Stochastic Model Shows How Cochlear Implants Process Azimuth in Real Auditory Space

Abstract

Interaural time difference (ITD) is a major cue for sound azimuth localization at lower sound frequencies. We review two theories of how the sound localization neural circuit works. One of them proposes labeling of sound direction in the array of delay lines by maximal response of the tuning curve (Jeffress model). The other proposes detection of the direction by calculating the maximum slope of tuning curves. We formulate a simple hypothesis from this that stochastic neural response infers sound direction from this maximum slope, which supports the second theory. We calculate the output spike time density used in the readout of sound direction analytically. We show that the numerical implementation of the model yields results similar to those observed in experiments in mammals. We then go one step further and show that our model also gives similar results when a detailed implementation of the cochlear implant processor and simulation of implant to auditory nerve transduction are used, instead of the simplified model of auditory nerve input. Our results are useful in explaining some recent puzzling observations on the binaural cochlear implantees.