DYNAMIC CHANGES IN TUNING IN THE GERBIL COCHLEA

Afferent axons of the gerbil cochlear nerve were studied with reverse correlation analyses carried out with movable time windows and with no ise that was modulated with a 10-Hz trapezoidal envelope that switched the noise amplitude between two levels, 20 dB apart. At the time of s witching, the attributes of the axonal tuning curves derived in this m anner switched very rapidly (e.g., within 10 ms) from those characteri stic of lower-level stimuli to those characteristic of higher-level st imuli and vice versa. As previous investigators have shown, the attrib utes of tuning curves at higher levels include broader bandwidth and a n accentuated low-frequency hump. Characteristic frequencies (CFs) of gerbil axons used in this study ranged from approximately 500 Hz to ap proximately 5 kHz. Over this range, the low-frequency hump was most pr onounced in our studies for units with higher CFs, each of which showe d a sharp high-frequency peak and a distinctly separate, broad low-fre quency hump (reminiscent of the tip and tail of a conventional frequen cy-threshold tuning curve). The amplitude of the peak relative to that of the hump, and the breadth of the peak, both changed rapidly and re versibly following sudden change of noise level. Observation of such r apid changes of tuning would be difficult to achieve with conventional frequency-threshold tuning curves, derived from tonal stimuli.