Sensitivity to oscillatory target motion in congenital nystagmus

Despite incessant to and fro motion of their eyes, persons with congenital nystagmus rarely report illusory motion of the visual world (oscillopsia). To determine whether insensitivity to retinal image motion prevents oscillopsia, thresholds for detecting horizontal and vertical oscillatory motion of a luminous dot target were measured in subjects with congenital idiopathic nystagmus and compared to normals. Thresholds for unreferenced and referenced oscillatory motion were elevated substantially in subjects with nystagmus over the range of motion frequencies tested (0.25-12 Hz), particularly for motion parallel to nystagmus eye movements. However, motion thresholds were increased comparably in normal subjects when the target underwent continuous sinusoidal or ramp motion to simulate the retinal image motion that occurs with nystagmus eye movements. These results indicate that sensitivity to motion in persons with congenital nystagmus is similar to normal as long as the ongoing motion of the retinal image, produced by the eye movements, is taken into account. Therefore, insensitivity to motion does not explain the lack of reported oscillopsia in subjects with congenital nystagmus. Perceptual adaptation is suggested as a possible mechanism that contributes to the prevention of oscillopsia.