PARADOXICAL RELATIONSHIP BETWEEN FREQUENCY-SELECTIVITY AND THRESHOLD-SENSITIVITY DURING AUDITORY-NERVE FIBER DEVELOPMENT

The acquisition of adultlike frequency selectivity is generally assume d to be tightly coupled to improvements in threshold sensitivity durin g cochlear development. In this study, frequency versus threshold (tun ing) curves obtained from 1108 auditory-nerve fibers were used to inve stigate the relationship between tuning and threshold at characteristi c frequency (CF) during postnatal development in kittens. At the earli est ages included in this study, sharpness was within the adult range, but thresholds were significantly higher than adult values. Tuning an d thresholds improved along different exponential time courses that va ried with CF. For units with CFs below 1 kHz, tuning curve slopes belo w CF matured earliest, followed by CF threshold, and then by slopes ab ove CF. In contrast, for CFs above 1 kHz, the high-frequency slopes ma tured first, followed by threshold and then by slope below CF. One int erpretation of these results is that tuning and thresholds are nor tig htly coupled in immature animals. Paradoxically, however, high-frequen cy slopes were correlated with threshold for individual units at all a ges, suggesting that the relationship between tuning and threshold is maintained during development. This contradiction can be resolved by a developmental model that features a functional separation between coc hlear nonlinearities and mechanical/electrical conversion. (C) 1998 Ac oustical Society of America.