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1 Department of Biomedical Engineering, Boston University, Boston, MA, USA; Hearing Research Center, Boston University, Boston, MA, USA
2 Department of Biomedical Engineering, Boston University, Boston, MA, USA; Center for BioDynamics, Boston University, Boston, MA, USA
3 Departments of Biomedical & Chemical Engineering and Electrical Engineering & Computer Science, Institute for Sensory Research, Syracuse University, Syracuse, NY, USA
* To whom correspondence should be addressed. E-mail: vdasika1974{at}yahoo.com.
The avian auditory brainstem consists of a network of specialized nuclei, including nucleus laminaris (NL) and superior olivary nucleus (SON). NL cells show sensitivity to interaural-time-difference (ITD), a critical cue that underlies spatial hearing. SON cells provide inhibitory feedback to the rest of the network. Empirical data suggest that feedback inhibition from SON could increase the ITD sensitivity of NL across sound-level. Using a bilateral network model, we assess the effects of SON feedback inhibition. Individual cells are specified as modified leaky-integrate-and-fire neurons whose time constants and thresholds vary with inhibitory input. Acoustic sound-level is reflected in the discharge rates of the model auditory-nerve fibers which innervate the network. Simulations show that with SON inhibitory feedback, ITD sensitivity is maintained in model NL cells over a threefold range in auditory-nerve discharge rate. In contrast, without SON feedback inhibition, ITD sensitivity is significantly reduced as input rates are increased. Feedback inhibition is most beneficial in maintaining ITD sensitivity at high input rates (simulating high sound-levels). With SON inhibition, ITD sensitivity is maintained for both interaurally balanced inputs (simulating an on-center sound source) and interaurally imbalanced inputs (simulating a lateralized source). Further, the empirically observed temporal buildup of SON inhibition and the presence of reciprocal inhibitory connections between the ipsilateral and contralateral SON both improve ITD sensitivity. In sum, our network model shows that inhibitory feedback can substantially increase the sensitivity and dynamic range of ITD coding in the avian auditory brainstem.
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