Changes in Human Infants' Sensitivity to Slow Displacements over the First 6 Months

Sensitivity to slow movement improves substantially during the early postnatal months. Thresholds were measured for slow oscillatory displacements for 68 infants aged 6, 12, 18, and 24 weeks. Standing wave line stimuli were used; 6-week-olds were tested at 1.2 Hz, and infants in the three older age groups were tested at either 0.6 or 1.2 Hz. Six-week-olds as a group were very insensitive to these slow displacements. Sensitivity increased systematically across the three older ages. Thresholds were marginally lower at 1.2 Hz than at 0.6 Hz, and there was some indication that these thresholds may reflect a mixture of detection by position-sensitive and motion-sensitive mechanisms. Several factors are hypothesized to be responsible for this development: (1) improvements in spatial resolution; (2) improvements in temporal contrast sensitivity; (3) decreases in the size of second order motion integration mechanisms; and (4) increased neural connectivity in the motion pathways.