Proprioceptive and retinal afference modify postsaccadic ocular drift

Drift of the eyes after saccades produces motion of images on the retina (r etinal slip) that degrades visual acuity. In this study, we examined the co ntributions of proprioceptive and retinal afference to the suppression of p ostsaccadic drift induced by a unilateral ocular muscle paresis. Eye moveme nts were recorded in three rhesus monkeys with a unilateral weakness of one vertical extraocular muscle before and after proprioceptive deafferentatio n of the paretic eye. Postsaccadic drift was examined in four visual states : monocular viewing with the normal eye (4-wk period), binocular viewing (2 -wk period): binocular viewing with a disparity-reducing prism (2-wk period ); and monocular viewing with the paretic eye (2-wk period). The muscle par esis produced vertical postsaccadic drift in the paretic eye, and this drif t was suppressed in the binocular viewing condition even when the animals c ould not fuse. When the animals viewed binocularly with a disparity-reducin g prism, the drift in the paretic eye was suppressed in two monkeys (with s uperior oblique pareses) but generally was enhance in one animal (with a te notomy of the inferior rectus). When drift movements were enhanced, they re duced the retinal disparity that was present at the end of the saccade. In the paretic-eye-viewing condition, postsaccadic drift was suppressed in the paretic eye and was induced in the normal eye. After deafferentation in th e normal-eye-viewing state, there was a change in the vertical postsaccadic drift of the paretic eye. This change in drift was idiosyncratic and varia bly affected the amplitude and velocity of the postsaccadic drift movements of the paretic eye. Deafferentation of the paretic eye did not affect the postsaccadic drift of the normal eye nor did it impair visually mediated ad aptation of postsaccadic drift. The results demonstrate several new finding s concerning the roles of visual and proprioceptive afference in the contro l of postsaccadic drift: disconjugate adaptation of postsaccadic drift does not require binocular fusion; sloe, postsaccadic drift movements that redu ce retinal disparity but concurrently increase retinal slip can be induced in the binocular viewing state; postsaccadic drift is modified by proprioce ption from the extraocular muscles, but these modifications do not serve to minimize retinal slip or to correct errors in saccade amplitude; and visua lly mediated adaptation of postsaccadic drift does not require propriocepti ve afference from the paretic eye.