FIGURE 1 Feature extraction pipeline. The four-wavelength fNIRS
data is collected from the subjects and several pre-processing methods
are applied before extracting the features.
Total of 209 features were
extracted from fNIRS and fNIRS derived chromophore relative
concentration signals [20]. The differential path length factor
(DPF) used in MBBL was 5.97 and the extinction coefficients were based
on the work of Cope [26]. The summary of the features is shown in
Table 1. Some of the features have been described in the latest fNIRS,
EEG and fMRI NDD studies while some are common features, such as signal
standard deviation and gender of the subject. The frequency bandwidths
of respiratory (0.1 - 0.6 Hz), cardiac (0.6 - 5 Hz) and full (0.008 - 5
Hz) were utilized when extracting fNIRS features. In addition to noise
attenuation, the bandwidth specific features are assumed to contain
information related to different physiological events and could contain
more predictive power in the brain aging assessment.
Spectral entropy, fractional amplitude of physiological fluctuations
(fAPF) and relative power derived features have been used in fNIRS and
EEG studies related to aging and AD research [18,27–29].
Furthermore, pulse shape derived from fNIRS has been used to distinguish
AD subjects from healthy controls [11]. Thus, the features were
selected as they are assumed to have potential predictive power in the
age group classification task. In addition, the features of differential
phase transfer entropy (dPhaseTE) [30,31], sample entropy (sampEn)
[32,33], and correlation between HbT and H2O
[34] were utilized. Unique feature of envelope-signal spectra
correlation (ESSC) was also used. The feature is calculated by computing
the correlation between the power spectral density (PSD) of the signal
and its envelope. The feature describes the relationship between the
spectral power of the original signal and its low frequency component.
TABLE 1 List of all features extracted from the fNIRS and
derived relative chromophore concentration signals used in the
study.