Transmission of Temporal Information Through the Auditory Pathway Measured with Cochlear-Implant Stimulation.

Abstract

Cochlear implants provide the sensation of hearing to deaf individuals through electric stimulation of the auditory nerve. Environmental sounds are band-pass filtered, and the envelope of each frequency band is used to modulate electric pulse trains that are presented through electrodes along the cochlea. Selection of cochlear-implant processing-strategy parameters influences how temporal information is transmitted to the listener. Increasing the rate of the pulsatile carrier has been hypothesized to improve temporal acuity, which is important for speech recognition. We used cochlear-implant stimulation to measure cortical coding of temporal features of the stimulus envelope. We tested the effect of pulse rates from 254-4069 pps on three measures of temporal acuity: gap detection, forward masking, and amplitude-modulation detection.

We observed that gap-detection thresholds decreased with increasing pulse rate in awake and anesthetized animals. We measured forward masking, the suppression of a probe sound by a preceding masker, and observed two exponential components of recovery from forward masking. Based on their time constants, we attributed the rapid component to the auditory nerve and the longer component, which was stronger at lower pulse rates, to a central auditory center. We concluded that shorter gap-detection thresholds at higher pulse rates resulted from reduced forward masking. In awake animals, we observed responses to a variety of gap features. OFF-responses to the pre-gap marker contributed to gap-detection thresholds at the lowest pulse rate, in conditions for which the ON-response to the post-gap marker was suppressed.

We measured the synchrony of cortical responses to 10.6- and 21.2-Hz sinusoidal amplitude modulation on one cochlear-implant electrode in the presence of a simultaneous, interleaved stimulus on another electrode. We hypothesized that higher pulse rates and shorter inter-pulse-train delays would maximize interference between the two stimuli. There was little effect of either parameter, however, for an unmodulated pulse-train masker. When the stimuli were modulated at different frequencies, we observed cortical representations that were dominated by the sum of the stimuli. Longer interleaving delays resulted in slightly more stimulus-selective responses.

These results reveal principles of temporal processing in the auditory pathway, and may guide development and selection of future cochlear-implant processing strategies.