The development of symmetrical optokinetic nystagmus during infancy

1. We examined the monocular optokinetic nystagmus (OKN) of normal 2-, 3-, and 6-month-olds and adults (n = 20 per group) for large (9°) stripes. 2. During seventy-eight 7-s trials, each subject was shown either a blank field or black-and-white vertical stripes moving horizontally across an 84 x 84° screen. 3. The results were similar for the two velocities of square-wave gratings (13 and 24°/s), and for the three measures of OKN: presence vs absence of OKN, duration of OKN per s of looking time, and number of beats of OKN per s of looking time. 4. We found significant asymmetries of OKN (more OKN to stripes moving temporally-to-nasally than nasally-to-temporally) in most (65-75%) 2-month-olds but in only a few (≤ 25%) older subjects. These data provide a standard for clinical assessment. 5. On one measure, 6-month-olds showed significantly larger asymmetries than adults and, on all measures, 6-month-olds showed significantly less OKN than adults for stripes moving in either direction. Thus, OKN is not yet adult-like at 6 months of age. 6. The substantial reduction of asymmetry between 2 and 3 months of age likely reflects developments in projections from the retina through the visual cortex to the midbrain.