A book by the neuroscientist Benjamin Libet is always worth reading. His experiments on the timing of psychological events are crucial to important debates in consciousness research and philosophy of mind. This book in particular provides a clear summary of Libet's views on consciousness and how they reflect on deeper philosophical issues to do with free will and the nature of mind. He gives a detailed account of his finding that consciousness lags behind the bodily initiative to act, and explains how and why he came to the conclusion that consciousness is a mental field that emerges out of specific neural activities — namely, those that last for about 500msec (more on this below). The result is at once informative and accessible to undergraduate level students, and engaging to more expert readers who will continue to find in Libet's work much to mull over. My own opinion of Mind Time is that it does little, if anything, to dispel the usual concerns raised by philosophers regarding Libet's interpretations and conclusions from his own data. In fact, I found little in his arguments to convince me that consciousness is in any way a distinguishing property of only certain mental events; i.e., something deserving its own scientific theory.

I'll provide justification for my last two claims in three stages. First, I will give a (very) brief summary of Libet's experiments and his views on consciousness. Next, I will present some of the concerns that have been voiced (particularly by philosophers) about Libet's chosen interpretation of the facts. I will also point out why his Mental Field Theory of Consciousness (still) doesn't have a convincing answer to the difficulties raised. Finally, I will offer my own criticism of Libet's enterprise: that he gives us no good reason to think that there is indeed a unique property (in this case, being a CMF) common to all and only those psychological events subjects report as of consciousness.

Starting back in the 1960s, Libet has done a series of experiments relating the temporal order of neural events to the reported timing of subjective experience. His more striking discovery is that the brain events directly implicated in the production of movement occur about 350 milliseconds before the time subjects report being aware of making the decision to act. In other words, our brain begins an action well before subjects report awareness of the intention to act.

The immediate impression left by Libet's results is that consciousness comes after the decision to act. The implication is that when we decide to perform an action it cannot be our conscious intention to act that causes us to so act (since the awareness of any such intention only appears 0.5 sec after the brain as initiated the act), but rather the decision to act is made unconsciously. The conclusion that appears inevitable, then, is that conscious decision making is an illusion.

Libet himself has repeatedly rejected this conclusion. He claims that even if the movements pertaining to an action are initiated by unconscious processes, there is still the possibility that conscious will can veto the performance of an action. The basic idea is the following: if it takes half a second of neuronal activity for consciousness to make its entrance, it must be possible to interfere with the processes which would normally produce a conscious decision. Experiments in retroactive masking and enhancement of skin sensation seem to lend some support to this. For example, if a stimulus is applied to the skin and up to 0.5 sec later another stimulus is applied to the somatosensory cortex, subjects report feeling the second stimulus, but not the first. Importantly, if the second stimulus is delayed by more than half a second, subjects have no difficulty in distinguishing the two sensations.

This, however, is not yet the whole story. Together with a temporal delay, Libet claims that awareness also serves a function of 'backward referral'. To explain, when a stimulus lasts long enough to become conscious there is an automatic (and unconscious) antedating of its experience back to the time of the primary evoked potential associated with its detection. However, we don't experience a delay in the onset of our conscious experience. According to Libet, the reason for this is that those experiences are subjectively referred back to the time they actually occurred (see p. 79). Libet's astonishing hypothesis at this point is that such subjective backward referrals are not realized by any neural mechanism in the brain. He subscribes to a version of Dual Aspect Theory, and posits what he calls a unified Conscious Mental Field to account for subjective referrals. The CMF, he says, "does not exist without the brain. It emerges from the appropriate system of neural activities."(p. 182) But crucially, it 'is [also] not describable in terms of any externally observable physical events or of any known physical theory as presently constituted." (p. 169) Put more simply, the CMF is sort of like an electromagnetic or a gravitational field, and at the same time nothing like them since it has only non-physical, mental properties. To sum up, consciousness is a distinctively non-physical, mental property of only certain neural events lasting at least 500 milliseconds.

It's fairly easy to see why many have expressed reservations against Libet's theory. Persistent doubts, for example, have been raised that Libet's 'conscious veto' function can offer the opportunity to exercise free will that he claims for it. The usual worry is that the decision to veto must itself arise unconsciously, and thus leave unexplained what it is that distinguishes conscious from unconscious processes. Libet, as we just saw, bets that the veto is a property of a mental field that doesn't operate via any known physical mechanisms. In other words, the decision to veto doesn't arise in the same way that our urges to act do; it doesn't have an unconscious (physical) beginning. The problem with this answer, as many will rightly anticipate, is that in its endorsement of Dual Aspect theory (to account for the Mind-brain relationship) it inherits the problems that make these objectionable in their own right. In particular, it is difficult to see how any distinction between the mental properties and the physical properties of an event can escape the charge of epiphenomenalism without falling into substance dualism, or vice versa (just think about how to explain the interactions among those two types of properties). Libet's defense in this book adds little to what he has already said in previous publications (although there is a clear advantage in having his views come together in a single book) and can, I think, be instructively summarized by the following quote: "You can, of course, ask the proponents of the 'anti-ghost' argument, How do they know there is no ghost in the machine? The answer is they do not really know. The emergence of conscious subjective experience from nerve cell activities is still a mystery. If you want to refer to subjective experience as a ghost, you can do so." (p. 183). I suppose those already disposed to believe in apparitions will find no qualms with such arguments.

My own objection to Libet's theory is concerned with his assumption that there is some scientifically interesting property common to all and only those events subjects report as being conscious. That is, unlike Libet (and many other friends of the consciousness hypothesis), I am not convinced that the phenomena Libet is referring to (those figuring in introspective reports) share some unique property (physical or mental) that makes them scientifically worthy of their own explanation. To me, what normally falls under the term 'conscious' is a very heterogeneous group of phenomena having little more in common than, say, what we also normally call 'dust'. Libet claims throughout his book that "there is no need to invent different kinds or categories of consciousness or of conscious experiences to deal with all the kinds of experiences. The common feature in all cases is awareness. The differences lie in the different contents of awareness." (p.13) He claims moreover, as we saw above, that the property which these reported phenomena have in common is a time requirement – neuronal activities need to last a minimum of half a second. But the problem with electing the time factor as the identifying property of consciousness is that it sits rather uncomfortably with the very 'evidence' it is trying to explain. For, firstly, Libet's evidence for a temporal delay is confined to sensory input and his putative category of consciousness is meant to include also phenomena like dreams, thoughts, desires, beliefs, and so forth. In this sense, his theory is very far from supporting the stronger claim that all and only reportedly conscious phenomena share the same property. Secondly, there are the numerous temporal anomalies that are not easily describable or explainable under his theory. Think for example of the phi-phenomenon. When two lights at different locations are flashed quickly one after the other, subjects report that they are aware of one light moving from one side to the other (on a known variation, one can also see the moving light change color half way through). According to Libet the illusion of movement is created because of the antedating (i.e., backward referral) of the stimuli to the time of their physical occurrence. But when exactly did the 'antedating' take place: before or after the second stimulus 'reached' consciousness? This is the now classic dilemma posed by Dennett (1991): 'Orwellian vs. Stalinesque'. It's clear that neither response is agreeable to Libet's position since claiming that the fabrication of the illusion occurs before the content enters awareness (Orwellian) is too early to avail himself of the antedating facility (a distinguishing feature consciousness), and claiming that it occurs after it reaches consciousness is already too late. Libet's theory is caught between the horns of this dilemma, and to me that is an indication that his reasoning must have gone wrong somewhere. My suspicion is with the assumption that the phenomena introspectively reported by subjects share some unique and scientifically interesting property in common. Other readers will no doubt form their own conclusions.

© 2007 Isabel Gois

Isabel Gois is a PhD student at King's College London working on Consciousness. Her research interests include Philosophy of Mind, Neuropsychology, and Mental Disorder. She has articles published on Emotions, Computationalism, and Consciousness.    

Categories: Philosophical, Psychology