Task design

The subjects performed the Eriksen flanker task^17,21, responding by either flexing or extending the index finger of their right hand to the appearance of a visual stimulus. Participants were seated in a comfortable chair, their eyes at a distance of 75 cm from a computer monitor. Their wrist was constrained during the experiment, and subjects were specifically instructed to perform the task only with their right index finger without moving any other joints. A custom-built apparatus supported the right forearm and hand. It was equipped with infrared position sensors at the central (rest) position of the index finger as well as at the left and right peripheral position (to monitor respectively left and right movements in response to the target). This allowed detection of both the finger release from the rest position and the actual choice (button pressed) made by the subject. The task was an arrow version of the classical Eriksen flanker task (Fig. 1B). Two different conditions (Fig. 1B and Table ​Table1)1) were used in the task: in the congruent condition, four flanker arrows appeared beside the central arrow, pointing in the same direction (<<<<< or >>>>>). In the incongruent condition, the flanker arrows pointed in the opposite direction (flexion for >> <>> and extension for <<><<). In both congruent and incongruent conditions, subjects were instructed to respond by flexing or extending their index according to the central arrow direction. All stimuli were presented in white on a black background. Task stimuli were manipulated and presented in a pseudo-random order in order to satisfy the following parameters: (1) congruent (C) and incongruent (I) trials were presented in equivalent proportions; (2) similar number of trials for movement direction demanded for responding to the central arrow (left pointing arrow = first dorsal interosseous (FDI) muscle contraction (FDI agonist) or right pointing arrow = extensor indicis (EI) muscle contraction (FDI antagonist movement)); (3) similar number of trials regarding the movement history (repetition, i.e. two successive movements in the same or changed direction); (4) similar number of trials regarding the cognitive condition sequence effect (congruent followed by congruent: cC; incongruent followed by congruent: iC; congruent followed by incongruent: cI; incongruent followed by incongruent: iI). Each trial (Fig. 1C) began with a 1000 ms rest period, followed by a 1000 ms warning stimulus (white circle on black screen, 5 cm diameter) displayed at the center of the screen. Then a 500–800 ms delay preceded the presentation of the cue arrows. The subject had 2000 ms to respond. Cue arrows were displayed until the movement onset and each trial ended approximately 1600 ms after movement completion.