Urges for food and money spill over into motor system excitability before action is taken

Much human behavior is driven by urges. Yet research into urges is hampered by a paucity of tools to objectively index their strength, timing and control. Here we used transcranial magnetic stimulation (TMS) and concurrent electromyography to examine whether urges for food and money are detectable via motor system excitability. In Experiment 1, we used a naturalistic food paradigm to show that food items that were most strongly wanted elicited the largest motor excitability, even before participants knew which response to make to get them. In Experiment 2a, we replicated the results using money - motor excitability was greater for larger monetary amounts. In Experiment 2b we show that monetary amount does not modulate motor excitability when participants simply observe, without having to take action. As the chief effect occurred prior to the subject knowing which motor response to make, it is not merely related to response preparation, and as the effect was present only when action was required, it is not merely related to increased arousal. Instead, the increased motor excitability likely indexes the degree of motivation a subject has to perform an action. Thus, we have used TMS to demonstrate that urges for food and money 'spill over' into the motor system. This is likely mediated by interactions between the limbic system (including the orbital frontal cortex) and the motor system, probably at the level of the basal ganglia. Implications are discussed for theories of embodied cognition and for methodological progress in studying urge control.