Residual stereopsis in anisometropia and small-angle strabismus

We have developed a custom test to measure stereopsis at locations across the visual field to yield a  “stereo map”— visual field locations that are capable of stereopsis. Our study shows that for individuals with anisometropia and small-angle strabismus, residual stereopsis is typically present in the periphery, but is absent in the fovea. This finding is consistent with our hypothesis that residual stereopsis in amblyopia and strabismus is mediated by peripheral loci with larger receptive fields at farther eccentricities that are less sensitive to blur and to small misalignments. As disparity drives fusional vergence responses, we hypothesized that if stereopsis in the central foveal region is impaired, then the fusional vergence response to a small stimulus confined to the near foveal region will be reduced. Our measurements indicate that individuals with anisometropia and micro-strabismus have low vergence gain for small stimuli confined to the foveal region, but near normal gain for larger stimuli that extend into the periphery. Taken together, our stereo mapping and vergence studies indicate that stereo-deficiency in the central retina is associated with poor fusional drive.