Using transcranial magnetic stimulation (TMS), we have tested the time needed for the perceived onset of spontaneous motor intention to be fully determined. We found that TMS applied over the presupplementary motor area after the execution of a simple spontaneous action shifted the perceived onset of the motor intention backward in time, and shifted the perceived time of action execution forward in time. The size of the effect was similar regardless of whether TMS was applied immediately after the action or 200 msec after. The results of three control studies suggest that this effect is time-limited, specific to modality, and also specific to the anatomical site of stimulation. We conclude that the perceived onset of intention depends, at least in part, on neural activity that takes place after the execution of action. A model, which is based on the mechanism of cue integration under the presence of noise, is offered to explain the results. The implications for the conscious control of spontaneous actions are discussed.

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