A novel MEG system has been customized to measure brain activity in cochlear implant recipients. The system uses two groups of irregular gradiometers to reduce electromagnetic interference from cochlear implants. The system will measure speech processing in the brain by analyzing the coherence between brain activity and the temporal envelope of speech. Auditory cortical responses were measured in normal hearing participants using a cochlear implant simulator to evaluate the system's ability to suppress interference from implants.

1. MEG system for Cochlear Implant Recipients and Auditory Entrainment
Qingqing Meng1,3,4, Catherine McMahon2,3,4 and Blake Johnson1,3,4
1 Macquarie University, Department of Cognitive Science, 2 Macquarie University, Department of Linguistics
3 The HEARing Cooperative Research Centre 4 ARC Centre of Excellence in Cognition and its Disorders
Introduction
A novel MEG system, customized to measure brain function of
cochlear implant recipients is in a second iteration of its design
phase. Two groups of irregular baseline 2nd order gradiometers have
been installed to reduce the effects of electromagnetic artefacts
produced by the cochlear implant device.
Speech entrainment, characterized by oscillatory activities in
auditory cortex (theta band) phase-locked to slow varying speech
temporal envelope, has been proposed to index speech processing
[1] and may underlie speech perception performance of cochlear
implant recipients.
Here, we review the MEG design improvements and present
methods for evaluating amplitude envelope entrainment as a
measure of speech processing in the cortex of cochlear implant
recipients.
creating sound value www.hearingcrc.org
Upgraded MEG System
Type A sensor: Measures environmental noise, has null or near-
null responsiveness at locations corresponding to auditory cortex.
Type B sensor: Measures auditory cortical activity, has null or
near-null responsiveness at locations corresponding to the cochlear
implant.
Conclusions
The custom-engineered MEG system, in combination with noise
reduction techniques, will be used to measure speech processing in
the brains of cochlear implant recipients.
Speech processing will be indexed by spectral coherence with the
temporal envelope of the speech signal.
Methods for measuring speech processing in
the brain
Speech entrainment is indexed by the spectral coherence
between brain recordings and the temporal envelope of speech [3].
EEG coherence with 6 Hz amplitude modulated white noise is
shown. 12 type A and 20 type B irregular
baseline 2nd order gradiometers are
contained in a cylindrical dewar. It
covers one side of a participant’s
head contralateral to a (unilateral)
cochlear implant.
MEG System Evaluation
Auditory cortical activity was acquired with normal hearing
participants and a cochlear implant simulator.
Result with CALM noise reduction technique [2] reveals capability
of suppressing interference introduced by cochlear implant.
References
[1] Nai Ding and Jonathan Z Simon. Cortical entrainment to continuous speech:
functional roles and interpretations. Front Human Neurosci, 8, 2014.
[2] Y Adachi, M Shimogawara, M Higuchi, Y Haruta, and M Ochiai. Reduction of
non-periodic environmental magnetic noise in meg measurement by continuously
adjusted least squares method. IEEE Trans Appl Supercond, 11(1):669-672, 2001.
[3] Jonathan E Peelle, Joachim Gross, and Matthew H Davis. Phase-locked
responses to speech in human auditory cortex are enhanced during comprehension.
Cerebral Cortex, 23:1378-1387, 2013.