Strabismic suppression is mediated by inhibitory interactions in the primary visual cortex

Most strabismic observers do not suffer from double vision because of suppression from conscious perception of 1 of the 2 eyes' conflicting views. Direct evidence for the site and neural substrate of strabismic suppression has not been available so far, although psychophysical data suggest a cortical origin. On the other hand, cross-orientation suppression among conflicting stimuli presented monocularly has recently been shown to have a strong thalamic component. Here we present evidence, using both visual stimulation and pharmacological techniques, that strabismic suppression occurs in the primary visual cortex and involves gamma-amino butyric acid (GABA)-mediated inhibition. We show that its dependency on the drift rate of the suppressing stimulus is consistent with a cortical origin; unlike monocular cross-orientation suppression, it cannot be evoked by very fast-moving stimuli. Furthermore, strabismic suppression is greatly reduced when GABAergic inhibition is locally blocked by the GABA(A) antagonist bicuculline.