Auditory Steady-State Responses in Cochlear Implant Recipients

Background: Auditory steady state responses (ASSRs) are a direct key to the phase-locking ability of neurons on the auditory pathway. The ASSRs are recorded in the EEG and the stimuli used for ASSRs can be a model for the speech envelope. The modulations in the speech envelope can be well encoded in cochlear implants (CI) and thus of particular relevance.

Objective: ASSRs open a window to auditory temporal processing from brainstem to cortex, depending on the modulation frequencies used. A few recent studies will be highlighted from neurosciences to applications in CI.

Methods: ASSR measurements reported here are recorded in post-lingually deaf CI users using a 64-channel EEG set-up. Stimuli are amplitude modulated pulse trains with modulation frequencies between 1-100 Hz. The electrical artifacts originating from the radio frequency transmission and from the stimulation pulses are a major drawback. Different advanced signal processing techniques are used to remove the artifacts from the EEG. The latter is a separate topic of research.

Results: EASSRs have increased our basic understanding of neuro-processing of speech-like signals, and lead as well to applications of clinical relevance. Automatic fitting of CI and development of improved electrical stimulation strategies are investigated using objective measures based on EASSRs. Recent results of EASSR-studies will be discussed: about the fitting of CI (automatic determination of objective stimulation thresholds, objective measure of loudness for matching purposes, …), the optimization of stimulation strategies (correlation of speech intelligibility in noise with the variability of EASSRs as a measure for modulation sensitivity across the electrode array), and basic temporal encoding in neural ensembles along the auditory pathway (functional connectivity and laterality of brain activity to electrical stimulation).

Conclusions: EASSRs are a reliable measure of the phase-locked neural activity from brainstem to auditory cortex, and quantify temporal processing in CI-users. EASSRs have the potential to become an important objective measure for research and clinical applications of electrical stimulation in cochlear implants.