Midbrain sensitivity to frequency “chirps:” Implications for coding voiced speech sounds

Abstract

Many neurons in the mammalian midbrain (inferior colliculus, IC) have strong selectivity for direction of frequency transitions (“chirps”), which often co-exist with the better-known sensitivity for amplitude-modulation (AM) frequency. Chirps occur when harmonics with a phase gradient are summed, as in Schroeder-phase harmonic stimuli. Chirps also occur in voiced speech due to the phase properties of vocal tract resonances. IC neurons are direction-selective for chirps in stimuli with fundamental frequencies in the voice-pitch range, and chirp selectivity can be achieved in an AM-tuned IC model by adding off-CF inhibition. Here, we will show physiological and model responses illustrating how chirps influence IC responses to speech in awake rabbit. In healthy auditory-nerve (AN) responses, the frequency extent, or size, of chirps vary with proximity of AN tuning to formant peaks due to inner ear nonlinearities. For example, synchrony capture causes one harmonic to dominate responses of AN fibers near spectral peaks, and responses that are dominated by one harmonic do not exhibit chirps. The systematic variation in chirp size across frequency channels and strong selectivity of IC neurons for chirps suggest a role for this feature in coding voiced speech. Importantly, this coding mechanism is vulnerable to sensorineural hearing loss.