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

Abstract 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.