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The Journal of Neuroscience, June 20, 2007, 27(25):6740-6750; doi:10.1523/JNEUROSCI.0052-07.2007

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Behavioral/Systems/Cognitive
Sensitivity to Interaural Time Differences in the Inferior Colliculus with Bilateral Cochlear Implants

Zachary M. Smith^1,2 and Bertrand Delgutte^1,2,3

¹Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, ²Speech and Hearing Bioscience and Technology Program, Harvard–Massachusetts Institute of Technology (MIT) Division of Health Sciences and Technology, Cambridge, Massachusetts 02139, and ³Research Laboratory of Electronics, MIT, Cambridge, Massachusetts 02139

Correspondence should be addressed to Dr. Zachary M. Smith, Department of Biomedical Engineering, Johns Hopkins University, 505 Traylor Building, 720 Rutland Avenue, Baltimore, MD 21205. Email: zsmith{at}alum.mit.edu

Bilateral cochlear implantation attempts to increase performance over a monaural prosthesis by harnessing the binaural processing of the auditory system. Although many bilaterally implanted human subjects discriminate interaural time differences (ITDs), a major cue for sound localization and signal detection in noise, their performance is typically poorer than that of normal-hearing listeners. We developed an animal model of bilateral cochlear implantation to study neural ITD sensitivity for trains of electric current pulses delivered via bilaterally implanted intracochlear electrodes. We found that a majority of single units in the inferior colliculus of acutely deafened, anesthetized cats are sensitive to ITD and that electric ITD tuning is as sharp as found for acoustic stimulation with broadband noise in normal-hearing animals. However, the sharpness and shape of ITD tuning often depended strongly on stimulus intensity; some neurons had dynamic ranges of ITD sensitivity as low as 1 dB. We also found that neural ITD sensitivity was best at pulse rates below 100 Hz and decreased with increasing pulse rate. This rate limitation parallels behavioral ITD discrimination in bilaterally implanted individuals. The sharp neural ITD sensitivity found with electric stimulation at the appropriate intensity is encouraging for the prospect of restoring the functional benefits of binaural hearing in bilaterally implanted human subjects and suggests that neural plasticity resulting from previous deafness and deprivation of binaural experience may play a role in the poor ITD discrimination with current bilateral implants.

Key words: binaural hearing; electric stimulation; neural prosthesis; cochlear implant; inferior colliculus; ITD

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Received Jan. 5, 2007; revised April 19, 2007; accepted May 4, 2007.

Correspondence should be addressed to Dr. Zachary M. Smith, Department of Biomedical Engineering, Johns Hopkins University, 505 Traylor Building, 720 Rutland Avenue, Baltimore, MD 21205. Email: zsmith{at}alum.mit.edu

This article has been cited by other articles:

				Z. M. Smith and B. Delgutte Sensitivity of Inferior Colliculus Neurons to Interaural Time Differences in the Envelope Versus the Fine Structure With Bilateral Cochlear Implants J Neurophysiol, May 1, 2008; 99(5): 2390 - 2407. [Abstract] [Full Text] [PDF]

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