Does inappropriate bilateral CI stimulation impair binaural hearing, and if so, can it be rehabilitated? – An animal study

Abstract

Prelingually deafened patients with bilateral cochlear implants (biCI) often have poor sensitivity to interaural time differences (ITD). The reason could be technical limitations as most clinical CI processors encode little or no temporal information in the timing of the electrical stimulus pulses. Here, we investigate whether uninformative pulse timing ITDs prevent the development of normal ITD sensitivity in biCI subjects, and, if so, whether training with informative ITDs rehabilitates it. Neonatally deafened biCI rats were trained on auditory discrimination designed to simulate the demands placed on CI patients in daily life. One cohort was stimulated with informative pulse ITDs, while a second cohort received stimulation with non-informative pulse ITDs. After five weeks of training, the animals were tested for ITD sensitivity and relative perceptual weighting. This was followed by a four-week rehabilitation phase and a final testing phase. As a result, we found that biCI rats used ITDs for lateralization when presented with informative ITDs immediately after CI implantation. In contrast, animals exposed to stimulation with random, uninformative ITDs showed a much-reduced ITD sensitivity. After rehabilitation training with informative pulse ITDs, these animals were able to use ITDs for lateralization. Overall, our results suggest that ITD sensitivity may decline or even be lost, if binaural CIs provide uninformative pulse ITDs. When presented with CI stimulation containing only useful, coherent ITDs, all rats achieved good ITD sensitivities at a clinical pulse rate.