Douglas C. Fitzpatrick, PhD, and his colleagues study the physiology and anatomy of hearing using animal models. Projects this past year include the start of a study on preserving residual hearing during cochlear implantation, as well as continuing studies on midbrain implantation, binaural hearing, and transformations of auditory information processing at different brain levels.
The new study on preserving residual hearing during cochlear implantation is being done in collaboration with Drs. Adunka and Buchman. In their surgical practices, a growing number of patients with severe hearing loss are being treated with cochlear implants. These patients retain some residual acoustic hearing, and the best outcomes occur if this residual hearing can be preserved. In particular, retaining residual hearing improves speech understanding in noise compared to electrical stimulation alone. However, residual hearing is often compromised during the implantation surgery. Our hypothesis is that the retention of acoustic hearing will be improved if the surgeon has real-time physiological information on the state of hearing preservation during the implantation process. Current generations of cochlear implants are capable of providing such physiological information, but knowledge of physiological changes as a result of electrode interaction with cochlear structures is limited. Consequently, our experiments are aimed to correlate changes in intracochlear potentials in response to acoustic stimuli as an electrode impacts cochlear structures. Funding for this project has been provided by the Med-El company.
The studies on midbrain implants are motivated by the need to provide auditory sensation to people who lack a functional auditory nerve. These patients cannot be helped by traditional cochlear implants. We have successfully implanted multichannel electrodes in the IC of rabbits, and have measured neural and behavioral thresholds to stimulation of the implant. Further work on this project requires major changes to hardware and software to provide electrical stimulation of complex stimuli including speech sounds. This work is being done in collaboration with Dr. Charles Finley, and is supported by an R21from NIH that began this year.
Studies on binaural hearing are continuing with a particular emphasis on the ability of the binaural auditory system at different brain levels to process temporal variations in cues to sound source location. These experiments are motivated by mismatch between behavioral and neural results, with behavioral measurements suggesting the binaural system is slow, or “sluggish”, while the neurons show no such sluggishness. Techniques include behavioral measurements and neural recordings in rabbits. To date, we have not observed differences in the ability of neurons at any brain level to follow temporal modulations of binaural compared to monaural cues. 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.
To study information processing at different brain levels, we are continuing research on processing between the inferior colliculus and the auditory cortex. In collaboration with Dr. Nell Cant from Duke, we have begun experiments to study the anatomy and physiology of the auditory thalamus in gerbils. The thalamus lies between the inferior colliculus in the midbrain and the auditory cortex. In the inferior colliculus, the central nucleus is the beginning of the core (or “lemniscal”) auditory pathway. It contains a single tonotopic representation. In contrast, the auditory cortex contains multiple “functional areas” each with a separate tonotopic organization. The transformation from a single to multiple tonotopic areas is typically thought to be due to divergence of pathways from the auditory thalamus to auditory cortex. However, Dr. Cant’s work in gerbils and our previous work in bats suggests that the transformation actually occurs in the output pathways of the IC to the auditory thalamus. Our hypothesis is therefore that multiple tonotopic representation will first occur in the auditory thalamus. Our experiments to test this are being done through a combination of electrophysiology and anatomical tract-tracing.
As always, Stephen Pulver has provided superb technical assistance throughout the year.