Electrophysiological and Psychophysical Measures of Amplitude Modulation Discrimination in Cochlear Implant Users

Cochlear implants (CIs) work by dividing the incoming acoustic signal into a limited number of frequency channels, extracting the slowly varying amplitude envelope in each channel and using this to modulate the level of electrical pulses delivered to the auditory nerve fibres. The amplitude modulation (AM) cues that are transmitted are crucial for speech understanding and an individual’s abilities to perceive different rates of AM is related to speech perception. The use of these cues can be affected by interference from adjacent channels caused by current spread which is further exacerbated if traumatic electrode placement or poor neural survival occurs. We have tested eleven adult CI users with a psychophysical measure of across-channel modulation interference (AMCI), to determine discrimination of different rates of AM in the presence of interferers and an electrophysiological correlate, the electrically-evoked auditory change complex (eACC).The eACC is a cortical potential in response to a change in an ongoing stimulus,i.e. AM rate change. Ability to discriminate AM rate (based on AMCI and eACC) is related to speech perception. If eACC to AM rate changes can be developed it can be used with children and also to identify poorly discriminated CI electrodes, to guide re-mapping.