Vertical and horizontal motion perception in congenital nystagmus

Abstract

We investigated whether individuals with congenital nystagmus (CN) have abnormalities in motion perception and whether any such abnormality could be due to their nystagmus or to adaptive mechanisms to avoid oscillopsia. CN and control subjects performed motion detection and discrimination tasks. In the detection tasks, subjects reported the onset of motion and drift direction in either a vertical or horizontal direction. In the discrimination task, the stimulus was a high-contrast grating and moved vertically. Subjects judged whether successively presented reference and test velocities were the same or different, using a forced choice instruction. Vertical velocity detection was normal in the patient group. The vertical velocity discrimination task showed that the patients were less accurate than the controls, especially when velocities were slow. Horizontal velocity detection thresholds were raised 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 highest when stimulus motion and spv were in the same direction. When nystagmus was absent due to a prolonged neutral zone, thresholds did not reduce to normal values. The findings show that the image motion caused by the nystagmus cannot account for all the abnormalities found. Deficits occurred in the absence of nystagmus and when motion was orthogonal to the meridian of the nystagmus suggesting that the suppression of motion perception is, in part, due to adaptive mechanisms used to avoid oscillopsia.