Submitted on July 5, 2007
Accepted on February 17, 2008
Sensitivity of Inferior Colliculus Neurons to Interaural Time Differences in the Envelope versus the Fine Structure with Bilateral Cochlear Implants
Zachary M. Smith1* and Bertrand Delgutte2
1 Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, United States; Speech and Hearing Bioscience and Technology Program, Harvard-MIT Division of Health Sciences and Technology, Cambridge, Massachusetts, United States; Research and Applications, Cochlear Americas, Englewood, Colorado, United States
2 Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, United States; Speech and Hearing Bioscience and Technology Program, Harvard-MIT Division of Health Sciences and Technology, Cambridge, Massachusetts, United States; Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
* To whom correspondence should be addressed. E-mail: zsmith{at}alum.mit.edu.
Bilateral cochlear implantation seeks to improve hearing by taking advantage of the binaural processing of the central auditory system. Cochlear implants typically encode sound in each spectral channel by amplitude modulating (AM) a fixed-rate pulse train, thus interaural time differences (ITD) are only delivered in the envelope. We investigated the ITD sensitivity of inferior colliculus (IC) neurons with sinusoidally AM pulse trains. ITD was introduced independently to the AM and/or carrier pulses in order to measure the relative efficacy of envelope and fine structure for delivering ITD information. We found that many IC cells are sensitive to ITD in both the envelope (ITDenv) and fine structure (ITDfs) for appropriate modulation frequencies and carrier rates. ITDenv sensitivity was generally similar to that seen in normal-hearing animals with AM tones. ITDenv tuning generally improved with increasing modulation frequency up to the maximum modulation frequency that elicited a sustained response in a neuron (tested up to 160 Hz). ITDfs sensitivity was present in about half the neurons for 1000 pulses-per-second (pps) carriers and was non-existent at 5000 pps. The neurons that were sensitive to ITDfs at 1000 pps were those that showed the best ITD sensitivity to low-rate pulse trains. Overall, the best ITD sensitivity was found for ITD contained in the fine structure of a moderate rate AM pulse train (1000 pps). These results suggest that the interaural timing of current pulses should be accurately controlled in a bilateral cochlear implant processing strategy that provides salient ITD cues.
