The Perception of Relative Motion by Young Infants

The perception of relations between stimuli is considered to be essential for perceptual grouping. The present study investigated the ability of five-month-old infants to perceive the relative motions between simultaneously moving dots. Two of the dots moved horizontally back and forth in parallel trajectories, and a third dot moved vertically up and down between the other two. As a consequence of the relative motions, adults perceived the vertically moving dot to be moving an average of 32° clockwise or counterclockwise from the vertical depending on the phase of motion between the vertical and horizontal dots. An habituation paradigm was used to determine whether infants also perceive these relative motions. A control group of infants was presented a display with the same phase of motion between the vertical and horizontally moving dots for twelve trials. The experimentals were presented with the same display as the controls during the first ten trials, but on the last two trials the phase of motion was reversed. Fixation of the display continued to decline during the last two trials for the control, but not for the experimental group. A second study in which the phase of motions could be varied in two simultaneously presented three-dot displays supports the results of the first study. By five months of age, infants seem to be sensitive to relative-motion cues, and presumably have the ability to group moving stimuli perceptually.