Project C-T2 - Coding of interaural time differences in electrically evoked responses

Thomas Stark (TUM), Werner Hemmert (TUM), Benedikt Grothe (LMU)

Bilateral cochlear implants (CI) offer patients the benefits of the binaural auditory system in everyday life. Thus the number of patients with bilateral cochlear implants (CI) is steadily increasing. However the use of interaul time delays (ITDs) is still severely limited in patients with bilateral CIs. ITD is one of the most crucial binaural cues for sound localization and speech comprehension.
Important progress in animal models of bilateral CIs has been achieved. These models illustrate neuronal sensitivity to binaural electrical stimulation and the role of acoustic deprivation for ITD discrimination with CIs. Though the reason for the deficit of bilateral CIs in ITD processing remains unknown. For instance, the use of a binaural jitter enhances the ITD sensitivity and increasing stimulation rates decreases the ITD sensitivity. Besides, the neuronal mechanism and the cue sensitivity underlying the processing of interaural time delays is so far not completely understood and still evolving.

Objective and description of the project

We will investigate under which conditions the decoding of ITDs for electrical hearing is possible. Extracellular recordings from responses of auditory midbrain neurons (IC) will be obtained. In particular, neuronal responses to binaural electrical stimulation with microsecond interaural time differences will be recorded and compared to acoustically evoked responses in normal hearing creatures.

Models will help us to understand why ITD sensivity degrades with increasing puls-rates of electrical stimulation and they will help us to find coding strategies to optimize ITD sensivity. Furthermore the modeling studies will systematically investigate the mechanism of ITD processing and differences between acoustic and electric ITD sensitivity. Predictions deriving from these models will contribute to our physiological recordings and will be experimentally tested. Our findings will be crucial in the development of a new generation of bilateral CIs allowing the use of ITD cues by the listeners.

References

Smith ZM, Delgutte B. (2007) Sensitivity to interaural time differences in the inferior colliculus with bilateral cochlear implants. J Neurosci, 27(25): 6740-50.
Smith ZM, Delgutte B. (2008) Sensitivity of inferior colliculus neurons to interaural time differences in the envelope versus the fine structure with bilateral cochlear implants. J Neurophysiol, (5): 2390-407.
Hancock KE, Noel V, Ryugo DK, Delgutte B. (2010) Neural coding of interaural time differences with bilateral cochlear implants: effects of congenital deafness. J Neurosci, 30(42): 14068-79.
Laback B, Majdak P. (2008) Binaural jitter improves interaural time-difference sensitivity of cochlear implantees at high pulse rates. Proc Natl Acad Sci U S A, 105(2): 814-7.
Grothe B, Pecka M, McAlpine D. (2010) Mechanisms of sound localization in mammals. Physiol Rev, 90(3): 983-1012.