Coding of temporally fluctuating interaural timing disparities in a binaural processing model based on phase differences.

A model of the effective processing of interaural timing disparities in the human auditory system is presented which provides modifications and extensions to existing models motivated by recent physiological findings. In particular, an established model of excitatory-inhibitory (EI) neuronal connectivity is complemented by a model that is based on a rate code derived from the interaural phase difference (IPD). The IPD model is shown to successfully simulate literature data on fine structure and envelope-based binaural detection and lateralization experiments. In order to investigate the processing of temporal fluctuations of interaural timing disparities, detection thresholds of broadband binaural-beat stimuli were measured in six normal-hearing listeners and were compared with model simulations. In a first experiment, the highest detectable beat frequency was found to be 96 Hz for a noise bandwidth of 550 Hz and 219 Hz for a bandwidth of 1100 Hz. Both models predicted lower thresholds, but performed increasingly better when the integration time constants of the binaural processors were reduced. In a second experiment, the signal-to-noise ratio at the detection threshold of binaural-beat stimuli mixed with interaurally uncorrelated noise was measured as a function of the beat frequency. The threshold increased about 1.7 dB per octave which was simulated similarly by both models. The results indicate that the primary temporal resolution of the binaural system for detecting interaural timing disparities is much higher than the temporal resolution found in higher auditory processes as supposedly involved in, e.g., masking.