Action Potentials in the Cochlea: Masking, Adaptation and Recruitment

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Abstract
Action potentials were picked up from the round window of guinea pigs. By averaging the cochlear responses to tone bursts, the sine wave polarity of which was reversed in half of the presentations, the CM was cancelled out. During steady tones of various frequencies and various intensities, the AP thresholds for short tone bursts of different frequencies were determined. In some experiments the short tone burst was presented shortly after a long tone burst: forward masking. The masking curves of the steady pure tones are compared with the psycho-acoustic masking curves as given in the literature. Another curve is the frequency response curve of a certain location of the cochlea measured by de Boer with the method of reverse correlation. The frequency response curve gives the ability of the cochlea to initiate nerve impulses for various frequencies. The shapes of all these curves are similar and they point to a rather high frequency-selectivity in the cochlea. Tuning curves of single nerve fibers have the steepest slopes compared with the other curves. The masking curves measured during forward masking have the same shape but the thresholds are less elevated for the same intensity of the masking tone. Above threshold AP responses were measured with the method of forward masking. They are reduced when compared with the responses in the no-masking situation. The reduction has the highest value when the frequency of the test stimulus is near the frequency of the masking tone. At a constant level of the masking tone the reduction diminishes with increasing level of the test stimulus showing a kind of recruitment phenomenon. This is valid for all frequencies of the test stimulus and at high levels of the test stimulus the reduction has almost disappeared.

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