It should be noted that in freezing trials, it was not the case that at all time points the subject was under freezing. Instead, a freezing trial contained both the episodes where the subject was freezing and episodes where the subject was walking stably. Thus, in each freezing trial, we extracted a continuous 5 s non-freezing-episode with the lowest average FI and termed it as the freezing trials’ non-freezing episode (FN), which best represented a period of clear, rhythmic walking in a freezing trial. Moreover, the freezing episode, where FI exceeded three, in a freezing trial was extracted and termed as the freezing trials’ freezing episode (FF). For non-freezing trials, a continuous 5 s episode with the lowest average FI was also extracted and termed as the non-freezing trials’ normal walking episode (NN), served as a control. The schematic diagram of the episode extraction is shown in Fig. 3A and B. Episodes with the same type extracted from trials in the same stimulation condition were concatenated for each subject. A 10 s sliding window with a 1 s step size
