The experiments of Libet, et al., and their implications for free will
In a ground-breaking series of experiments first reported in 1973, Benjamin Libet, et al. [Subjective referral of the timing for a conscious sensory experience: a functional role for the somatosensory specific projection system in man, by Libet, Wright, Jr., Feinstein, and Pearl, Brain 102 (1979) 193-224] showed that the earliest experiential awareness of a sensory stimulus occurs about 500 msec (0.5 sec) after the stimulus itself (see diagram below). These experiments involved applying small electrical pulses to the skin of the hands of patients who were undergoing brain surgery, and then measuring the resulting electrical signals from electrodes implanted in the sensory cortex. The initial negative pulse is the primary evoked potential resulting from the nerve impulse traveling from the hand to the brain---it appears 10-30 msec after the skin stimulus. The subsequent wave (average evoked response AER) is the brain's response to the stimulus.
The experiments showed that none of our experiences of perception are in objective time (time as measured by a clock or other instrument), but in fact are delayed by about one-half second after the objective events. This delay is the time required for the AER to rise to the level necessary for experiential awareness (neuronal adequacy). (Other experiments showed the necessity of neuronal adequacy for subjective experience to occur.) This means that it is impossible to respond volitionally in less than 500 msec to any external stimulus since our experience is always delayed by that much. [Libet. et al. also showed that meaningful but unconscious, reflexive behavioral responses can occur in as little as 100 msec after a stimulus, showing that meaningful behavior need not be conscious behavior (e.g., a sprinter exploding from the blocks after the starter's gun fires).]
[In addition, Libet, et al. showed that our experience of a skin stimulus precedes neuronal adequacy because the brain refers the experience retroactively to the time of stimulus, as is shown in the diagram. This required an experiment in which pulses were applied directly to the sensory cortex simultaneously with pulses applied to the hand. When this was done, the skin pulses were felt by the subject to occur before the cortex pulses (which were also felt in a hand, not in the cortex) even though it was known that the brain required the same time to process the skin pulses as the cortex pulses. Only when the skin pulses were delayed about 500 ms relative to the cortex pulses were the two pulses felt simultaneously.]
In 1983, Libet, et al. [Unconscious cerebral initiative and the role of conscious will in voluntary action, The Behavioral and Brain Sciences (1985) 529-566] reported an even more profound set of experiences in which a different set of subjects, these without implanted electrodes, were "volitionally" initiating muscular acts rather than responding to sensory stimuli. Electromyogram signals from a designated trigger finger were used to initiate computer storage of the EEG responses (the readiness potential, RP) that had appeared on the scalp prior to the triggers [see diagram below from Alexander Riegler, Whose Anticipations? in Butz, M., Sigaud, O., and Gerard, P. (eds), Anticipatory Behavior in Adaptive Learning Systems: Foundations, Theories, and Systems. Lecture Notes in Artificial Intelligence, Springer-Verlag (2003) 11-22].
The results showed that the onset of the readiness potential RP preceded the finger action A by 550-1050 msec, but the experiential awareness of the willingness to perform the action preceded the finger action by only about 200 msec. (This awareness could not be signaled by finger motion because that would require another decision for muscular action. It was measured by having the subject associate his reading of an electronic clock with the onset of his awareness of the decision.) Thus, the decision to perform a muscle act is made prior to the awareness of the decision. In other words, we become aware of a decision only after the decision has already been made. Libet speculated that it may be possible to consciously veto such an unconscious decision if it is done within the last 100-200 msec before the action is to occur. However, because there is no muscle action to trigger the recording of a veto event, experimental verification of conscious veto decisions is not possible. Regardless of that, the possibility of volitional veto decisions is overruled by the considerations in the following paragraph, and by those in Sections 5.10 and 5.12.
A simplified diagram of these results is shown below:
Libet’s experiments point to a general concept that a little thought shows must always be valid. This is that everything that happens must happen before we can become aware of it. Any neurological or sensory process always happens before our awareness of the thought, feeling, or sensation that represents it. In Libet’s experiments, the lag of awareness was between 350 msec and 500 msec, but the exact value is unimportant. So long as this lag exists, no matter how large or small, whether it is one hour or one microsecond, our subjective experience of an event must always come after the objective measurement of the event. In other words, the subjective present always lags the objective present, or subjective time always lags objective time. [Because the brain requires about 500 msec to process an event before we can become aware of it, it is impossible for us to be aware of any instant in which the brain ceases to function, such as the instant we fall asleep (either naturally or under anesthesia), or the instant we die.]
The consequences of this insight are extraordinary, revolutionary, and far-ranging. Every thought, feeling, sensation, or action always occurs objectively before we become aware of it subjectively and hence there is no possibility that we can avoid it. This includes any choices or decisions that are made. We inescapably live in the objective past so that the objective present and future are completely beyond our awareness and control.