The development of motion sensitivity during the first year of life

Using the sweep visual evoked potential (VEP), we have measured oscillatory displacement thresholds (OMTs) in 49 infants ranging in age from 7 to 54 weeks of age. The stimuli were high-contrast (80%), sine-wave gratings (1 c/deg) undergoing oscillatory displacements at 6 Hz. In addition to the motion thresholds, contrast thresholds for phase-reversing (6 Hz), 1 c/deg gratings were measured in the same session for 26 infants. In the main experiment, responses were recorded at the second harmonic (F2) of the stimulus frequency (12 Hz) under binocular viewing conditions. Our main finding is that, over the age range during which infants' peak contrast sensitivity (CS) first develops to within a factor of 2 of adult CS (9-12 weeks), infants' sensitivity to grating displacement is a factor of approximately 10 less than adults'. Moreover, infants' sensitivity to oscillatory motion undergoes relatively little development over the period between 2 and 15 months postnatal, gradually achieving a factor of 4.5 below adult values by 1 yr of age. Averaged over the entire age range tested, infants' OMTs were 167 sec arc, a factor of 6.4 times higher than the average OMT (26 sec arc) for 13 adults tested under identical conditions. In contrast, the infants' average CS for reversing gratings averaged only a factor of 2.5 less than the adults' average CS. In a second experiment, we took advantage of a developmental asymmetry in the monocular oscillatory motion VEP which allows for unambiguous identification of direction selective responses from very young infants. Monocular motion VEPs were measured in five infants (8-14 weeks) and their data analyzed at the fundamental frequency (F1). Responses at F1 were present in the monocular motion VEP from each infant and were 180 deg out of phase between the two eyes, identifying them as directional cortical responses with a nasalward/temporalward bias. These directional thresholds were equal to or lower than the symmetric (F2) thresholds. The presence of directional asymmetry in the motion VEP and the similarity of the monocular F1 and F2 OMTs support the notion that the OMTs measured in the main experiment were, in fact, derived from the responses of directionally selective cells in visual cortex. These data also imply that the OMTs are not derived from local contrast-reversal responses. Other models to explain infants' relative insensitivity to oscillatory motion are discussed.