Using interleaved stimulation to measure the size and selectivity of the sustained phaselocked neural response to cochlear-implant stimulation

Abstract

We measured the sustained neural response to electrical stimulation by a cochlear implant(CI). To do so we interleaved two stimuli with frequencies F1 and F2 Hz and recorded a neuraldistortion response (NDR) at F2-F1 Hz. We show that, because any one time-point containsonly the F1 or F2 stimulus, the instantaneous nonlinearities typical of electrical artefactshould not produce distortion at this frequency. However if the stimulus is smoothed, such asby charge integration at the nerve membrane, subsequent (neural) nonlinearities can producea component at F2-F1 Hz. We stimulated a single CI electrode with interleaved sinusoids orinterleaved amplitude-modulated pulse trains such that F2=1.5F1, and found no evidence foran NDR when F2-F1 was between 90-120 Hz. However, interleaved amplitude-modulated pulsetrains with F2-F1 40 Hz revealed a substantial NDR with a group delay of about 45 ms,consistent with a thalamic and/or cortical response. The NDR could be measured even fromrecording electrodes adjacent to the implant and at the highest pulse rates (>4000 pps) usedclinically. We then measured the selectivity of this sustained response by presenting F1 andF2 to different electrodes and at different between-electrode distances. This revealed abroad tuning that, we argue, reflects the overlap between the excitation elicited by the twoelectrodes. Our results also provide a glimpse of the neural nonlinearity in the auditorysystem, unobscured by the biomechanical cochlear nonlinearities that accompany acousticstimulation. Several potential clinical applications of our findings are discussed.