Douglas C. Fitzpatrick, PhD

Douglas C. Fitzpatrick, PhD, and his colleagues study the neuronal basis of binaural hearing. The two ears encode the frequency and timing of sounds, and dedicated neural pathways then compare the information from the two sides to improve the signal to noise ratio and to extract specific information such as the location of the sound source. The difference in the time of arrival of sounds at each ear is a major binaural cue for localizing sounds on the azimuth. Human listeners can discriminate interaural time differences (ITDs) as small as 10-20 microseconds, or about 1/100 the width of a single action potential. As might be expected, the neural basis for discriminating ITDs has received considerable attention. Our approach has been to combine neurophysiological and behavioral techniques to measure the thresholds for ITD discrimination achieved by neurons for comparison with human behavioral thresholds and with behavioral thresholds measured in the species from which the neurons are recorded. In addition, we are applying our basic understanding of binaural processing and the auditory system to test devices that may be used to provide hearing to the deaf. Some subjects are not candidates for cochlear implants because their condition has not left a patent auditory nerve. In these individuals the only way to provide auditory input is through stimulation of the central auditory system. We are using our animal model to test the feasibility of an implant in the auditory midbrain.

This past year we made progress in understanding the ability of neurons in the inferior colliculus and auditory cortex or rabbits to follow temporal fluctuations of the ITD, such as would be associated with a moving sound source. Currently, the literature suggests a mismatch between behavioral and neural results, with behavioral measurements suggesting the binaural system is slow, or “sluggish”, while the neurons show no such sluggishness. However, physiological experiments have generally used a different methodology and have not been done at the cortical level. Because binaural sluggishness is measured relative to the ability to encode fluctuating cues in monaural channels, such as amplitude modulation (AM), our experiments compare neural responses to modulation of the ITD or of AM. We also perform parallel behavioral experiments in rabbits to determine if they show binaural sluggishness as seen behaviorally in humans. To date, we have not observed differences in the ability of neurons to follow temporal modulations of the ITD or AM. Significantly, our preliminary behavioral results also show no differences in temporal processing of these two signals. Consequently, our current hypothesis is that binaural sluggishness is not due to an inherent limitation in processing binaural signals, but rather an enhancement in processing AM in humans compared to rabbits. This enhancement is likely to be the result of the need to encode AM for extracting information in complex signals such as speech.

This work is being done in collaboration with the psychoacousticians (Drs. Joe Hall, John Grose and Emily Buss) and has benefited from the participation of medical (Jason Roberts and Katherine Sebastian) and undergraduate (Stuart Owens) students.

We have continued a project previously supported by the Advanced Bionics Corporation to investigate the ability of an implant in the midbrain rather than cochlea to provide the percept of hearing. We successfully implanted multichannel electrodes in the IC of rabbits, and have measured neural and behavioral thresholds to stimulation of the implant. The major result to date is that the neural and behavioral thresholds are in close agreement. Future work in this project will be to determine the ability of the rabbits to detect cues for speech such as formant frequency and voice onset time based on electrical stimulation of the IC. This work is being done in collaboration with Dr. Charles Finley.

As always, Stephen Pulver has provided superb technical assistance throughout the year.