Among the avalanche of new books on consciousness, it would be hard to find two whose authors hold more dramatically different views than these. While Benjamin Libet describes his own famous experiments and concludes that consciousness is a field with powerful effects, Gerald Edelman builds his theory on the assumption that the world is causally closed and consciousness is devoid of causal efficacy.
I have to confess to opening Edelman's latest book with some trepidation, for I have found most of his previous books incomprehensible.
This one was, therefore, a pleasant surprise. Edelman skims over the details of his well-known theory of neuronal group selection but clearly explains the basics and sets the theory in context. He explains that any effective theory of consciousness must take a global approach (dealing with the whole brain) and must be based on selection rather than instruction. He emphasises that "the brain is not a computer, and the world is not a piece of tape". This is important because of the role of noise in such complex systems. While computers have to get rid of noise, brains depend on it.
They use their enormous variability to construct patterned responses to ever-changing environments that are full of novelty.
Of course, the real test for any theory of consciousness is how it deals with subjectivity or qualia (those private experiences of redness or the indescribable smell of the sea). Here, Edelman is explicit. The brain's complex looping neural circuits make multiple discriminations, and the qualia are those discriminations. To explain this further, he refers to the brain processes as C1 and the experiences as C. Now, C always and necessarily accompanies C1 but, given the laws of physics, C itself cannot be causal. This allows him to take a strong stand on some classic issues such as the function of consciousness and the possibility of zombies - those imagined creatures that look and act just like us but have no conscious experience. As far as function is concerned, consciousness would necessarily have evolved as the brain's capacity to make discriminations evolved; so there would be no sense in asking what the function of consciousness itself is or whether we humans might have evolved as zombies.
Zombies are logically impossible. All of this, he concludes, allows us to speak as if consciousness is causal when really it is not.
Libet takes the opposite view, concluding that consciousness really does have causal powers, and free will is no illusion. His book is greatly to be welcomed because it provides the first full and detailed account of his famous experiments, explaining how and why he carried them out, and how he came to his conclusions.
The experiments concern what has come to be known as the "half-second delay in consciousness" or "Libet's delay". The earliest of these were carried out in the 1970s, using patients who were having brain surgery and had the surface of their brains exposed while awake and fully conscious. Libet was able to stimulate their sensory cortex with longer or shorter series of electrical pulses and ask them whether they felt anything. To cut a long and controversial story short, he demonstrated that it requires about half a second of continuous stimulation in sensory cortex for the person to report a conscious sensation.
This seems deeply weird. It seems to imply that we must live our lives half a second behind the events of the real world. The reason we do not, according to Libet, is that once "neuronal adequacy" has been reached (the brain activity has gone on long enough) the events are subjectively antedated to the time of their first effect on the brain - the evoked potential. This means we never notice the half second that the brain needs to build a conscious experience.
Not surprisingly, Libet's delay has provoked long and heated debates. Libet mentions some of these in the book, but his coverage is patchy and superficial. He is summarily dismissive of all materialist and reductionist arguments, and does not deal with objections to the very idea of a separate time at which consciousness itself happens.
His later, and equally controversial, experiments concern a question that must have occurred to us all; how can our powerful sense of free will be reconciled with a scientific understanding of the brain? His experimental question was, "Does the conscious will to act precede or follow the brain's action?" Libet's genius lay in devising an experiment to find out.
In these experiments, subjects had to flex their wrists, deliberately and consciously, whenever they felt like it. The action was not freely chosen, but the timing was. Libet then timed three things: the wrist movement; the start of the readiness potential in the brain (showing the motor cortex starting to plan the action); and the conscious desire to move. Measuring the first two is easy, but the last is not, so he devised a method by which subjects had to call out the position of a revolving spot at the time they decided to move. The results were unequivocal: unconscious brain processes began nearly half a second before the conscious decision to act.
These results are generally accepted by other scientists. What is not agreed on is the interpretation, and again Libet only sketches the many arguments that have raged over the years. His own interpretation is somewhat curious, to say the least. He argues that these results prove that we cannot have free will in the sense of consciously initiating actions, but we can consciously intervene to veto actions that have started unconsciously. This implies that we cannot help thinking of bad or wicked actions, but we can be held responsible for not preventing ourselves carrying them out. This way he retains a role for consciousness as a power or force that influences what the brain does.
This much is well known, although it is nice to see all the experiments described together in one book. What is new is Libet's "conscious mental field theory", which is startlingly different from any other current theory of consciousness. The idea is that conscious, subjective experience is a unique and fundamental property in nature; a so-far-unrecognised field that emerges from brain activity and can, in turn, act on and influence that brain activity. This unified and powerful field can, he claims, explain the two most difficult features of consciousness - the unity of our mental life and our sense of having free will. He even proposes an experiment to test the theory. If a small part of a living brain were to be isolated from the rest, he would expect activity in the isolated part to give rise to conscious experiences even without any neuronal connection to the rest of the brain. I agree with his concluding remark that if he were proved right, this would have a profound impact on neuroscience.
Susan Blackmore is visiting lecturer in psychology, University of the West of England, and a freelance writer, lecturer and broadcaster on consciousness studies.
Mind Time: The Temporal Factor in Consciousness
Author - Benjamin Libet
Publisher - Harvard University Press
Pages - 248
Price - £19.95
ISBN - 0 674 01320 4