Using a laser velocimeter, basilar membrane (BM) responses to tones were measured in neonatal gerbils at a site near the round window of the cochlea. In adult gerbils, 'active' BM responses at this site are most sensitive at 34-37 kHz and exhibit a compressive non-linearity. Postmortem, BM responses in adults become 'passive', i.e. linear and insensitive, and the best frequency (BF) shifts downwards by about 0.5 octaves. At 14 and 16 days after birth (DAB), BM responses in neonatal gerbils were passive but otherwise very different from postmortem responses in adult gerbils: BF was more than an octave lower, the steep slopes of the phase vs. frequency curves were shifted downwards in frequency by nearly 1 octave, and the maximum phase lags amounted to only 180 deg relative to stapes. BFs and phase lags increased systematically between 14 and 20 DAB, implying drastic alterations of the passive material properties of cochlear tissues and accounting for a large part of the shift in BF that characterizes maturation of auditory nerve responses during the same period.
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