Benjamin Libet's 1985 paper titled "Unconscious cerebral initiative and the role of conscious will in voluntary action" was the first ever attempt to correlate conscious decision making with real physiological data. The centerpiece of the Libet experiment was the famed bereitschaftspotential (readiness potential), which is a buildup of electrical activity originating in the secondary motor area, immediately rostral to the motor cortex.
The potential builds for around 500-1000 ms and peaks right at the onset of activity. In this paper, Libet argued that the readiness potential is an indication of certain areas of the brain preparing to perform a voluntary task. The individual only becomes conscious of the imminent action 200 ms before the action actually occurs.