The development of position sensitivity in the infant visual system: Evoked potentials, motion and blur

The ability to detect differences in the relative spatial position of contour elements is a necessary requirement for both motion and pattern perception. This ability may be quantified by measures of vernier acuity, which determine the smallest resolvable relative position difference in the elements of a visual pattern. In this dissertation, four experiments concerning the development of vernier acuity in human infants were performed. The first three experiments centered on the study of task demands in measures of infant vernier acuity, and the final experiment investigated the mechanisms underlying vernier acuity development.

In Experiment 1, estimates of vernier acuity development were obtained from human infants using the visual evoked potentials (VEP) technique. These estimates of threshold development indicated that VEP vernier acuity is very immature throughout the first postnatal year, relative to adults' VEP vernier acuity. Specifically, VEP acuity measures paralleled behavioral measures of grating and vernier acuity in that the VEP technique estimated a different rate of development for vernier acuity relative to grating acuity; moreover, at all ages tested (5-75 weeks postnatal) VEP grating acuity was relatively more mature (closer to adult values) than VEP vernier acuity.

Experiments 2 and 3 investigated task variables that may contribute to behavioral estimates of vernier acuity. These experiments demonstrated that, when a vernier acuity stimulus is spatiotemporally modulated, threshold may be governed by processing in a local motion mechanism. This suggests that behavioral measures of vernier acuity should use stationary stimuli.

Experiment 4 used stationary single-bar stimuli to measure the effects of different levels of two-dimensional Gaussian blur upon 3- and 5-month-old infants' vernier acuity. For both ages, threshold was unaffected by low levels of stimulus blur. As blur increased, 5-month-olds' average threshold was significantly degraded by lower levels of blur, relative to 3-month-olds' threshold. Moreover, at high levels of blur, average threshold for the two ages converged, and was proportional to the level of stimulus blur. This experiment suggests that vernier acuity development depends, in part, upon the reduction of intrinsic blur during the first postnatal year.