Infant eye movement asymmetries: stationary counterphase gratings elicit temporal-to-nasal optokinetic nystagmus in two-month-old infants under monocular test conditions.

Tested monocularly, young infants classically show directional eye movement asymmetries, with optokinetic nystagmus (OKN) being more readily elicited by stimuli moving in the temporal-to-nasal (T-N) than in the nasal-to-temporal (N-T) direction. Since a counterphase grating is physically identical to two superimposed sinusoidal components moving in opposite directions, we wondered whether or not a counterphase grating would elicit T-N OKN in monocularly tested infants. Two-month-old infants were tested in a motion nulling paradigm. Under monocular test conditions, all infants showed T-N OKN in response to the counterphase grating. The results suggest that the young infant's visual system represents the counterphase grating in terms of its T-N and N-T components, and reduces the effective contrast of the N-T component prior to the generation of OKN. The results are discussed in relation to models of OKN asymmetries and the responses of direction-selective neurons to counterphase gratings.