Assessment of vernier acuity development using the "equivalent intrinsic blur" paradigm

Vernier acuity in human infants is more than two orders of magnitude poorer than in adults and does not appear to reach adult levels until well beyond the first postnatal year. One source of these developmental differences in vernier acuity may be the presence of levels of intrinsic blur that are higher in infants than in adults. We investigated this hypothesis by measuring vernier acuity in 3-month-olds, 5-month-olds, and adults using stimuli blurred by two-dimensional Gaussian filters. Experiment 1 showed that more stimulus blur is required to degrade vernier acuity in infants than in adults. From these data we estimated that the level of equivalent intrinsic blur for this vernier acuity task decreased by approx. 1.5 log units between 3 months of age and adulthood. These results also suggested that this reduction of equivalent intrinsic blur can account entirely for the improvement in vernier acuity between 3 and 5 months postnatal. However, the large further improvement which occurs between 5 months of age and adulthood cannot be explained solely by equivalent intrinsic blur. In Expt 2, we measured vernier acuity in 3-month-olds and adults using a non-blurred stimulus with the same luminance contrast as the most-blurred stimulus used in Expt 1. Infants' and adults' thresholds were degraded slightly, relative to the non-blurred stimulus from Expt 1, but still were significantly better than the most blurred condition of Expt 1. This suggests that the results from the first experiment were not due simply to the reduction in overall luminance contrast which occurs when stimuli are blurred.