Optically pumped magnetometers for a brain-computer interface based on event-related desynchronization
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2022-10-07
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en
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Abstract
Background Stroke is one of the leading causes of disability worldwide and often responsible for
impairments of hand motor function. Rehabilitation and restoration of motor functions can be
significantly improved using devices controlled by brain signals, so called brain-computer interfaces
(BCIs). Most current BCI systems are based on electroencephalography (EEG), which provides
only limited spatial resolution and thus limited versatility of control commands. Compared to
EEG, optically pumped magnetometers (OPMs) measure cortical magnetic fields without contact
to the scalp and provide a higher spatial resolution and bandwidth. In contrast to superconducting
quantum interference device (SQUID)-based magnetoencephalography (MEG), OPMs have low
maintenance cost and allow movement in the scanner, making them more applicable in clinical
contexts.
Methods We quantify the signal characteristics of a commercially available OPM system
(FieldLine Inc., USA) in terms of noise floor, dynamic range and bandwidth to verify its suitability
for cortical measurements. We then develop an experiment contrasting resting and right hand
grasping imagery to measure modulations of the sensorimotor rhythm (SMR) with 17 OPMs over
the left motor cortex of 18 healthy participants. The BCI capabilities of the OPM acquisition
system are evaluated with a modular near real-time classification pipeline, which provides visual
feedback to the user.
Results The sensor characterization revealed a system noise floor of about 27 fT/
√
Hz at 10 Hz,
a bandwidth of 400 Hz and a dynamic range of ±15 nT, fulfilling the minimum requirements for
cortical measurements. In 10 of 16 eligible participants, a difference in SMR power between resting
and grasping condition could be identified. We show that OPMs are suitable to measure SMR
modulations in near real-time and that the classification performance of our pipeline significantly
exceeds chance level.
Discussion OPMs allow for the online quantification of voluntary modulations of the sensorimotor
rhythm on single-trial basis, a central requirement for many BCI systems used in the rehabilitation
of stroke survivors. With their higher spatial resolution compared to EEG, OPMs could be used
for more complex classification paradigms and ultimately facilitate a development towards more
versatile BCI applications. The increasing availability and sensitivity of commercialized OPM
systems allows for the exploration of MEG in new research areas. OPMs are projected to become
an important tool in the field of cognitive neuroscience within the next few years.
Keywords: brain-computer interface, optically pumped
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Faculteit der Sociale Wetenschappen