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Neuronal coding of interaural time differences in the long deaf auditory system: Effects of age at
deafness onset
Kempe M. , Wiegner A. , Vollmer M.
Univ.-HNO-Klinik Würzburg, Comprehensive Hearing Center Würzburg, Würzburg, Germany
Introduction: Interaural time differences (ITDs) are important cues for directional hearing and speech
understanding in noise. Precise ITD discrimination in the microsecond range is assumed to be dependent on
normal auditory experience during development. To explore the effects of severely distorted auditory experience
on neural ITD coding, the present study investigates the effects of age at deafness onset on neural ITD coding in
long deaf animals.
Methods: Mongolian gerbils were deafened either around hearing onset (P12, early-onset deafness) or as
adults (~P70, late-onset deafness). After prolonged deafness durations of ~8 weeks, animals were bilaterally
implanted with cochlear prostheses (CI), and single neuron responses to electric ITDs were recorded in the
dorsal nucleus of the lateral lemniscus and in the inferior colliculus. Adult, acutely deafened gerbils with prior
normal auditory experience served as controls.
Results: The incidence of ITD-sensitive neurons in the two long-deafened groups was similar to that in normal
hearing controls. Independent of age at deafness onset, long-term deafness resulted in a greater variability of all
ITD parameters tested (ITD at maximum spike rate, best ITD; ITD at maximum slope, ITDms; physiological
modulation depth, PMD; half width; half rise). In contrast to the narrow distribution of best ITDs that are biased
towards contralateral-leading in normal hearing animals, both deafened groups had broad distributions of best
ITDs around the midline. These results were paralleled by a lower incidence of ITDms, thus a reduced sensitivity
to changes in ITDs, within the physiological range. Moreover, both deafened groups demonstrated a reduced
sharpness in ITD tuning (broader halfwidth and halfrise), lower PMD, and a reduced neuronal ITD discrimination
ability. When compared across the two deafened groups, neurons in juvenile deafened animals revealed
significantly poorer ITD sensitivities (ITD signal to total variance ratio) than those in adult deafened animals.
Conclusion: Results indicate that prolonged periods of both early- and late-onset deafness cause various
degradations in neuronal ITD coding. These deficits may be physiological correlates for the poor ITD
discrimination performance in human bilateral CI users. Moreover, the findings suggest the importance of
temporally correlated binaural hearing experience early in life in order to optimally benefit from binaural
Supported by Deutsche Forschungsgemeinschaft PP 1608, VO 640/2-1, and MED-EL.