November 8, 2020
In subjects with alternating strabismus, either eye can be used to saccade to visual targets. The brain must calculate the correct vector for
each saccade, which will depend on the eye chosen to make it. The superior colliculus, amajormid brain center for saccade generation, was
examined to determine whether the maps serving each eye were shifted to compensate for strabismus. Alternating exotropia was induced
in two male macaques at age 1 month by sectioning the tendons of the medial recti. Once the animals grew to maturity, they were trained
to fixate targets with either eye. Receptive fields were mapped in the superior colliculus using a sparse noise stimulus while the monkeys
alternated fixation. For some neurons, sparse noise was presented dichoptically to probe for anomalous retinal correspondence. After
recordings, microstimulation was applied to compare sensory and motor maps. The data showed that receptive fields were offset in
position by the ocular deviation, but otherwise remained aligned. In one animal, the left eye’s coordinates were rotated 20° clockwise
with respect to those of the right eye. This was explained by a corresponding cyclorotation of the ocular fundi, which produced an
A-pattern deviation. Microstimulation drove the eyes accurately to the site of receptive fields, as in normal animals. Single-cell recordings
uncovered no evidence for anomalous retinal correspondence. Despite strabismus, neurons remained responsive to stimulation of either
eye. Misalignment of the eyes early in life does not alter the organization of topographic maps or disrupt binocular convergence in the
superior colliculus.