Interocular Suppression in Primary Visual Cortex in Strabismus

Area of Science:

• Neuroscience
• Ophthalmology
• Visual Perception

Background:

• Strabismus, or eye misalignment, acquired in childhood prevents diplopia (double vision).
• The mechanism of suppressing duplicate visual information to avoid double vision in strabismus is not fully understood.
• This suppression, while preventing diplopia, can impede visual feedback necessary for correcting eye alignment.

Purpose of the Study:

• To investigate the neural basis of strabismic suppression in the primary visual cortex (V1).
• To determine if V1 neurons exhibit suppression of visual input from the deviating eye in a model of strabismus.
• To explore how the brain avoids perceiving double images in individuals with strabismus.

Main Methods:

• Two male monkeys were raised with surgically induced alternating exotropia from four weeks of age.
• Electrophysiological recordings were made from single neurons in the primary visual cortex (V1) of mature, awake monkeys.
• Neuronal responses were analyzed under various monocular and binocular viewing conditions, including fixation switches and presentation of drifting gratings.

Main Results:

• Ocular dominance histograms revealed a monocular bias in V1 neurons compared to normal animals, yet many cells remained binocularly responsive.
• Neuronal responses were generally unaffected by changes in ocular fixation.
• No net interocular suppression or facilitation was observed at the population level in V1, even in areas with reduced cytochrome oxidase activity associated with suppressed visual fields.

Conclusions:

• Primary visual cortex (V1) neurons in this strabismus model do not appear to mediate the suppression of visual input that prevents diplopia.
• The findings suggest that the elimination of double vision in strabismus is likely regulated at a higher cortical level than V1.
• This study provides the first electrophysiological evidence from V1 in awake, strabismic primates regarding the neural correlates of strabismic suppression.