EEG-based and psychophysical test methods for characterizing the impact of electrode interactions on the auditory processing of Cochlear Implant users

Cochlear implant (CI) users display a considerable and largely unexplained
variability in speech recognition: some users reach more than 80% word
recognition scores, for others, scores are disappointingly low (e.g., 50% or
less). The variability is even increased in noisy environments. Possible
sources of this variability are multifold. These can be device-related, for
example, differences in the precise locations of CI electrodes in the cochlea,
and interference between electrodes because of overlapping auditory nerve
populations, together with suboptimal individual settings of the sound
processor. Alternatively, or in addition, the cause of variability may be
subject-related, for example degeneration of the auditory nerve, deficiencies
in the more central auditory processing, or cognitive factors. Spectro-temporal
resolution seems to be an important factor in speech recognition. We
investigate two new methods to measure the spectro-temporal resolution,
including * in a controlled way - the impact of CI electrode interactions on
the processing of temporal and spectral information. Two CI electrodes will be
stimulated simultaneously employing basic pulse patterns. The response of the
auditory system to this known input is then measured either with EEG, without
any active involvement of the subject, or with a simple, but information-rich,
reaction-time task, designed to largely eliminate cognitive factors.

The main objective of the study is to better explain disappointing speech
recognition in individual cases, with as ultimate goal to find optimal
individual settings for the sound processor (outside the scope of this
project). We test two new methods to measure spectro-temporal resolution on
electrode-level, excluding influences of the sound processor and largely
excluding cognitive influences. Results will be compared to results with a more
conventional spectro-temporal test including the sound processor, and with
speech recognition tests.
The secondary objective is to evaluate an App for obtaining data. Being able to
perform a part of the measurements at home is now more relevant than ever.
Further we aim to contribute to disentangling the impact of the above-mentioned
sources of variability on speech recognition.

This is an explorative study with between-subject and within-subject
comparisons. Outcomes will be compared with speech recognition results and with
standard objective measures as electrode impedance and electrically evoked
compound action potentials (eCAPs), an indication of the response of the
auditory nerve.

Full participation involves three visits to Nijmegen, each with a duration of 3
hours. The third visit is only necessary for the participants in the EEG
measurements (10 of the 16 subjects). In addition, all subjects will be asked
to perform measurements at home using an App, this will in total take about 10
hours. Measurements are non-invasive and have negligible risks. EEG devices
employed in this study are CE approved. The research interface used for the CI
stimulation in a part of the measurements, although not CE approved, guarantees
electrical current levels below physiological safety limits, and has low risk,
as described in the attached investigational medical device dossier. There will
be no direct benefit for the participants.