Rod-mediated increment threshold functions in infants

PURPOSE. To obtain and analyze scotopic increment threshold functions to te st the hypothesis that rod photoreceptor immaturity accounts for the elevat ion of infants' over controls' dark-adapted thresholds and elevation of par afoveal over peripheral thresholds in infants. METHODS. Using a preferential looking method, thresholds for detection of 2 degrees, 50 msec, blue stimuli presented 10 degrees (parafoveal) or 30 deg rees (peripheral) eccentric were measured in the dark and in the presence o f steady red backgrounds. Ten 10-week-old infants and four control subjects (8-35 years) were tested. To evaluate pre- and postadaptation site determi nants of threshold, a model of the increment threshold function was fit to the data, and the dark-adapted threshold (T-D) and eigengrau (A(O)) were ca lculated. The values of T-D and A(O) were compared between infants and cont rols and between parafoveal and peripheral eccentricities. RESULTS. At both parafoveal and peripheral eccentricities, infants' values of T-D and A(O) were significantly higher than those of controls. The locus of the coordinates (A(O), T-D) differed significantly between parafoveal a nd peripheral eccentricities. In every infant, the parafoveal value of T-D was higher (by 0.3-0.6 log unit) and A(O) lower (by 0.2-0.5 log unit) than the peripheral value, whereas controls had no difference in T-D and A(O) at the two eccentricities. CONCLUSIONS. The results indicate that both receptoral and postreceptoral i mmaturities have a role in the elevation of infants' over controls' thresho lds. In infants, rod photoreceptor immaturity before the site of adaptation accounts for elevation of parafoveal over peripheral thresholds.