Temporally directed deficits for the detection of visual motion in latent nystagmus: evidence for adaptive processing

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Abstract
The purpose of this study was to investigate if visual motion detection deficits are caused by exposure to latent nystagmus (LN). Four subjects with infantile esotropia (two with and two without LN) and six age-matched control subjects were required to detect the onset of motion and drift direction of a linearly increasing velocity spatial frequency grating, monocularly presented just above threshold for contrast in paracentral vision (either in the nasal or temporal retina). LN was damped during testing by ensuring that nystagmic subjects held their viewing eye in an adducted position. In a separate session after the initial testing, all strabismic subjects and two of the six age-matched control subjects repeated the motion detection task. Subjects were then monocularly occluded for three hours, thereby increasing the nystagmus intensity in the LN subjects. After this time, the motion detection task was repeated. The results from each session were consistent and showed motion detection asymmetries for each strabismic subject. The LN subjects had a reduced sensitivity to temporally directed motion, and the observed asymmetry increased after prolonged monocular occlusion. No asymmetries were found in the age-matched control subjects. Since increasing nystagmus intensity through prolonged occlusion significantly increased the asymmetry in the LN subject, the temporally directed motion detection deficit in LN is most likely due to an adaptation to retinal slip information and is evidence against the hypothesis that the nystagmus might be caused by cortical motion processing deficits.