Page 145 - Handbook of Biomechatronics

142 Domen Novak

1.4 Combining Multiple Sensor Types
The different BCI signal modalities (EEG, ECoG, and fNIRS) can also be
combined with each other or with other signals (not originating in the brain)
in order to improve BCI performance. Such approaches are called hybrid
BCIs, and have been reviewed in detail in a recent paper by Hong and
Khan (2017); a few representative examples are provided in the following
sections.

1.4.1 EEG and fNIRS
EEG offers a rapid response to stimuli but poor spatial resolution; conversely,
fNIRS offers poor temporal resolution but good spatial resolution. Thus,
combining them has the potential to harness the advantages of each modality
and increase overall BCI performance. One of the first studies on this topic
indeed showed that simultaneously recording both EEG and fNIRS during
motor imagery allows better classification of different motor images (left vs
right arm) than using either modality alone (Fazli et al., 2012). As such clas-
sification of motor imagery requires both EEG and fNIRS sensors to be
placed over roughly the same area of the brain (motor cortex), it necessitates
the use of specialized devices designed to measure both modalities
simultaneously.
As an alternative to measuring both EEG and fNIRS from the same part
of the brain (e.g., the motor cortex), it is possible to use different paradigms
for each modality and thus measure each signal from a different region. For
example, a user can send one type of command by performing mental arith-
metic (which is monitored at the prefrontal lobe using fNIRS) and send
another by imagining left or right-hand movements (which are monitored
at the motor cortex using EEG) (Khan et al., 2014). While this does not nec-
essarily increase the speed with which the user must send commands (since it
can be difficult to simultaneously perform mental arithmetic and imagine
hand movements), it can increase overall BCI accuracy by making it easier
to differentiate between different types of commands.

1.4.2 EEG and EOG
The EOG measures the electrical activity generated by the eyes using elec-
trodes placed to the left/right as well as above/below the eyes. This results in
two different EOG channels, of which one is proportional to the vertical
angle while the other is proportional to the horizontal angle of the eyes.