Libet's experiments and the urge to act - what do you think?
Posted: Tue Aug 25, 2015 10:40 am
Many may be familiar with the Libet experiments in the early 80s that suggested that the brain begins the process of preparing for a voluntary motor action before the subject decides to act.
Essentially Libet had subjects undertake a motor act - usually a flick of a wrist or finger - and note when they did this. He also recorded via ECG the brain activity to pinpoint any correspondence between subjective sense of the act and the objective neural activity underlying the act.
The times that were recorded were W, M, S and 0, where W is the time the subject perceived the urge to act, M is the time that the act was perceived to occur, S was the time that the subject perceived an electronic stimulus of the skin and 0 was the objective time of the act.
Subjects were to allow the act (wrist or finger flexion) to simply occur - that is, to be voluntary and not in response to any external stimulus - and not to prepare or pre-plan the act. It was found that the brain began preparation for the act via a Readiness Potential (RP) that arises some 500ms before the act. The W moment was typically noted at about 200ms before the act, and the M moment at around 85ms before the act.
Now, if I try this myself, I am simply not aware of any 'urge to act'. I can just sit here and then make my finger flex and hence clearly identify the moment it happens (M), but an 'urge to act' (W)? Not at all. Even if I try to keep my mind 'clear', I am simply not aware of anything other than the movement. How could I detect an urge to act, especially when you think it's within 200ms of the act occurring? That's a level of fine temporal distinction I don't think I can achieve.
Does anyone have any thoughts on what exactly Libet's subjects might be reporting?
Essentially Libet had subjects undertake a motor act - usually a flick of a wrist or finger - and note when they did this. He also recorded via ECG the brain activity to pinpoint any correspondence between subjective sense of the act and the objective neural activity underlying the act.
The times that were recorded were W, M, S and 0, where W is the time the subject perceived the urge to act, M is the time that the act was perceived to occur, S was the time that the subject perceived an electronic stimulus of the skin and 0 was the objective time of the act.
Subjects were to allow the act (wrist or finger flexion) to simply occur - that is, to be voluntary and not in response to any external stimulus - and not to prepare or pre-plan the act. It was found that the brain began preparation for the act via a Readiness Potential (RP) that arises some 500ms before the act. The W moment was typically noted at about 200ms before the act, and the M moment at around 85ms before the act.
Now, if I try this myself, I am simply not aware of any 'urge to act'. I can just sit here and then make my finger flex and hence clearly identify the moment it happens (M), but an 'urge to act' (W)? Not at all. Even if I try to keep my mind 'clear', I am simply not aware of anything other than the movement. How could I detect an urge to act, especially when you think it's within 200ms of the act occurring? That's a level of fine temporal distinction I don't think I can achieve.
Does anyone have any thoughts on what exactly Libet's subjects might be reporting?