2.4 EEG Time-Frequency analysis
EEG epochs underwent spectral decomposition using Morlet wavelets to
extract power data for frequencies from 4 to 30 Hz (with approximately 1
Hz intervals). Three cycles were set for the lowest frequency and cycles
increased linearly with frequency scaled at 0.5 using EEGLAB newtimef()
function (‘cycles’, [3 0.5]; Delorme and Makeig, 2004). Temporal
resolution was around 36-37 ms per time frame. Baseline correction
within each 1 Hz frequency interval was performed for each single trial
by subtracting the average power between -600 and -100 ms pre-stimulus
onset from each time point post-stimulus onset to calculate
event-related spectral perturbation (ERSP) using the gain model
(Grandchamp and Delorme, 2011). The power data were then logarithmically
converted to decibel (dB) for further statistical analysis. Increase in
power relative to baseline represents event-related synchronization
(ERS), while lower power relative to baseline represents event-related
desynchronization (ERD).