Libet

Libet's experiments suggest unconscious processes in the brain are the true initiator of volitional acts, therefore, little room remains for the operations of free will. If the brain has already taken steps to initiate an action before we are aware of any desire to perform it, the causal role of consciousness in volition is all but eliminated.
Libet finds room for free will in the interpretation of his results only in the form of 'the power of veto'; conscious acquiescence is required to allow the unconscious buildup of the readiness potential to be actualized as a movement. While consciousness plays no part in the instigation of volitional acts, it retains a part to play in the form of suppressing or withholding from certain acts instigated by the unconscious. Libet noted that everyone has experienced the withholding from performing an unconscious urge. Since the subjective experience of the conscious will to act preceded the action by only 200 milliseconds, this leaves consciousness only 100-150 milliseconds to veto an action (this is because the final 50 milliseconds prior to an act are occupied by the activation of the spinal motor neurones by the primary motor cortex, and the margin of error indicated by tests utilizing the oscillator must also be considered).
It has been suggested that consciousness is merely a side-effect of neuronal functions, an epiphenomenon of brain states. Libet's experiments are proffered in support of this theory; our reports of conscious instigation of our own acts are, in this view, a mistake of retrospection. However, some philosophers have disputed this conclusion:
"in short, the [neuronal] causes and correlates of conscious experience should not be confused with their ontology [...] the only evidence about what conscious experiences are like comes from first-person sources, which consistently suggest consciousness to be something other than or additional to neuronal activity."[4]
It is also possible to dispute the meaning of the experiments with regard to free will on a more simple methodological question: Libet asked his subjects to note the position of the dot the moment at which they became aware of making a decision. The assumption contained in the interpretation of the results, is that it took no time to note the position of the dot. Another account would be that in fact, given their instructions, subjects had to make a decision to note the position of the dot, but that this itself would take some time, and would interfere with the decision to move the wrist.