THE EFFECTS OF RATE AND FORWARD MASKING ON HUMAN ADULT AND NEWBORN AUDITORY EVOKED BRAIN-STEM RESPONSE THRESHOLDS

Several investigators have reported that human newborn auditory brainstem evoked response (ABR) thresholds to clicks are about 10-20 dB higher than human adult thresholds to the same stimuli (Galambos & Hecox, 1978; Shulman-Galambos & Galambos, 1979; Lasky, Rupert, & Waller, 1987). The present study replicated these results, but, in addition, addressed developmental differences in ABR thresholds due to forward masking and changes in stimulus repetition rate. Eight human newborns and seven human adults served as subjects. Newborn ABR thresholds to a 100-mu-s click which followed a 82 db(A) SPL 50 ms broadband forward masker by 10 ms were significantly more elevated relative to their unmasked ABR thresholds than were those of the adults. Relatively loud sounds seem to have a greater effect on the newborn's ability to process subsequently presented soft sounds than they do on adult processing. In contrast, changes in repetition rate of 100-mu-s clicks from 10/s to 50/s had no greater effect on newborn than adult ABR thresholds. At near threshold stimulus levels, rate effects on the ABR are not apparent in newborns nor adults.