Visual responses of neurons from areas V1 and MT in a monkey with late onset strabismus: a case study

One adult monkey (Macaca fascicularis) was investigated psychophysically and electrophysiologically after at least 5 years of late onset esotropic macrostrabismus (squint angle 52 deg). Behavioural tests revealed normal monocular visual and visuomotor functions. No indications of deep amblyopia or oculomotor asymmetry were found. The monkey used the left or right eye alternately at about equal frequencies. Single unit recordings from area VI disclosed a normal ocular dominance distribution. Most VI neurons from both hemispheres received binocular input. Thus, discordant visual information from corresponding retinal locations of the two eyes converged onto the cortical neurons. No evidence for anomalous retinal correspondence was found. Diplopia and confusion must therefore be avoided by suppression of vision through one eye to allow stable, unambiguous perception. Possible suppression was investigated by stimulating a neuron through the same eye when it was actively used for fixation in one set of trials, and when it was not used for fixation in another set of trials. Significant differences in these two stimulus conditions were found in 20/39 neurons from area VI and in 11/34 motion sensitive neurons recorded in the middle superior temporal area (MT). The normalized population activity in VI and MT was higher if cells were stimulated through the fixating eye. The data are discussed with respect to possible suppressive mechanisms helping to prevent double vision in strabismus and in binocular rivalry.