2.3 EEG data acquisition and preprocessing
EEG was continuously recorded from a 64-electrode EEG cap (Neuroscan
Quikcap) via a Neuroscan SynAmps2 amplifier using Scan 4.5 software
(Compumedics Neuroscan, USA; sampling rate: 1 kHz, DC-200 Hz) while
subjects performed the SORT task. The reference electrode was placed
between Cz and CPz at the midline. A bipolar vertical electrooculogram
(VEOG) was recorded for the left eye. EEG data from electrodes with
impedance exceeding 10–20 kΩ were discarded from further processing and
most impedances were less than 5–10 kΩ. Poorly functioning electrodes
were excluded manually by visual inspection of the raw data. Data from
fewer than 5% of electrodes were rejected; the number of rejected
electrodes did not differ significantly across groups (YA vs HA,t (49) = 0.15, p = 0.881; HA vs MCI, t (40) = 0.44,p = 0.663). An algorithm computing the average based on spherical
splines fitted to the data was then applied to interpolate EEG data to
the sites of the bad electrodes identified (Ferree et al., 2009).
The continuous EEG data were high-pass filtered at 1 Hz and subsequently
low-pass filtered at 40 Hz using a finite impulse response filter.
Independent component analysis (ICA) was then applied to the filtered
continuous EEG data to identify artifacts (muscle, eye, and heart) using
the EEGLAB toolbox (Delorme and Makeig, 2004) running under MATLAB 2021
(MathWorks, Natick, MA, USA). Components with >70%
probability of representing an artifact were automatically removed
(ICLabel, Pion-Tonachini et al., 2019). Subsequently, ICA components of
each individual’s data were visually examined and artifacts not
identified previously by the algorithm were removed manually to complete
data cleaning. After this step, EEG data were segmented for retrieval
and non-retrieval conditions into multiple EEG epochs (-1000 to 3000 ms,
time-locked to the stimulus onset). Epochs having a peak amplitude of
±75 µV between -500ms and 2000 ms post stimulus onset (highly associated
with artifacts) were rejected and epochs with extreme values (based on
standard deviation > 5) were excluded by rejection
algorithms in EEGLAB using joint probability and kurtosis of activity.
The average (accepted trials / all correct trials) epochs retained for
retrieval and non-retrieval conditions respectively were 88.3 ± 5.8 %
(42.3 ± 4.1 trials) and 89 ± 6.6 % (42.7 ± 5.2 trials) in the YA group,
91 ± 5.3 %(44.7 ± 4.2 trials) and 91.9 ± 4.1 % (44.8 ± 4.4 trials) in
the HA group, and 88.6 ± 5.8% (39.3 ± 4 trials) and 88.3 7.5 ±% (40.3
± 7.6 trials) in the MCI group.