brainstem: Implications for sound localization"
Dr. Vasant Dasika, Boston University Abstract: 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 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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