Fig. 1

Relationships between placebos and biological system. There are two possible placebos (“0” and “1”) and two possible states for the biological system: no change (“resting” state or background) which is noted “↓” and biological change above background (“activated” state) which is noted “↑”. As a consequence, there are two possible relationships defined as: direct relationship with “placebo 0” associated with “↓” and “placebo” 1 associated with “↑”; reverse relationship with “placebo 0” associated with “↑” and “placebo 1” associated with “↓”