From their paper in Trends in Cognitive Sciences, 20(2), 2016.

“Now a series of new developments has begun to unravel what we thought we knew about the brain activity preceding spontaneous voluntary movements (SVMs). The main new revelation is that the apparent build-up of this activity, up until about 200 ms pre-movement, may reflect the ebb and flow of background neuronal noise, rather than the outcome of a specific neuronal event corresponding to a ‘decision’ to initiate movement.”