Oscillopsia and tolerance to retinal image movement in congenital nystagmus.

PURPOSE To determine the relationship between retinal image movement (RIM) and oscillopsia in subjects with idiopathic congenital nystagmus (CN). METHODS Eye movements were recorded using an IRIS infrared system. The eye movement signal was fed back to move an otherwise stationary target on a screen and thereby modify the RIM experienced by each of the five CN subjects. The target was present with either no background (the absolute condition) or a textured background (the relative condition). Feedback gains were varied from -1.0 (i.e., 100% retinal image increase) to +1.0 (i.e., 100% retinal image decrease or complete stabilization), with 0 representing the zero feedback or stationary target condition. In the first experiment, RIM thresholds were determined for a range of feedback values. Using zero feedback, a second experiment measured the detection threshold for absolute and relative motions to a ramp-generated target movement for five CN and five control subjects. RESULTS Under feedback control spatial constancy broke down for both increased and reduced RIM. The range of spatial constancy was greater for absolute (-0.56 to +0.44) compared with relative (-0.18 to +0.18) RIM. Motion detection thresholds for the CN group were 8 times less sensitive to the absolute and 17 times less sensitive to the relative motion of the target compared with the control group. CONCLUSIONS These results suggest that in CN subjects perceptual stability is achieved primarily by extraretinal signals.