A, Example data of two responders (P02 and P03). Blue and red vectors show the phases of the alternating stimulation pattern at the time of the left and right heel strikes, respectively. The heel strikes were clustered around one point of the stimulation cycle (between Π/2 and Π for the left heel strike). The black vectors show the average preferred phase (scaled to unit length on the left two plots to enable a better visual comparison of the similarity between the two patients). The two plots to the right show the preferred phase and strength of entrainment (indicated by the length of the black vector) for each of the separate sequences of 20 s stepping (n = 10 sequences with altDBS in each patient, with an average of 22 left and right heel strikes per sequence to calculate the phase and strength of entrainment; note that some arrows are short or overlap with each other and are thus difficult to see). Here the vectors also point relatively consistently to the same quarter. B, No consistent clustering was present in non-responders (P04).