Abstract
A longstanding debate in developmental psychology concerns the mechanisms of development which allow infants to perceive partially occluded objects as complete. One candidate explanation involves the role of motion. It has long been known that young infants achieve perceptual completion only when the visible portions of a partly occluded surface undergo common motion, implying that failure to perceive connectedness may be rooted in an insensitivity to visible motion.
Perceptual completion, motion sensitivity, and smooth pursuit emerge at about the same age in human infants (2 months), leading to the hypothesis that smooth pursuit and perceptual completion arise from developments in motion sensitivity. We observed 11 infants, 52–99 days of age, in three tasks on the same day. In the first, infants viewed small moving targets as their eye movements were recorded with a corneal-reflection eye tracker. We isolated segments of smooth pursuit and compared its speed with object speed to obtain individual infants' gain. In the second, we tested object unity perception by habituating infants to a partially occluded moving rod and recorded looking times at unoccluded rods, either broken or complete. A novelty preference for the broken rod was interpreted as evidence of perceptual completion. In the third, infants observed side-by-side random-dot kinematograms in which dots moved either uni- or bi-directionally.
Infants who showed a preference for bi-directional motion displays were better able to smoothly pursue a moving target (p < .03). In contrast, neither performance in the motion task nor the smooth pursuit task predicted perceptual completion. Smooth pursuit and motion sensitivity performance were correlated with infant age (r = .75, p < .01 and r = .50, p = .11), but unity perception was not (r = -.06, ns). We conclude that motion sensitivity is more strongly involved in smooth pursuit than it is in the development of the perception of object unity.
This research was supported by NIH R01-HD40432 and NSF BCS-0418103. We would like to thank our infant participants and their parents.