The detection of slow stimulus movement in 2- to 5-month-olds.

Preferences for moving over static bars were assessed in 8-, 16-, and 20-week-old human infants. The display consisted of two adjacent horizontal bars one of which moved with one of four velocities (0.70, 1.4, 2.8 or 5.6 degrees/s) through one of four distances (11, 22, 44, or 88 arcmin) before reversing and traveling in the other direction at the same velocity. No significant preferences for motion were obtained at 8 weeks. At both 16 and 20 weeks of age, however, preferences for motion were determined exclusively by the velocity of the movement and were unaffected by the excursion of the bar. The minimum velocities that elicited significant preferences for motion were 5.08 degrees/s and 2.32 degrees/s at 16 and 20 weeks of age, respectively. The more attentive 20-week-olds, however, showed significant preferences for motion above a velocity of approximately 1.8 degrees/s. The addition of static reference bars had little influence on these preferences for motion in 20-week-olds; preferences were again related exclusively to the velocity of the bar's movement. The results are discussed in terms of the development of motion-sensitive mechanisms within the visual system.