For each gait condition (standing, slower walking, faster walking, running) and each
method of artifact removal (before artifact removal, after channel-based removal, after
further IC-based removal), we computed the power spectral density for each EEG
channel using Welch’s method. For illustrative purposes, we computed the power
spectral envelope for each subject, defined as the maximum and minimum spectral
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density at each frequency over all EEG channels. We analyzed spectral power in the 1.5
Hz to 8.5 Hz range to assess the efficacy of the gait-related artifact removal methods.
This frequency band was selected because: 1) in all gait conditions for all subjects it
encompassed the step frequency and the first two harmonics of the step frequency, 2)
its lower cutoff (1.5 Hz) was greater than the high-pass filter cutoff frequency (1 Hz) that
was applied to all EEG signals prior to analysis, and 3) frequencies above 8.5 Hz
accounted for less than 6% of the total spectral power in the artifact templates for all
gait conditions (slow walking, 6.2%; faster walking, 5.1%; running, 5.5%)(Figure 2-3).