DEVELOPMENT OF AUDITORY FUNCTION IN THE TAMMAR WALLABY MACROPUS-EUGENII

Auditory brainstem responses (ABRs) were evoked in developing wallabie s by click and tone burst stimuli delivered by bone conduction and air conduction, al progressive stages of post-natal (pouch) life. ABRs we re recorded through the onset of auditory responses (95-110 days), the opening of the external ear canal (125-130 days) and the maturation o f ABR thresholds and latencies to values corresponding to those in adu lts(> 180 days). ABRs were evoked in response to bone-conducted clicks some days prior to the age at which an acoustically evoked response w as first observed (around 95 days of pouch life). ABRs could be evoked by bone-conducted and intense air-conducted stimuli prior to opening of the ear canal. A trend of decreasing threshold and latency with age was observed for both modes of stimulation. The morphology of the ABR became more complex, according to both increased age and increased st imulus intensity. The ABR waveforms indicated relatively greater mecha nosensitivity to bone-conducted stimuli than to air-conducted stimuli, prior to opening of the ear canal. Following opening of the ear canal , thresholds to air-conducted clicks and tones were substantially redu ced and decreased further over the next 10-20 days, while thresholds t o bone-conducted clicks continued slowly to decrease. Thresholds to to ne bursts in the centre frequency range (4-12 kHz) remained less than those for low (0.5-1.5 kHz) and higher (16 kHz) frequencies. Latencies of an identified peak in ABR waveforms characteristically decreased w ith age (at constant stimulus intensity) and with stimulus intensity ( for a given age). ABR waveforms obtained at progressive ages, but judg ed to be at corresponding sensation levels, underwent maturational cha nges, independent of conductive aspects of the wallabies' hearing, for 2-3 weeks after opening of the ear canal. (C) 1998 Elsevier Science B .V.