Frequency contribution to the click-evoked auditory brain-stem response in human adults and infants.

The results of previous research reports have led some investigators to hypothesize that frequency contribution to the infant click-evoked auditory brain-stem response (ABR) is low-frequency dominated and derived primarily from the apical cochlea. This is in contrast to latency and morphology of the adult click-evoked ABR which reflects contributions from the basal cochlea. Recent research, however, has suggested that a simple low-frequency first model of development does not adequately describe the infant auditory brain-stem response. This experiment was conducted as a carefully controlled comparison of infant and adult click-evoked ABRs restricted to narrow frequency ranges with notched-noise masking. The primary objective of this experiment was to define frequency contribution to wave I and V click-evoked ABR latency and morphology in adults and 3-month-old infants. Results indicate that 3-month-old infants have adultlike latency shifts (re: unmasked latency) when the ABR is recorded in the presence of notched-noise masking with center frequencies ranging from 500-8000 Hz. With high-frequency centered notches, latency, and morphology change are similar to the unmasked response, while low-frequency centered notches induce an average latency shift of approximately 3.5 ms for wave I and V of both infant and adult subjects. These data suggest that by 3 months of age, in normal hearing infants, ABR latency and appearance are determined by high-frequency spectral components in the broadband click which activate the basal cochlea. The adultlike pattern of latency shift observed in the ABR of these infants suggests that relatively mature tonotopic organization is established by 3 months of age.