Functional development of the auditory brainstem in the tammar wallaby (Macropus eugenii): the superior olivary complex and its relationship with the auditory brainstem response (ABR)

Twenty pouch-young tammar wallabies (Macropus eugenii) were used to determine the generator of the auditory brainstem response (ABR) during development through ABR and focal superior olivary complex (SO) recordings. A click response from the SO in the wallaby was recorded from postnatal day (PND) 112 when the ABR was only a positive-negative deflection. Before PND 120, the SO response did not contribute to the ABR as it occurred outside the ABR time-span. After PND 140, the SO response was correlated with multiple waves of the ABR with its dominant component corresponding to the ABR P3 wave. The latency, threshold, and amplitude of the SO response developed to the adult-like level at PND 140, while the rate-following ability in the SO response reached the adult level at PND 160. Presumably this was due to more complicated mechanisms underlying the auditory adaptation. The adaptation of the SO response was directly proportional to the stimulus rate and intensity as well as developmental status. Developmental comparison between the ABR and the focal responses from four auditory brainstem nuclei indicated that each ABR component may have a dominant contributor from the auditory brainstem, but there was no simple and exclusive association between the ABR component and the auditory brainstem nuclei