2.6 Data Processing
Data were processed offline using in-house MATLAB scripts leveraging
EEGlab functions (Delorme and Makeig, 2004). Data were high-pass
filtered at 0.1 Hz (order: 16,896; cutoff frequency: 0.05 Hz; transition
band width: 0.1 Hz) and low-pass filtered at 45 Hz (order: 153; cutoff
frequency: 50.625; transition band width: 11.25 Hz) using a finite
impulse response filter implemented through EEGlab (pop_eegfiltnew).
Bad channels were detected automatically based on joint-probability and
kurtosis. A channel was defined as “bad” when recorded data from that
electrode exceeded 5 SD from all other electrodes. For eye artifact
removal, Independent Component Analysis (ICA) was applied to continuous
data high-pass filtered at 1 Hz to identify components corresponding to
eye movements and blinks. Based on output from an ICA automated
component classifier (ICLabel) (Pion-Tonachini et al., 2019), components
with > 80% probability of originating from eye movements
and a < 5% probability of origination from a brain source
were rejected.
Data were re-referenced offline to the common average. EEG
response-locked epochs were constructed from -350 to +300 ms surrounding
the button press. A baseline was defined from -350 to -300 ms prior to
the button press. An automatic artifact rejection criterion of ±150
microvolts was used to remove trials containing excessive muscular
activity and eyeblinks. An additional artifact rejection threshold was
calculated based on an array of maximum amplitudes for each trial (the
largest absolute value recorded in a given epoch across all channels).
Epochs containing values > 3 SD from the median of this
array of maximum values were removed. Trials in which participants
responded earlier than 150 ms after the cue and trials in which
participants responded greater than 1000 ms following the cue were
excluded. For TD participants, an average of 4.48% of trials met
criteria for removal based on this response profile, while for ASD
participants, 7.74% of trials met these criteria (t(166) = -3.963, p
< 0.001). In addition, trials with RTs > 3 SD
faster or slower than the mean RT of all remaining trials for that
individual were excluded from ERP waveforms.