A recent study of a Parkinson patient with alien hand syndrome has been published by Schaefer, Heinze and Galazky (citation below). This patient offered an interesting opportunity because his left hand made both involuntary and voluntary (but with effort) movements. What is more, a particular sort of involuntary movement could be triggered in a predictable manner. Blocks of fMRI scans were made under 4 conditions and rest.
left handed triggered to move without volition by pushing it slightly resulting in a counter movement (Gegenarbeiten) it gave activity as compared with 2 in primary motor cortex, left premotor cortex, precuneus, right inferior frontal gyrus, and cerebellum;
a similar trigger on the right hand that does not result in any movement it gave no significant activity compared to 1;
a voluntary movement of the left hand which took extra effort to achieve it gave activity as compared with rest in primary motor cortex, supplementary motor area, ventromedial prefrontal cortex, precuneus and cerebellum;
a voluntary movement of the right hand similar activity to 3 compared to rest except for inactivity in the precuneus and inferior frontal gyrus.
A difference between conscious and unconscious movement was found in the precuneus, where it would likely be due to conflicts of agency.
Similar to the precuneus, the IFG is not typically activated in motor paradigms. This brain area has also been reported to be uniquely activated during alien movements in the fMRI study by Assal et al. Recent lesion studies discuss a role for the right IFG in inhibitory control over motor responses. Since the IFG was active only in the condition when we elicited involuntary movements, involvement of this brain region may reflect attemptions to control and inhibit movements of the alien hand. Thus, we argue that both precuneus as well as the IFG may reflect conflicts of agency in unconscious and unwanted movements.
So the main difference between conscious and ‘alien’ movement seems to be attempting and/or resisting the sense of agency or ownership of the movement and attempting/resisting inhibition of the movement. There appears to be no difference in the planning, specifying and execution of the movement.
Schaefer, M., Heinze, H., & Galazky, I. (2010). Alien Hand Syndrome: Neural Correlates of Movements without Conscious Will PLoS ONE, 5 (12) DOI: 10.1371/journal.pone.0015010