Conference on Implantable Auditory Prostheses (CIAP) location:Granlibakken Conference Center, Lake Tahoe, California, USA date:12-17 July 2009
Steady state potentials evoked by periodic acoustic stimuli like AM/FM-modulated tones, beats or low-rate click trains can be recorded with scalp electrodes. Up to now such responses have not been recorded in cochlear implant (CI) users.
The aims of this study were: (1) to show that auditory steady state potentials to electrical pulse trains (EASSRs) can be reliably recorded by electrodes placed on the scalp of a CI user, (2) to develop a measurement setup to facilitate stimulation and reliable recordings of such responses and (3) to analyze the predictive value of thresholds obtained using EASSRs for behaviorally hearing thresholds.
For six users of a Nucleus cochlear implant, EASSRs to symmetric biphasic pulse trains with rates between 35 and 44 Hz were recorded with seven scalp electrodes. The influence of various stimulus parameters was characterized: pulse rate, stimulus intensity, monopolar or bipolar stimulation mode and presentation of either one pulse train on one electrode or interleaved pulse trains on multiple electrodes. To compensate for the influence of the artifacts introduced by the stimulation pulses and RF transmission, different methods of artifact reduction were evaluated. The validity of the recorded responses was confirmed by (1) recording on/off responses, (2) determination of apparent latency across the measured pulse rates, and (3) comparison of amplitude growth of stimulus artifact and response amplitude.
The results show that EASSRs in CI users evoked by low-rate electrical pulse trains can be successfully recorded and separated from the artifacts generated by the stimulation pulses and RF transmission. The obtained response amplitudes and apparent latencies are comparable to the ones obtained in acoustically stimulated normal hearing patients. Hearing thresholds determined from EASSR amplitude growth correlate well with behaviorally determined threshold levels.