Reduced duration of a visual motion aftereffect in congenital nystagmus

Congenital nystagmus (CN) is a primarily horizontal, involuntary, conjugate eye movement which can be observed soon after birth or during the first ha lf-year of life. Individuals with CN rarely complain of oscillopsia. Using a motion aftereffect (MAE), we investigated if individuals with CN have abn ormalities in motion perception and if any such abnormality could be due to nystagmus or to compensatory mechanisms to avoid oscillopsia. In task A, patients (n= 10) and control subjects (n= 10) indicated the dire ction, duration and relative velocity of MAEs. The subjects binocularly vie wed a high contrast, grey scale grating (0.23 cyc/deg; visual angle: 18.3 d eg) moving upward or downward at 1, 3, and 6 deg/sec for 60 sec. Vertical o ptokinetic nystagmus (OKN) was monitored. In task B, patients (n=8) and con trol subjects (n=8) viewed similar spatial frequency gratings (visual angle : 40.7 degs; 0.5, 0.2, 0.08 cyc/deg) which mov ed at 4, 10, and 16 deg/sec for 60 sec. In task C, five control subjects, with induced vestibular nysta gmus, viewed a grating (0.2 cyc/deg; visual acuity: 28.5 deg), moving upwar d for 40 sec. In all three tasks, after adaptation with the moving grating, subjects viewed the then static grating and reported the duration and dire ction of the MAE. One CN patient and eight control subjects reported MABs a t all three test velocities in task A. When patients exhibited OKN, the gai n was close to one, as in the control group. In task B, seven of the eight patients and all of the control subjects had MABs at the faster adaptation velocities. CN patients had less MAEs at an adaptation velocity of 4 deg/se c and when MAEs were observed, the duration of the illusory motion was redu ced by approximately 48%. Control subjects, with induced vestibular nystagm us, reported MAEs at 4 deg/sec (task C). These findings indicate that nystagmus cannot be the only factor accounting for the suppressi on of motion perception and suggest that compensatory me chanisms used to avoid oscillopsia contribute to the differences found betw een the groups.