Development of motion-specific cortical responses in infancy.

The development of visual motion mechanisms has been studied in infants with a visual evoked potential (VEP) technique which isolates responses from directionally-selective mechanisms. In adults, the amplitude of this directional VEP increased with velocity up to a maximum at 15-20 deg/sec, and then declined with further increases in velocity. In a group of infants tested longitudinally, directional responses were first found at a median age of 74 days with a stimulus velocity of 5 deg/sec, and 90 days with a velocity of 20 deg/sec; this age difference was statistically significant. Initially, VEP amplitudes were significantly greater at 5 deg/sec than at 20 deg/sec. By the end of the longitudinal study, there was no significant difference in amplitudes at the two velocities. In a second group of infants, simultaneous recording of VEPs and electrooculograms indicated that eye movements tracking the stimulus were not a significant factor in the development of the directional VEP. It is concluded that the development of directional selectivity starts at low velocities, and extends to higher velocities with age.