Auditory Model-Based Sound Direction Estimation With Bilateral Cochlear Implants

Users of bilateral cochlear implants (CIs) show above-chance performance in localizing the source of a sound in the azimuthal (horizontal) plane; although localization errors are far worse than for normal-hearing listeners, they are considerably better than for CI listeners with only one implant. In most previous studies, subjects had access to interaural level differences and to interaural time differences conveyed in the temporal envelope. Here, we present a binaural model that predicts the azimuthal direction of sound arrival from a two-channel input signal as it is received at the left and right CI processor. The model includes a replication of a clinical speech-coding strategy, a model of the electrode-nerve interface and binaural brainstem neurons, and three different prediction stages that are trained to map the neural response rate to an azimuthal angle. The model is trained and tested with various noise and speech stimuli created by means of virtual acoustics. Localization error patterns of the model match experimental data and are explicable largely in terms of the nonmonotonic relationship between interaural level difference and azimuthal angle.