Magnitude and phase-frequency response to single tones in the auditory nerve

Abstract

Magnitude and phase measurements obtained from primary single unit recordings in the cat auditory nerve are described. Levels range from threshold to 100 dB SPL [sound pressure level] with frequencies from 0.1-30.0 kHz. The upper limit on the phase measurements was limited by the loss of neural phase locking at 4-5 kHz. For each unit, the frequency tuning curve (FTC) was measured by the method of Kiang et Moxon (1978) to establish the threshold frequency response of the unit. Data from several selected animals, organized by characterized frequency (CF), are presented showing phase response, group delay, frequency tuning, and tuning slope for each CF range. The major emphasis is on the linear aspects of the data as characterized by the filter properties of the single unit response; a number of nonlinear (level-dependent) effects are described. Data are presented showing the phase response normalized by the cochlear microphonic (CM) recorded at the round window membrane. This normalization simplifies the phase data since it produces a constant phase slope with respect to frequency (constant group delay) for high CF units (fCF > 1 kHz) for frequencies more than 1 octave below their characteristic frequencies. A model of CM, as measured at the round window (RW), is presented and compared to experimental CM measurements. The CM model gives a reasonable fit to the experimental data > 500 Hz. The CM normalization may remove driver and middle ear effects. In the model CM is assumed to be generated by the displacement of the basilar membrane near the round window recording site.