"Time to See" in Infantile Nystagmus

Purpose: Interventions that dampen the involuntary eye oscillation in infantile nystagmus (IN) often only elicit modest improvement in visual acuity (VA), even when subjective improvement is reported. Unique to nystagmus, the repeated movement of the fovea away from the intended visual target introduces a temporal dimension to vision. We hypothesized that the viewing time required to resolve optotype stimuli is increased in people with IN and that this measure of "time to see" increases with IN severity. Methods: Exposure duration thresholds (viewing times) were measured psychophysically for 18 individuals with IN and 14 controls for an optotype orientation discrimination task at spatial frequencies varying relative to each participant's VA. Viewing times were measured at two gaze positions (toward the IN null zone, where waveform intensity is lower, and away from the null zone, where intensity is higher) to establish the effect of IN severity as quantified by intensity. Results: Exposure duration thresholds were significantly longer in those with IN than controls (P = 0.027) and increased significantly with IN intensity (P = 0.034). Both effects were greatest for optotypes sized at participant-relative VA. Conclusions: "Time to See" is increased in IN and is sensitive to within-participant changes in IN severity, suggesting that exposure duration threshold could be a useful measure of visual function in people with IN.