H-reflex measurement

To determine the changes induced to the H-reflex by tsDCS in one of the three configurations, we chose to characterize the H-reflex recruitment curve at four characteristic points (Figure ​(Figure2):2): H-Reflex threshold (H_thresh), 50% of H-reflex maximum (H_max50%), the point at which the ascending part of the recruitment curve begins to settle (H_settle) as well as the maximum H-wave (H_max). These points were chosen for their ability: (1) to reflect the anticipated changes of the H-reflex recruitment curve (left/right shift, based on Lamy et al., 2012), or overall amplitude modulation, as well as (2) to sufficiently approximate the ascending part of the recruitment curve. The points were determined from a detailed H-reflex recruitment curve, recorded at the start of each experiment. The recruitment curve was sampled at stimulation intervals I (see: Experimental Protocol). Thereby, M_max, H_max, and H_thresh were determined manually, with H_max defined as the peak value of the average H-wave recruitment curve and M_max the amplitude immediately after peak settling amplitude of the M-wave recruitment curve. H_thresh was defined as the first visible H-Wave, in response to a stimulus. H_settle and H_max50% were determined via fitting of a sigmoid function f(s) to the recorded recruitment curve: f(s) = H_max/(1 + e^{-m(s-S_max50%)}). Thereby, m is the function slope at f(S_max50%), S_max50% the stimulus needed to evoke 50% of H_max, H_max the maximum value of the recruitment curve and S_max the corresponding stimulation amplitude. H_max50% and H_settle are then defined as f(S_max50%) and f(S_settle) respectively, given f^″(S_settle) = min (f^″(s)). For each point, the closest multiple of I was chosen as a stimulation amplitude.

Overview of the distinct points measured within the H- and M- Wave recruitment curves: H-Reflex threshold (H_thresh), 50% of H-reflex maximum (H_max50%), the point at which the ascending part of the recruitment curve begins to settle (H_settle), the maximum H wave (H_max) as well as the maximum M-wave (M_max).