We compared sensitivity to first- and second-order information in 3-month-olds, 5-year-olds and adults by measuring thresholds for the detection of motion at two velocities. During each trial, participants viewed two 15x15 deg vertical 0.5 c/deg sine-wave gratings that were separated by a 5 deg gap. The gratings were added to (first-order condition) or multiplied with (second-order condition) binary noise. Randomly, on each trial, one of the gratings was stationary and the other moved outward at 1.5 or 6 deg/s. First- and second-order information was modulated over trials in separate conditions to measure thresholds. For 3-month-olds (n=24), a trained tester, who was unaware of the side of motion, decided whether the moving grating was on the left or right based on the infant’s eye movements. Each infant completed 16 trials during which either first- or second-order information was varied by the method-of-constant stimuli. Separate group thresholds were calculated for the two conditions. Older subjects (n=24 for each age and speed) indicated the side with the moving grating, and a staircase procedure (Harvey, 1986) was used to calculate individual thresholds. Three-month-olds, who were tested only at the faster speed, had group thresholds 1.3 times higher than 5-year-olds for first-order gratings, but 2.3 times higher than 5-year-olds for second-order gratings. At both speeds, 5-year-olds were as sensitive as adults to first-order gratings (ps>0.1) but were less sensitive than adults to second-order gratings (1.1-1.3 times higher than adults; ps<0.001). Thus, the results indicate that, under the parameters tested, sensitivity to second-order information develops more slowly than sensitivity to first-order information both during infancy and early childhood. Because participants were required to detect motion rather than discriminate between directions of motion, sensitivity may have been mediated either by the motion system or by mechanisms sensitive to flicker or position.

Supported by Canadian Institutes of Health Research grant # MOP-36430