Encoding of vowel-like sounds in the auditory nerve: model predictions of discrimination performance

The sensitivity of listeners to changes in the center frequency of vowel-like harmonic complexes as a function of the center frequency of the complex cannot be explained by changes in the level of the stimulus [Lyzenga and Horst, J. Acoust. Soc. Am. 98, 1943-1955 (1995)]. Rather, a complex pattern of sensitivity is seen; for a spectrum with a triangular envelope, the greatest sensitivity occurs when the center frequency falls between harmonics, whereas for a spectrum with a trapezoidal envelope, greatest sensitivity occurs when the center frequency is aligned with a harmonic. In this study, the thresholds of a population model of auditory-nerve (AN) fibers were quantitatively compared to these trends in psychophysical thresholds. Single-fiber and population model responses were evaluated in terms of both average discharge rate and the combination of rate and timing information. Results indicate that phase-locked responses of AN fibers encode phase transitions associated with minima in these amplitude-modulated stimuli. The temporal response properties of a single AN fiber, tuned to a frequency slightly above the center frequency of the harmonic complex, were able to explain the trends in thresholds for both triangular- and trapezoidal-shaped spectra.