The experiments carried out by Benjamin Libet into the timing of conscious awareness have provoked, and go on provoking, a vast amount of discussion. His own theory of consciousness as a kind of 'field' has received somewhat less attention; and the strange brain-cutting experiment he proposed to test it seems likely to remain unperformed for the foreseeable future. A large number of papers and discussions have been published: in 2004, Libet finally summarised his own account in the book 'Mind Time'.
Libet's early research was actually intended to explore what the minimum stimulus giving rise to a conscious sensation might be. He had an enviable opportunity to study the response of the brain to direct stimulation (using trains of electrical pulses) through the help of a friendly neurosurgeon and the co-operation of a series of patients, who remained conscious and able to report their sensations throughout the experiment. There are several ways of varying electrical stimuli, of course, but a curious fact emerged: whatever the voltage or frequency of the pulses, the stimulus had to persist for about 500 milliseconds before the subject became consciously aware of it. Actually, this is not quite true: above a certain level of voltage, the interval decreased, but the current involved was by then well above anything likely to occur in the brain normally.
The result was unexpected, because it had already been established that stimuli applied to the skin, rather than directly to the brain, could be detected consciously even if they were much shorter than 500 milliseconds. Libet was able to demonstrate that although the stimulus to the skin might be brief, it was still the case that the resultant brain activity had to persist for 500 milliseconds before the subject became consciously aware of it. Where a patient was anaesthetised, the initial brain response to a stimulus was the same as for a fully conscious subject, but it failed to continue for the required period: moreover, a stimulus applied directly to the brain 500 milliseconds after one applied to the skin could cancel (or in some circumstances, enhance) it.
Now, it isn't surprising that there should be some delay between an event, and our becoming aware of it: indeed, if the normal process of cause and effect is to be sustained, the event has to precede the awareness it causes. If we were passive spectators of the world, simply watching the way we might watch a film, the constant delay would be irrelevant -we should never notice that we were half a second behind reality. But we also respond to events, and here a delay is highly relevant and noticeable. The really surprising thing, therefore, was the length of the delay which seemed to be involved. 500 milliseconds - half a second - is a noticeable period of time, and it is evident that human beings often respond to events far more quickly than that. If we had to wait half a second before responding to events, we should never be able to play a good game of tennis, and we should be dangerous (or extremely cautious) drivers.
The answer appeared to be that our unconscious responses are far quicker than our conscious ones. A stimulus applied to the skin produces an 'evoked potential' or EP in the brain within tens of milliseconds, and that seems to be enough for it to register unconsciously but effectively. A series of experiments have shown that we register unconsciously a whole host of things which may influence our response to events but which never cross the threshold into consciousness. Among other evidence, Libet quotes experiments which show that a conditioned response - a blink - can be created to events which the subject is never actually conscious of. The remarkable phenomenon of blindsight might perhaps be seen as a related case.
That still leaves us with a considerable problem. If the foregoing is true, we ought to be aware of it, surely? On the tennis court we would find to our surprise that we returned serve competently before we actually saw the ball, and certainly without thinking about where in the opposite court we might want to put it. Our conscious and unconscious behaviour would be strangely unsynchronised. Libet's hypothesis was that conscious awareness is subjectively referred backwards in time. We consciously perceive the stimulus as occuring at the same moment it registers unconsciously, even though it doesn't in fact enter our awareness until it has persisted for half a second. Subjectively we backdate it to match the EP at the beginning rather than the end of the 500 millisecond span.
Libet was able to provide some direct evidence through experiments which, instead of comparing skin stimuli with direct stimulation of the cortex, instead made a comparison with stimulation of the medial lemniscus, part of the incoming neural pathway. Each pulse delivered here generates its own EP, but the sensation is nevertheless referred back to the time of the first in the train. Backdating remains controversial, however. Perhaps the sensations actually enter conscious awareness immediately, and the half-second delay merely allows time for them to become reportable, or fixed in short term memory? Perhaps we are merely dealing with the difference between being aware of the stimulus, and being aware that we are aware of it? Libet contends that awareness and memory, especially declarative, explicit memory, are different and independent phenomena.
Primary source material: