Activity of near response cells during disconjugate saccades in strabismic monkeys.

Infantile strabismus is a common disorder, characterized by a chronic misalignment of the eyes, impairment of binocular vision, and oculomotor abnormalities. Nonhuman primates with strabismus, induced in infancy, show a pattern of abnormalities similar to those of strabismic children. This allows strabismic nonhuman primates to serve as an ideal animal model to examine neural mechanisms associated with aberrant oculomotor behavior. Here, we test the hypothesis that impairment of disparity vergence and horizontal saccade disconjugacy in exotropia and esotropia are associated with disrupted tuning of near and far response neurons in the supraoculomotor area (SOA). In normal animals these neurons carry signals related to vergence position and/or velocity. We hypothesized that, in strabismus, these neurons modulate inappropriately in association with saccades between equidistant targets. We recorded from 62 SOA neurons from 4 strabismic animals (two esotropes and two exotropes) during visually guided saccades to a target that stepped to different locations on a tangent screen. Under these same conditions SOA neurons in normal animals show no detectable modulation. In our strabismic subjects we found that a subset of SOA neurons carry weak vergence velocity signals during saccades. In addition, a subset of SOA neurons showed clear modulation associated with slow fluctuations of horizontal strabismus angle in the absence of a saccade. We suggest that abnormal SOA activity contributes to fixation instability but plays only a minor role in the horizontal disconjugacy of saccades that do not switch fixation from one eye to the other.