g. to seeing $5 as opposed to 10 cents), but also reflects something about action. We started out defining an ‘urge’ as ‘how much someone wants something’. Based on the current paradigms

at least, the concept of urge could be refined to ‘how much someone wants something when action is required to get it’. In respect of the need for action, our findings are at selleck odds with those of (Kapogiannis et al., 2008), who identified an effect of reward on the motor cortex using paired-pulse TMS in a paradigm in which the participants did not make any response. However, with the task used in their study, they could not identify whether the effect was determined by the size of the reward or the probability of receiving it (in that check details study the effect related to a strong reduction in uncertainty when observing whether a reward materialized over a specific time interval). We suspect that the changing reward probabilities drove the observed effect and, therefore, having an action was not critical in their paradigm. Here, in our money paradigm, the task was set up to measure the strength of the urge, manipulated solely by the size of the monetary reward ($5 or $0.1), while keeping the probability of seeing $5 or $0.1 on any trial exactly the same. In these experiments, the MEPs likely reflect multiple contributing factors,

including not only the urge (determined by the value of the stimulus), but also action preparation. Hence, we caution the reader that comparing MEPs (raw or normalized) across different experiments might be misleading. It is only the relative difference between MEPs observed for different levels of urge (within an experiment, when all other factors are controlled) that can be reliably interpreted. Even a comparison of MEPs between baseline and non-baseline trials within

the same experiment is difficult to interpret because the probability of seeing a baseline trial was lower than the probability of seeing a non-baseline trial in all three experiments and, therefore, differences in the probabilities could confound such a comparison. We used baseline trials only for normalizing the MEPs within each participant (to reduce variance between participants). Thus, we have shown that the strength of an urge can be indexed via ‘spill over’ into motor system excitability, at one time-point and not another, and only when a response is needed Fossariinae for satisfying the urge. Moreover, unlike prior studies, we have separated the preparation to make a response from response execution itself. Further, by recording motor excitability before the participant knew which response to prepare, we also show that the effect on motor excitability is not purely one of motor preparation but must also reflect a motivational component. Further, by manipulating the response-requirement in the money task, we show that the effect is also not purely related to general brain arousal but must also include an action-relevant component.