Where do all the pulses go? Some limitations of high-rate stimulation in Cochlear Implant and Auditory Brainstem Implant Patients Event

Speaker: Dr. Bob Carlyon, Deputy Director of the MRC CBU

For cochlear implant (CI) users, thresholds and comfortable loudness levels drop as the rate of electrical stimulation on each channel is increased up to several thousand pulses per second (pps). However, for most patients, pitch increases with increases in pulse rate only up to about 300 pps. This limitation is likely to hamper attempts by CI companies to improve pitch perception. To investigate the neural basis of this limitation, we have obtained electrically evoked compounad action potentials (ECAPs) and behavioural responses with the same subjects and stimuli. The results indicate that the primary limitation is likely to arise at sites central to the auditory nerve.

For Auditory Brainstem Implant (ABI) users, increases in pulse rate beyond about 200-300 pps produce no, or very small, changes in threshold or loudness. This is very different to the large effects observed at high rates for CI users. We have performed psychophysical experiments showing that this difference is mediated by a mechanism having a very short time constant. We also show that ABI users are, like CI users, preferentially sensitive to anodic (as opposed to cathodic) current. We propose that it may be possible to exploit this finding to provide greater control over place of excitation in ABI users.