, 2002). This suggests that an important normal function of these areas may be to tonically inhibit unwanted actions. Second,

a recent neuroimaging study showed increased pre-SMA activation when an external stop signal successfully triggered inhibition of movement (Sharp et al., 2010). Finally, and compellingly, medial frontal areas that produced movement arrest during intracranial selleck products stimulation were also identified as the source of readiness potentials during action generation (Yazawa et al., 2000). A recent model of volition identified the decision of whether to act or not as an important component of volition (Brass and Haggard, 2008). 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 (Cisek, 2007). Libet thought that “veto decisions” could represent a form of pure mind-brain causation, with consciousness directly intervening check details to interrupt the buildup of the readiness potential. Competition between populations of medial frontal neurons may provide a simpler explanation, though for it still leaves us hunting for potential “decision” areas that may modulate the

competition. Not surprisingly, several questions remain unanswered. One is the possible contribution to volition of other cortical areas not studied here. Fried et al. highlight recent reports of an experience of urge to move following parietal stimulation (Desmurget et al., 2009). They briefly present one parietal recording from their own data set, which shows an increase in firing rate prior to W very similar to medial frontal neurons. The division of labor between medial frontal and parietal cortex in volition is a topic of current debate. Neuropsychological studies of patients with focal frontal and parietal lesions suggest that both areas are involved in volition (Haggard, 2008). It seems likely that they act as a concerted network: the pre-SMA might generate action plans, and the parietal cortex might monitor their progression to execution. However, we have little insight into the detailed operation of this network. In neurosurgical studies, the sites of stimulation and placement of recording electrodes are, of course, determined by clinical need alone. Therefore, the crucial data required to resolve the debate, such as simultaneous recordings from parietal and frontal electrodes in the same individual, may not be forthcoming. A second remaining question is the activity of these neurons in the absence of voluntary action.