Effects of nonlinearities on speech encoding in the auditory nerve

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Abstract
In this paper we will consider some effects of auditory-nerve nonlinearities on the representation of complex stimuli such as vowels. At low sound levels, profiles of discharge rate versus characteristic frequency in populations of auditory-nerve fibers show well-defined peaks at frequencies corresponding to the formants of a vowel stimulus. At levels above about 60 dB SPL, these peaks are not seen, primarily because of effects of rate saturation. In addition, effects related to two-tone suppression act strongly on units with characteristic frequencies in the vicinity of the second and third formants, suppressing their responses at high levels and contributing to the loss of distinct peaks in this region. Units with spontaneous rates less than about 1/s show the effects of suppression more dramatically than do higher spontaneous units; despite the suppression, however, this population retains formant peaks in its rate profiles up to levels at least 20 dB higher than does the higher spontaneous rate population. The temporal patterns of response of auditory-nerve fibers contain considerable information about the spectrum of a steady state vowel. In general, phaselocking to the formant frequencies is stronger than locking to other harmonics of the stimulus. As sound level increases, synchrony to the formant frequencies saturates. However, responses to nonformant harmonics are suppressed by responses to the formant harmonics. This synchrony suppression allows the phaselocking to the formant frequencies to remain dominant even at high levels. The exception to this behavior is locking to harmonics and intermodulation products of the first two formant frequencies (2F1, 3F1, F2−F1, etc.) which increases rapidly in amplitude at higher sound levels. These distortion products are, to some extent, a consequence of the rectification inherent in hair cell/nerve fiber transduction and may also reflect the presence of propagating combination tones. [Supported by grants from the National Institute of Neurological and Communicative Disorders and Stroke.]

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