Sounds reach the two ears at slightly different times, creating an interaural time difference (ITD) cue to the location of the sound source. We propose to use extracellular and intracellular techniques to record from ITD sensitive neurons in the medial superior olive (MSO) of Mongolian gerbils. MSO principal cells function as coincidence detectors of binaural excitatory inputs, and as a result show ITD sensitivity in their firing rates. The proposed research focuses on two mechanisms that may tune different MSO neurons to different ITDs. First, stereausis exploits the dependence of the cochlear traveling wave delay on characteristic frequency (CF) to create ITD tuning by systematically mismatching the CFs of the binaural inputs to the MSO. We will test the stereausis model by quantitatively evaluating the extent to which neural best ITD is correlated with the CF difference between the ears. Second, inhibitory inputs may affect ITD coding in the MSO by partially canceling excitatory inputs, by causing the neural membrane to integrate excitatory events more rapidly, and/or by preventing the coincidence detection mechanism from saturating at high sound levels. Inhibitory synaptic potentials have been measured in a few studies using slice preparations, but never in vivo. We will make intracellular recordings to characterize the effects of acoustically-evoked inhibition on MSO neurons in the gerbil. The ITD code is useful not just as a cue to sound source location, but also for extracting signals from background noise. A familiar example is the """"""""cocktail party effect,"""""""" in which speech from one speaker can be understood easily despite the presence of many other speakers. Comprehending speech in such environments is particularly troublesome for hearing impaired listeners. A more complete understanding of how the auditory system encodes ITD may help improve hearing aids or signal processing strategies for cochlear implants and thus improve the experience of hearing impaired listeners in challenging acoustic environments.
| Hancock, Kenneth E; Delgutte, Bertrand (2004) A physiologically based model of interaural time difference discrimination. J Neurosci 24:7110-7 |
