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.