RATE AND ADAPTATION EFFECTS ON THE AUDITORY-EVOKED BRAIN-STEM RESPONSE IN HUMAN NEWBORNS AND ADULTS

Auditory evoked brainstem response (ABR) latencies increased and ampli tudes decreased with increasing stimulus repetition rate for human new borns and adults. The wave V latency increases were larger for newborn s than adults. The wave V amplitude decreases were smaller for newborn s than adults. These differences could not be explained by development al differences in frequency responsivity. The transition from the unad apted to the fully adapted response was less rapid in newborns than ad ults at short (=10 ms) inter stimulus intervals (ISIs). At longer ISIs (=20 ms) there were no developmental differences in the transition to the fully adapted response. The newborn transition occurred in a two stage process. The rapid initial stage observed in adults and newborns was complete by about 40 ms. A second slower stage was observed only in newborns although it has been observed in adults in other studies ( Weatherby and Hecox, 1982; Lightfoot, 1991; Lasky et al., 1996). These effects were replicated at different stimulus intensities. After the termination of stimulation the return to the wave V unadapted response took nearly 500 ms in newborns. Neither the newborn nor the adult dat a can be explained by forward masking of one click on the next click. These results indicate human developmental differences in adaptation t o repetitive auditory stimulation at the level of the brainstem.