Orientational asymmetries in small-field optokinetic nystagmus in human infants

Optokinetic nystagmus (OKN) is a pattern of reflexive eye movements which occurs when portions of the visual field are in continuous motion. Gratings moving at 7 deg/s either horizontally (left or right) or vertically (up or down) were presented on a viewing screen subtending 30 degrees by 22 degrees. Horizontal (HOKN) and vertical (VOKN) OKN were recorded under binocular viewing conditions from infants and adults. Eye movements were recorded by means of an infrared corneal reflection eye movement recorder. OKN to horizontal and vertical stimuli was different in pattern for infants. Infants' HOKN was of significantly higher frequency and lower amplitude than their VOKN. Infants below 4 months of age also showed an asymmetry within VOKN between upward and downward stimulus motion, with markedly lower gains and more variable slow phase following movements for downward moving stimuli. No differences were found in HOKN to the right and left. There was also no evidence of a build-up of slow phase velocity over time. Infants' fast phases showed peak velocity/amplitude relationships like those of adults, and like those of infant saccades recorded in a previous study of infants' fast eye movements. Across all directions of stimulus movement, infants had lower slow phase gains and OKN frequencies, and larger slow phase amplitudes than adults. The characteristics of infants' OKN are discussed in relation to those observed in other species and in adult clinical patients with eye movement disorders.