Auditory Nerve

Each auditory nerve fiber in the auditory portion of the VIII cranial nerve innervates a small number of inner hair cells, with nerve fibers near the center of the auditory nerve bundle coming from the cochlear apex and those on the outside of the bundle coming from the cochlear base. Thus, the fibers in the auditory nerve bundle are topographically organized on the basis of their cochlear innervation, and they transmit the neural code for sound from the cochlea to the cochlear nucleus in the auditory brain stem.

The neural response of auditory nerve fibers increases in firing rate over a 30- to 40-dB range. Each

Figure 6 A schematic diagram of the traveling wave for three frequencies. The solid line is an instantaneous traveling wave, whereas the dotted curve represents the envelope of the traveling wave outlining its overall shape. The cochlea is shown as if it were unrolled. For low frequencies, the traveling wave travels to the apex, where maximal stimulation occurs. For high frequencies the wave travels a short distance from the base and maximal displacement is at the base (from Yost, 2000).

Figure 6 A schematic diagram of the traveling wave for three frequencies. The solid line is an instantaneous traveling wave, whereas the dotted curve represents the envelope of the traveling wave outlining its overall shape. The cochlea is shown as if it were unrolled. For low frequencies, the traveling wave travels to the apex, where maximal stimulation occurs. For high frequencies the wave travels a short distance from the base and maximal displacement is at the base (from Yost, 2000).

nerve fiber is highly selective to the frequency of the sound, reflecting the selectivity of the traveling wave (Fig. 7). Thus, each nerve fiber carries information about a narrow region of the spectrum and as such each fiber is tuned to a particular frequency. Because of the relationship between the location along the co-chlear partition that each nerve fiber comes from and the activity of the traveling wave, auditory nerve fibers are tonotopically organized so that fibers near the middle of the nerve bundle carry information about low frequencies and those toward the outside of the bundle carry high-frequency information. The tuning curves of Fig. 7 are obtained by determining for each frequency the tonal level necessary to elicit a threshold neural discharge rate. The frequency that requires the lowest level to reach this threshold (the tip of the tuning curve) is the tuning curve's center frequency (CF).

The nerve fibers discharge in synchrony (Fig. 8) to the pressure waveform, such that the neural output represents a half-wave rectified version of the stimulus waveform. Thus, the neural output of the auditory nerve can follow the temporal structure of the waveform up to frequencies of about 5000 Hz. The upper frequency limitation of such temporal resolution is dictated by the refractory properties of neuronal function.

Figure 9 represents the output of a computational model that simulates the properties of the auditory periphery. Each line represents the aggregate neural response of fibers tuned to particular frequencies (low frequencies for fibers coming from the cochlear apex at the bottom of Fig. 9 and those for high frequencies coming from the cochlear base at the top). The model uses bandpass filtering to simulate the frequency selectivity of the tuning curves (Fig. 7), and a model of the hair cell-nerual interaction that provides a rectified, compressed, and adapted version of the filtered sound is used as the cochlear simulation. Thus, Fig. 9 represents the neural information flowing from the cochlea to the auditory brain stem for the vowel /e/, as in the word bead. The vertical bands of high output represent those fibers that are firing to the regions of

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