International audience The Readiness Potential (RP) is a slowly increasing surface-negative cortical potential that precedes spontaneous voluntary movements. A recent interpretation provided by the Stochastic Decision Model (Schurger, 2012) suggests that this slow buildup could be the result of event-locked averaging of ongoing sub-threshold fluctuations in neural activity. According to the model, autocorrelated background activity plays an important role in the preparation of actions when the external imperative to act is weak or absent: slow fluctuations continuously drift randomly closer to or farther from the decision-threshold for initiating action in an integration-to-bound fashion where 'noise' in the brain is integrated over time. In particular, the model predicts that movement is more likely to happen at a 'crest' in these ongoing fluctuations, and less likely at a trough. In classical RP studies subjects are instructed that they have an unlimited amount of time in which to perform the movement. We developed a new experimental paradigm in order to investigate the effect of varying amounts of temporal freedom on the shape of the RP/RF (Readiness Field, for MEG recordings). We perform a variant of Libet's (1983) task in which subjects are asked to initiate a finger tap within a given time window on each trial. Participants are free to make the movement whenever they want as long as they do it before the time has elapsed. The time limit variable, signalled by an animated clock, will vary among blocks in a counterbalanced way across subjects. Our main prediction is that the movement-preceding activity in pre-motor areas of the frontal lobe will appear to begin earlier, and be more prominent in the time-locked average, as the window of time within which the subject is allowed to move becomes longer. The temporal constraint is predicted to affect the Early but not the Late component of the RP/ RF. We would like to investigate parametric variations of the shape of the Readiness Potential/ Field under temporal uncertainty, to unveil the timing mechanisms behind the non-movement and the movement states in the brain.