression. We used 96-electrode arrays to record neurons and neuronal groups in areas V1 and V2 of six female macaque monkeys (Macaca nemestrina) made amblyopic by artificial strabismus or anisometropia in early life, as well as two visually normal female controls. To measure suppressive binocular interactions directly, we recorded neuronal responses to dichoptic stimulation. We stimulated both eyes simultaneously with large sinusoidal gratings, controlling their contrast independently with raised-cosine modulators of different orientations and spatial frequencies. We modeled each eye's receptive field at each cortical site using a difference of Gaussian envelopes and derived estimates of the strength of central excitation and surround suppression. We used these estimates to calculate ocular dominance separately for excitation and suppression. Excitatory drive from the FE dominated amblyopic visual cortex, especially in more severe amblyopes, but suppression from both the FE and AEs was prevalent in all animals. This imbalance created strong interocular suppression in deep amblyopes: increasing contrast in the AE decreased responses at binocular cortical sites. These response patterns reveal mechanisms that likely contribute to the interocular suppression that disrupts vision in amblyopes.SIGNIFICANCE STATEMENT Amblyopia is a developmental visual disorder that alters both monocular vision and binocular interaction. Using microelectrode arrays, we examined binocular interaction in primary visual cortex and V2 of six amblyopic macaque monkeys (Macaca nemestrina) and two visually normal controls. By stimulating the eyes dichoptically, we showed that, in amblyopic cortex, the binocular combination of signals is altered. The excitatory influence of the two eyes is imbalanced to a degree that can be predicted from the severity of amblyopia, whereas suppression from both eyes is prevalent in all animals. This altered balance of excitation and suppression reflects mechanisms that may contribute to the interocular perceptual suppression that disrupts vision in amblyopes.

Population representation of visual information in areas V1 and V2 of amblyopic macaques

Amblyopia is a developmental disorder resulting in poor vision in one eye. The mechanism by which input to the affected eye is prevented from reaching the level of awareness remains poorly understood. We recorded simultaneously from large populations of neurons in the supragranular layers of areas V1 and V2 in 6 macaques that were made amblyopic by rearing with artificial strabismus or anisometropia, and 1 normally reared control. In agreement with previous reports, we found that cortical neuronal signals driven through the amblyopic eyes were reduced, and that cortical neurons were on average more strongly driven by the non-amblyopic than by the amblyopic eyes. We analyzed multiunit recordings using standard population decoding methods, and found that visual signals from the amblyopic eye, while weakened, were not degraded enough to explain the behavioral deficits. Thus additional losses must arise in downstream processing. We tested the idea that under monocular viewing conditions, only signals from neurons dominated by - rather than driven by - the open eye might be used. This reduces the proportion of neuronal signals available from the amblyopic eye, and amplifies the interocular difference observed at the level of single neurons. We conclude that amblyopia might arise in part from degradation in the neuronal signals from the amblyopic eye, and in part from a reduction in the number of signals processed by downstream areas.

Gaze-stabilizing deficits and latent nystagmus in monkeys with brief, early-onset visual deprivation: eye movement recordings

The normal development and the capacity to calibrate gaze-stabilizing systems may depend on normal vision during infancy. At the end of 1 yr of dark rearing, cats have gaze-stabilizing deficits similar to that of the newborn human infant including decreased monocular optokinetic nystagmus (OKN) in the nasal to temporal (N-T) direction and decreased velocity storage in the vestibuloocular reflex (VOR). The purpose of this study is to determine to what extent restricted vision during the first 2 mo of life in monkeys affects the development of gaze-stabilizing systems. The eyelids of both eyes were sutured closed in three rhesus monkeys (Macaca mulatta) at birth. Eyelids were opened at 25 days in one monkey and 40 and 55 days in the other two animals. Eye movements were recorded from each eye using scleral search coils. The VOR, OKN, and fixation were examined at 6 and 12 mo of age. We also examined ocular alignment, refraction, and visual acuity in these animals. At 1 yr of age, visual acuity ranged from 0.3 to 0.6 LogMAR (20/40-20/80). All animals showed a defect in monocular OKN in the N-T direction. The velocity-storage component of OKN (i.e., OKAN) was the most impaired. All animals had a mild reduction in VOR gain but had a normal time constant. The animals deprived for 40 and 55 days had a persistent strabismus. All animals showed a nystagmus similar to latent nystagmus (LN) in human subjects. The amount of LN and OKN defect correlated positively with the duration of deprivation. In addition, the animal deprived for 55 days demonstrated a pattern of nystagmus similar to congenital nystagmus in human subjects. We found that restricted visual input during the first 2 mo of life impairs certain gaze-stabilizing systems and causes LN in primates.