Fried et al. have now taken Libet's classic experiment (electodes on the surface of the head reporting activity in motor cortex in preparation for a movement before the subject is aware of deciding to act) to a whole new level. They recorded the activity of 1019 neurons while twelve subjects performed self-initiated finger movement. (In some cases of intractable epilepsy, intracranial electrodes are used for evaluation prior to neurosurgery. When depth electrodes are inserted into the cortical tissue itself, it is then possible to record the firing patterns of single neurons in awake, behaving humans.) Activity increases prior to volition were mapped with greater detail, and areas that decreased in firing in preparation for movement were also found, suggesting an inhibitory component to volition. Here is the author's summary:
* Progressive changes in firing rates precede self-initiated movements
* Medial frontal cortex units signal volition onset before subjects' awareness
* Prediction level is high (90%) based on neuronal responses in single trials
* Volition could arise from accumulation of ensemble activity crossing a threshold
Understanding how self-initiated behavior is encoded by neuronal circuits in the human brain remains elusive. We recorded the activity of 1019 neurons while twelve subjects performed self-initiated finger movement. We report progressive neuronal recruitment over ∼1500 ms before subjects report making the decision to move. We observed progressive increase or decrease in neuronal firing rate, particularly in the supplementary motor area (SMA), as the reported time of decision was approached. A population of 256 SMA neurons is sufficient to predict in single trials the impending decision to move with accuracy greater than 80% already 700 ms prior to subjects' awareness. Furthermore, we predict, with a precision of a few hundred ms, the actual time point of this voluntary decision to move. We implement a computational model whereby volition emerges once a change in internally generated firing rate of neuronal assemblies crosses a threshold.
From an introductory review by Haggard, a segment relevant to Libet's idea that "free will" might correspond to "free won't" - inhibition of an intended action:
A recent model of volition identified the decision of whether to act or not as an important component of volition. Fried et al.’s data suggest one mechanism that might be involved in this decision. Decreasing neurons might withhold actions until they become appropriate through tonic inhibition and then help to trigger voluntary actions by gradually removing this tonic inhibition. Competitive inhibitory interaction between decreasing and increasing neurons could then provide a circuit for resolving whether to act or withhold action. A similar model has already been proposed for decisions between alternative stimulus-driven actions in lateral premotor cortex. Libet thought that ‘‘veto decisions’’ could represent a form of pure mind-brain causation, with consciousness directly intervening to interrupt the buildup of the readiness potential. Competition between populations of medial frontal neurons may provide a simpler explanation, though it still leaves us hunting for potential "decision" areas that may modulate the competition.