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To discover the developmental relationship between the auditory brainstem response (ABR) and the focal inferior colliculus (IC) response, 32 young tammar wallabies were used, by the application of simultaneous ABR and focal brainstem recordings, in response to acoustic clicks and tone bursts of seven frequencies. The IC of the tammar wallaby undergoes a rapid functional development from postnatal day (PND) 114 to 160. The earliest (PND 114) auditory evoked response was recorded from the rostral IC. With development, more caudal parts of the IC became functional until age about PND 127, when all parts of the IC were responsive to sound. Along a dorsoventral direction, the duration of the IC response decreased, the peak latency shortened, while the amplitude increased, reaching a maximum value at the central IC, then decreased. After PND 160, the best frequency (BF) of the ventral IC was the highest, with values between 12.5 and 16 kHz, the BF of the dorsal IC was the lowest, varying between 3.2 and 6.4 kHz, while the BF of the central IC was between 6.4 and 12.5 kHz. Between PND 114 and 125, the IC response did not have temporal correlation with the ABR. Between PND 140 and 160, only the early components of the responses from the ventral and central IC correlated with the P4 waves of the ABR. After PND 160, responses recorded from different depths of the IC had a temporal correlation with the ABR.

Auditory brainstem responses (ABRs) were evoked in developing wallabies by click and tone burst stimuli delivered by bone conduction and air conduction, at progressive stages of post-natal (pouch) life. ABRs were recorded through the onset of auditory responses (95-110 days), the opening of the external ear canal (125-130 days) and the maturation of ABR thresholds and latencies to values corresponding to those in adults ( > 180 days). ABRs were evoked in response to bone-conducted clicks some days prior to the age at which an acoustically evoked response was 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 stimulus intensity. The ABR waveforms indicated relatively greater mechanosensitivity 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 reduced and decreased further over the next 10-20 days, while thresholds to bone-conducted clicks continued slowly to decrease. Thresholds to tone 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 with age (at constant stimulus intensity) and with stimulus intensity (for a given age). ABR waveforms obtained at progressive ages, but judged to be at corresponding sensation levels, underwent maturational changes, independent of conductive aspects of the wallabies' hearing, for 2-3 weeks after opening of the ear canal.

Auditory brainstem responses (ABR) elicited by click and tonal stimuli were recorded from the tammar wallaby (Macropus eugenii), a marsupial mammal. The morphology, threshold, amplitude, and latency of ABRs recorded in the tammar wallaby are similar to those of other marsupials and mammals used in auditory research, including humans. Thresholds determined by an algorithm employing cross-correlation and by conventional visual detection methods were comparable. The findings from this study indicate that tammar wallaby is a suitable model for auditory research and that algorithms employing cross-correlation are useful for detection of the ABR waveform.

Auditory responses to free-field broad band stimulation from different directions were recorded from clusters of neurones in the superior colliculus (SC) of the anaesthetized tammar wallaby. The auditory responses were found approximately 2 mm beneath the first recording of visually evoked responses in the superficial layers, the vast majority being solely auditory in nature; only one recording responded to both auditory and visual stimulation. Responses to suprathreshold intensities displayed sharp spatial tuning to sound in the contralateral hemifield. Those from the rostral pole of the SC disclosed a preference for auditory stimuli in the azimuthal anterior field, whereas those in the caudal SC preferentially responded to sounds in the posterior field. A continuum of directionally tuned responses was seen along the rostrocaudal axis of the SC so that the entire azimuthal contralateral auditory hemifield was represented in the SC. Furthermore, tight spatial alignment was evident between the best position of the visual responses in the superficial layers in azimuth and the peak angle of the auditory response in the deeper layers.