Auditory-nerve encoding of pinna-based spectral cues: rate representation of high-frequency stimuli.

The elevation of sound sources and their front-back position is encoded in spectral cues produced by direction-dependent filtering in the pinna. Auditory-nerve (AN) fiber population recordings were used to analyze the neural representation of the acoustic features which carry this information. The most prominent pinna-produced spectral features occur at frequencies greater than 5 kHz, so this information must be encoded in AN discharge rates and not in measures of phase locking. However, profiles of discharge rate versus fiber best frequency (BF) reveal a poor representation of the spectra of the stimuli, primarily because of fiber-to-fiber variation in rate. The variation is not controlled by rate normalization, but a clear representation of the ratio of the magnitude spectra of two stimuli is seen when responses are plotted as the difference between the rates in response to the two stimuli. This results suggests that precise information about stimulus spectrum is present in discharge rate, which could be revealed in rate profiles constructed with suitable normalization. When binaural stimuli are presented, a weak inhibitory effect, due to the olivocochlear bundle or the middle ear muscle reflex, is observed. The rate changes are small and are not correlated with the spectrum of the contralateral ear stimulus.