VERTICAL AND HORIZONTAL MOTION PERCEPTION IN CONGENITAL NYSTAGMUS

We investigated whether individuals with congenital nystagmus (CN) hav e abnormalities in motion perception and whether any such abnormality could be due to their nystagmus or to adaptive mechanisms to avoid osc illopsia. CN and control subjects performed motion detection and discr imination tasks. In the detection tasks, subjects reported the onset o f motion and drift direction in either a vertical or horizontal direct ion. In the discrimination task, the stimulus was a high-contrast grat ing and moved vertically. Subjects judged whether successively present ed reference and test velocities were the same or different, using a f orced choice instruction. Vertical velocity detection was normal in th e patient group. The vertical velocity discrimination task showed that the patients were less accurate than the controls, especially when ve locities were slow. Horizontal velocity detection thresholds were rais ed in the patient group regardless of the direction of the slow phase velocity (spv) of the nystagmus. Evaluation of eye movement recordings performed during the task demonstrated that detection velocity was hi ghest when stimulus motion and spy were in the same direction. When ny stagmus was absent due to a prolonged neutral zone, thresholds did not reduce to normal values. The findings show that the image motion caus ed by the nystagmus cannot account for all the abnormalities found. De ficits occurred in the absence of nystagmus and when motion was orthog onal to the meridian of the nystagmus suggesting that the suppression of motion perception is, in part, due to adaptive mechanisms used to a void oscillopsia.