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 noise that was modulated with a 10-Hz trapezoidal envelope that switched the noise amplitude between two levels, 20 dB apart. At the time of switching, the attributes of the axonal tuning curves derived in this manner switched very rapidly (e.g., within 10 ms) from those characteristic of lower-level stimuli to those characteristic of higher-level stimuli and vice versa. As previous investigators have shown, the attributes of tuning curves at higher levels include broader bandwidth and an accentuated low-frequency hump. Characteristic frequencies (CFs) of gerbil axons used in this study ranged from approximately 500 Hz to approximately 5 kHz. Over this range, the low-frequency hump was most pronounced in our studies for units with higher CFs, each of which showed a sharp high-frequency peak and a distinctly separate, broad low-frequency hump (reminiscent of the tip and tail of a conventional frequency-threshold tuning curve). The amplitude of the peak relative to that of the hump, and the breadth of the peak, both changed rapidly and reversibly following sudden change of noise level. Observation of such rapid changes of tuning would be difficult to achieve with conventional frequency-threshold tuning curves, derived from tonal stimuli.