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, Edelman builds his theory on the assumption that the
world is causally closed and consciousness is devoid of casual
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 actively 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 C’ and the experiences as C. Now, C always and
necessarily accompanies C’ 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 500
msec (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 don’t, according to Libet, is that once “neuronal
adequacy” has been reached (i.e. 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 unfortunately 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 once
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 upon 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 can only 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, Bristol, and a freelance writer,
lecturer and broadcaster on consciousness studies.
