ONSET AND DEVELOPMENT OF AUDITORY BRAIN-STEM RESPONSES IN THE MONGOLIAN GERBIL (MERIONES-UNGUICULATUS)

The ontogeny of auditory function in the Mongolian gerbil (Meriones un guiculatus) was examined by measuring auditory brainstem response (ABR ) thresholds for clicks and tone bursts at frequencies between 0.125 a nd 32 kHz. ABRs were elicited as early as 10 postnatal days (PND) in a small proportion of animals (28.6%). Three phases of development were identified based on the appearance of frequency-threshold curves: (a) an onset period (10-12 PND) during which ABRs could be elicited only by intense stimuli (95-110 dB SPL) within a restricted range of freque ncies (0.25-8 kHz); (b) a transition period (13-14 PND) during which t hresholds improved but frequency-threshold curves remained relatively flat; and (c) a refinement period (15-30 PND) during which frequency-t hreshold curves exhibited an adult-like ('V') shape and thresholds imp roved steadily to adult-like values. Opening of the external auditory meatus was associated with threshold improvements but not with changes in the frequency region of greatest sensitivity. Between the ages of 13 and 21 PND, low-frequency (0.125-1 kHz) and high-frequency (10-32 k Hz) thresholds improved at statistically equivalent rates (4.9 and 5.6 dB/day, respectively), while thresholds for mid-frequency stimuli (2- 8 kHz) improved at a significantly faster rate of 9.2 dB/day.