Purpose: To measure the developmental course of infants' rod-mediated thresholds.
Methods: Thresholds for detecting stimuli (2 degrees diameter, 50 msec duration) presented at 10 degrees (parafoveal site) or 30 degrees (peripheral site) from a central fixation target were estimated using a preferential-looking method. Nine infants were tested at both stimulus positions at ages 10, 18, and 26 weeks.
Results: At 10 weeks, infants' thresholds at both sites were significantly higher than those of adults. The infants' average threshold at 10 degrees was 0.5 log unit higher than the infants' average threshold at 30 degrees. Adults' thresholds at the two sites were equal. Thresholds of all infants decreased with age until by age 26 weeks the parafoveal and peripheral thresholds were equal and were the same as those of adults. The rate of change of parafoveal thresholds was significantly faster than the rate of change of peripheral thresholds.
Conclusions: Although postreceptoral factors cannot be ruled out, the results suggest that developmental increases in rod outer segment length and rhodopsin density account for most of the threshold changes during infancy.