Development of visual motion processing: phase and peak latencies of direction-specific visual evoked potential.

The direction-reversal visual evoked potential (DR-VEP) latency is a key measure of the development of motion processing in infancy. However, the latency of this response has not been previously investigated. For other stimuli, both the latency of an initial peak and a latency measure calculated from steady-state phase as a function of frequency have been shown to be important and distinctive indicators of development. The latter measure is hypothesized to reflect the time course of cortical processing beyond the initial response that generates the first positive peak. DR-VEP was tested in 61 adults at 1-16 reversals per second (r/s) and 76 infants (age 7.7-79.0 weeks) at 2-8 r/s. In addition to measuring the transient peak latency at 1-3 r/s, latencies from the gradient of phase against reversal rates were also calculated from steady-state recordings at 1-16 r/s. For both adults and infants, peak latencies were similar for 1-3 r/s, while the calculated latency was substantially longer. Thirty-nine percent of adults and 17% of infants showed additional early transient peaks. We suggest that this early peak may reflect activation of extrastriate areas by motion, by a route that bypasses V1. While both transient latencies were similar to adult values around the onset of DR responses at 10 weeks of age, the latency calculated from phase values did not asymptote to the adult value of 207 ms until around 30 weeks. The overall time course of the response to direction reversal is prolonged compared with the transmission delay that generates the initial transient peak, presumably reflecting feed-forward, lateral, and recurrent connections that refine and elaborate the directional response of cortical neurons. While the peak latencies stay relatively unchanged throughout development, the dynamics of further motion processing are not mature until after 8 months of age. These measures may prove important indicators of motion development in future clinical evaluations.