gms | German Medical Science

Interaural time difference (ITD) discrimination is a major challenge for bilateral cochlear implant (biCI) patients, especially after early deafness. State of the art clinical CI processors employ asynchronous pulsatile stimulation in each ear, so that biCI users only experience ITD in pulse train envelopes (envITD) but not on the pulse-timing (ptITD). Previous work from our groups has shown that neonatally deafened (ND) rats with synchronized biCIs providing informative ptITD from the onset of stimulation can develop excellent ITD sensitivity [1] and are much more sensitive to ptITD than envITD in a sound lateralization task [2]. To characterize the neurophysiological sensitivity of the auditory system for ptITD compared to envITD, we used multi-channel silicon probes to record neurons from the auditory midbrain in three different cohorts: ND rats without hearing experience (n=9), normal hearing (NH) rats (n=6), and ND rats that were supplied with biCIs in young adulthood with at least two months of CI hearing experience and ITD sensitivity (n=7).

To investigate the neural sensitivity of the three cohorts to ptITD and envITD, we designed three different stimuli comprising pulse trains of either 900 or 4,500 pulses per second, which were modulated by either 0.01, 0.05 or 0.2 second long raised cosine windows (equivalent to one cycle of 100, 20 or 5 Hz sinusoidal amplitude modulation, respectively). The ptITD and the envITD could vary independently from the values [-0.1, 0, +0.1] ms. All stimuli were presented via biCIs, which were chronically implanted at least two months prior to the experiment in the ND biCI cohort with CI hearing experience and acutely implanted in the ND and NH cohorts immediately prior to the recording experiment.

For each recorded multi-unit, we first removed electrical stimulation artifacts through averaging. We then computed the analog multi-unit activity (AMUA) [3] and identified responsive units by comparing the AMUA during stimulation with baseline AMUA values. Finally, the proportion of variance in neural responses explained by ptITD and envITD was computed to quantify the effect of ptITD and envITD on AMUA intensity.

The variance explained (VE) values for ptITD and envITD, respectively, varied from multi-unit to multi-unit, but while VE by ptITD was often large and could reach values in excess of 80%, VE by envITD was typically very much smaller and only rarely exceeded values expected by chance. Consequently, this means that neurons in the auditory midbrain are more sensitive to ptITDs than to envITDs. This was the case in all cohorts, regardless of their previous hearing experience.

These findings underline the importance of a fine structure stimulation strategy in CI users and could thus demonstrate neuronally the importance of presenting spatial ITD information on the pulse timing of current CI devices in order to improve spatial hearing of biCI patients.