Representation of Vowel-like Spectra by Discharge Rate Responses of Individual Auditory-Nerve Fibers.

Neural representations of complex vowel-like spectra have been extensively characterized by population studies of single fiber responses in the auditory nerve of anesthetized cats. With traditional population measures, neural rate responses to formants (energy peaks) and troughs (energy minima) in a vowel's amplitude spectrum are measured by sampling several fibers, each tuned to one spectral feature. Similar analyses are rarely performed on structures in the central auditory system primarily due to the difficulty of obtaining the samples of neurons that are needed to construct complete population response profiles. As an alternative to population measures, this study introduces a method for estimating population measures from the responses of individual auditory-nerve fibers. With our spectrum manipulation procedure (SMP), a response profile was created by sampling the responses of individual fibers as important spectral features were shifted to the units' best frequency (BF, the frequency to which a neuron is most sensitive). Observed SMP rate profiles showed the same effects of rate saturation, two-tone suppression, and spontaneous rate as population measures. In addition, when analyzed with signal detection methods, changes in rate responses within individual neurons revealed new insights into how the neural representation of vowel stimuli may be influenced by unit threshold and best frequency. SMP sampling techniques should prove useful in future studies of speech encoding in the central auditory system.