Instrumentation & Measurement Magazine 24-4 - 83

mechanisms. The most
commonly observed intervals
for each band in the
literature are delta: < 4 Hz;
theta: 4-7.5 Hz; alpha: 7.5-
12.5 Hz; beta: 12.5-30 Hz;
and gamma: 30-40 Hz. The
most relevant frequency
bands for MF-related studies
are theta, alpha and
beta.
Theta rhythm (θ) tends to
Fig. 1. MF assessment system applied to complex maritime operation.
Myriad types of tasks performed on board vessels could
benefit from such a monitoring system, which brings an
additional requirement: the MF assessment should be taskindependent.
These tasks may have specific needs regarding
where they take place, movement freedom, equipment and
people involved, duration, etc. This is especially relevant
when dealing with EEG sensors, since they are very sensitive
and can be affected by external signal noises and even body
movement. To be task agnostic, the MF assessment system
should be based on biomarkers that do not rely on resting state
EEG. This system should also operate in real-time to support
management decisions while the operation takes place. An
example of such a system is presented in Fig. 1, where a management
system receives information about the MF level of
different operators involved in a safety critical operation and
emits warning feedback when some MF threshold is exceeded.
In this paper, we propose the use of a portable EEG sensor
to assess maritime operators' MF state. The system has low intrusiveness,
and the analysis of EEG frequency domain data
can be carried out in real time. To make the system subject and
task-independent, we proposed a relative MF indicator based
on the operator's initial MF state. The proposed approach was
tested in a vessel simulator, for both multi-subject and multisubject
and multi-task scenarios.
EEG and Frequency Band Analysis
EEG is an electrophysiological sensor used to measure electrical
activity in the brain by means of electrodes placed along
the subject's scalp. EEG is applied in the study and diagnostics
of neurological conditions and has also been applied in
cognitive science studies. The most common approach to analyzing
EEG data is subdividing it according to broad frequency
bands, which mainly include delta, theta, alpha, beta, and
gamma rhythms. The patterns in brain activity in each one of
these bands are associated with different mental and cognitive
June 2021
increase with the process
of encoding information,
focused attention, and
with increasing task difficulty,
being commonly
associated with mental
workload. It is also present
during tiredness and early
signals of drowsiness. It
can be recorded across the cerebral cortex, especially the frontal
region, where theta activity is likely to increase as a person
fatigues [9].
Alpha rhythm (α) correlates with sensorial, motor, and memory
tasks. It is best observed during eyes closure and relaxation
periods and is reduced with eyes opened and during mental
or bodily efforts. Alpha blocking correlates well with mental
activity, engagement, perceptual awareness, and distraction.
Alpha activity, especially on the occipital and parietal regions
of the brain, tends to increase with fatigue [9].
Beta rhythm (β) is the most frequent rhythm in awake,
healthy adults and children. It plays a big role in sensorimotor
behavior during both planning and execution of movement.
Higher beta levels can be found in anxious, active, and concentrating
individuals and usually indicates higher arousal levels
[7]. The beta rhythm is more accentuated over the frontal and
central regions of the brain.
Material and Methods
To assess the performance of the proposed MF assessment algorithm,
we performed experiments in a vessel simulator. The
assessment algorithm and the experimental procedure are described
here.
Data Collection and Preprocessing
To ensure maximum movement flexibility for the participant,
we opted to use a wireless EEG headset sensor, the Emotiv
Epoc+, from Emotiv [11]. It is a consumer grade EEG device
and has a fast setup time and accessible price. Although not
a clinical grade device, its performance has been validated in
different contexts and has been widely used in the research
literature. The sensor has 14 felt pad electrodes positioned
according to the extend 10-20 EEG electrode placement standard.
The limited number of electrodes made it impossible to
have a fine coverage of the whole scalp, but their position was
IEEE Instrumentation & Measurement Magazine
83

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