Fig. 1.

FMRI paradigm.

Each block of trials started with the presentation of a fixation cross. ‘Free’- condition: Upon appearance of the fixation cross, subjects were instructed to press the left or right button with the respective index finger at any self-chosen time. Every response was followed by a visual feedback pointing to the side of the button-press. Thereafter, the fixation cross re-appeared until the next response was given by the subject. Thus, subjects were free in terms of both movement lateralization and timing. ‘Intern’- condition: Subjects were instructed to react as fast as possible and press the left or right button upon appearance of a double-headed arrow pointing to both sides. Hence, subjects were restricted with re. to the timing of movement initiation but free in terms of movement lateralization. The fixation cross re-appeared for the time between stimuli. ‘Extern’- condition: Subjects were instructed to react as fast as possible and press the left button upon appearance of an arrow pointing to the left or the right button upon appearance of an arrow pointing to the right. Thus, subjects were restricted with re. to both timing and movement lateralization.