THE DEVELOPMENT OF MAXIMUM DISPLACEMENT LIMITS FOR DISCRIMINATION OF MOTION DIRECTION IN INFANCY

The development of visual motion mechanisms has been studied in infants by using forced-choice preferential looking to measure maximum displacement limits (d(max)) for the detection of coherent motion in random-dot patterns. The motion consisted of a sequence of coherent displacements at intervals of 20 msec. Between 8 and 15 weeks, d(max) for discrimination of coherent from incoherent motion, and for discrimination of opposite directions of coherent motion, increased with age; in both conditions, d(max) for the oldest infants was less than a third of the value found in adults. For 10-11-week-old infants, and for an adult subject, reducing stimulus contrast from 88 to 48% had no effect on d(max) for direction discrimination, which indicates that the rise in d(max) with age is not simply a result of improving contrast sensitivity. When the displacement interval was increased from 20 to 40 msec, d(max) increased significantly in 8-11-week-olds, but remained unchanged in 12-15-week-olds. These results show that while directional mechanisms are present in the visual system by 8 weeks, they operate over a restricted velocity range. The upper limit of this velocity range increases with age. After about 12 weeks, the increase is mainly due to changes in the spatial properties of motion mechanisms; however, in younger infants changes in their temporal properties are also important.