John Middlebrooks – middlebj@hs.uci.edu
University of California, Irvine
Irvine, CA 92697-5310
United States
Matthew Richardson and Harrison Lin
University of California, Irvine
Robert Carlyon
University of Cambridge
Popular version of 2aPP6 – Temporal pitch processing in an animal model of normal and electrical hearing
Presented at the 184 ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0018352
A cochlear implant can restore reasonable speech perception to a deaf individual. Sensitivity to the pitches of sounds, however, typically is negligible. Lack of pitch sensitivity deprives implant users of appreciation of musical melodies, disrupts pitch cues that are important for picking out a voice amid competing sounds, and impairs understanding of lexical tones in tonal languages (like Mandarin or Vietnamese, for example). Efforts to improve pitch perception by cochlear-implant users could benefit from studies in experimental animals, in which the investigator can control the history of deafness and electrical stimulation and can evaluate novel implanted devices. We are evaluating cats for studies of pitch perception in normal and electrical hearing.
We train normal-hearing cats to detect changes in the pitches of trains of sound pulses – this is “temporal pitch” sensitivity. The cat presses a pedal to start a pulse train at a particular base rate. After a random delay, the pulse rate is changed and the cat can release the pedal to receive a food reward. The range of temporal pitch sensitivity by cats corresponds well to that of humans, although the pitch range of cats is shifted somewhat higher in frequency in keeping with the cat’s higher frequency range of hearing.
We record small voltages from the scalps of sedated cats. The frequency-following response (FFR) consists of voltages originating in the brainstem that synchronize to the stimulus pulses. We can detect FFR signals across the range of pulse rates that is relevant for temporal pitch sensitivity. The acoustic change complex (ACC) is a voltage that arises from the auditory cortex in response to a change in an ongoing stimulus. We can record ACC signals in response to pitch changes across ranges similar to the sensitive ranges seen in the behavioral trials in normal-hearing cats.
We have implanted cats with devices like cochlear implants used by humans. Both FFR and ACC could be recorded in response to electrical stimulation of the implants.
The ACC could serve as a surrogate for behavioral training for conditions in which a cat’s learning might not keep up with changes in stimulation strategies, like when a cochlear implant is newly implanted or a novel stimulating pattern is tested.
We have found previously in short-term experiments in anesthetized cats that an electrode inserted into the auditory (hearing) nerve can selectively stimulate pathways that are specialized for transmission of timing information, e.g., for pitch sensation. In ongoing experiments, we plan to place long-term indwelling electrodes in the auditory nerve. Pitch sensitivity with those electrodes will be evaluated with FFR and ACC recording. If performance of the auditory nerve electrodes in the animal model turns out as anticipated, such electrode could offer improved pitch sensitivity to human cochlear implant users.