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Interaural time difference sensitivity in a new animal model of bilateral cochlear implant usersThe work leading to this publication was supported by the German Academic Exchange Service (DAAD) with funds from the German Federal Ministry of Education and Research (BMBF) and the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007 – 2013) under REA grant agreement n° 605728 (P.R.I.M.E. – Postdoctoral Researchers International Mobility Experience).
18 April 2018 (online)
Sound localization is one of the major challenges for bilateral cochlear implant (CI) users. To derive maximum benefit from two CIs, further research is needed in animal models to understand how parameters such as interaural synchronization influence the binaural processing in central auditory hubs of CI patients.
We present the neonatally deafened rat as a new model to investigate binaural hearing under electrical intracochlear stimulation. Rats received bilateral CIs in young adulthood, and were either trained in a self-developed setup for interaural time difference (ITD) based sound localization tasks, or they were prepared for acute recordings of ITD tuning from neurons in their inferior colliculus (IC), a major hub for sound localization.
For both methods ITDs varied over the rat's physiological range, from 0.16 ms left ear leading to 0.16 ms right ear leading. In this manner, electrical ITD tuning curves were recorded for 898 multi-units in the IC of 4 deaf rats fitted with bilateral CIs. All units were driven by the electrical stimuli, and the majority (87.6%) of the units were significantly sensitive to ITDs and mostly tuned to contralateral or central locations. In a behavioral two-alternative forced-choice ITD discrimination task the CI-fitted rats learned quickly to localize sounds by ITD. On average their probability of a correct choice increased up to 90% per 100µs of ITD, a behavioral sensitivity similar with that seen in normally hearing rats.
We have demonstrated that deaf rats can use ITD to localize sound sources with electric auditory stimulation through bilateral CIs. Overall, our results have shown that it should in principle be possible to incorporate ITD coding into future CI designs, and that rats are a good model to pioneer this research area.