Seeking Free Will in Our Brains
manuelm...@gmail.com
from which dessert to select to whether to save a life or commit a
crime, seems part of our basic human nature. Long the province of
philosophers and theologians, recently free will has become a question
that fascinates neuroscientists. Looking for its basis in the brain
has led some to argue that free will is only an illusion-a perception
not congruent with the unconscious biochemical processes that they see
as leading to thought and action. Two senior scientists, a neurologist
and a psychiatrist, debate the meaning of free will and whether brain
science can, now or ever, fully explain it. Each scientist first wrote
a position statement; they then exchanged statements to write
rejoinders.
We are constantly making movements. While we certainly think we
choose them freely, do we really? What would it mean if we did? The
physiology of movement has been the object of intense study by
scientists, and we now know the drivers of movement. These drivers
include sensory input from the external world, our emotions, our
biological drive for homeostasis-for balance of our physiological
systems-and our past experience, including rewards and punishments
that resulted from previous actions. Do these fully determine our
choice or can we identify another factor, which we call free will?
The answers to these questions are easy only for the dualist, who
believes in a mind separate from the brain and who thinks that free
will comes from the mind. No evidence for this position can be found,
however, and therefore most scientists reject it. The mind
(consciousness) is a product of the brain, so if free will can be a
driver of movement, we have to be able to find it in the brain. All
the tools of modern neuroscience provide ways of studying this
question.
Looking for free will in the brain not only is interesting for its own
sake, but it is also important for understanding a number of
neurological and psychiatric conditions.1 We can observe in patients
with certain disorders that a relationship between movement genesis
and a sense of volition is not mandatory. For example, people with
Tourette syndrome often say that they cannot not act out their tics.
With psychogenic movement disorders-also called conversion disorders
or the old term, hysteria-the movements look voluntary, but patients
say they are involuntary. In schizophrenia, movements also may look
normal, but patients might say that these movements are controlled by
external agents. In early Huntington's disease, the apparently
involuntary chorea (rapid jerky movement) is sometimes interpreted as
being voluntary. And in anosognosia (a condition in which a person who
suffers disability due to brain injury seems unaware of an
impairment), patients may think that they have made a movement when
they have not.
Do We Freely Choose to Move?
In general, scientists need to study what we call "simplified
preparations" in which it is possible to control all the variables in
a situation. One such experimental situation is making a single
movement of the hand or finger. People can be asked to move whenever
they want to; the commonsense view is that a person consciously
decides to make a movement and then makes it. Free choice has
preceded the movement.
This was put to the test first in the classic experiments in the 1970s
by Benjamin Libet, Ph.D., and colleagues at the University of
California, San Francisco.2 In these experiments, study participants
sat in front of a clock-like timer, with a ball moving around the
periphery once every 3 seconds. They were told to perform a simple
motor task, such as flexing a finger or wrist, whenever they wanted to
and, afterwards, tell the investigator where the ball was (that is,
what time it was) when they decided to move. An electroencephalogram
(EEG) recorded their brain activity, while electromyography (EMG)
recorded the electrical activity of their muscles.
What Libet learned was that participants in the study had the first
conscious sense of willing the movement (called "time W") about 300
milliseconds before the onset of muscle activity (as measured by the
EMG). But the EEG showe brain activity in the motor cortex beginning
about 1,000 milliseconds before movement could be measured-in other
words, earlier than conscious awareness of the intention to move. This
early brain activity measured by the EEG probably takes place in the
supplementary motor area and premotor cortex in preparation for
initiating movement. This experimental result, which has been
duplicated many times, appears to indicate that the movement begins
unconsciously. This does not jibe with our ordinary sense of how we
operate.
Can the data from the Libet experiment be interpreted in other ways?
Libet himself argued that we still have free choice, but it is
confined to the ability to veto actually making a movement after the
intention to move becomes conscious. This is not a strong argument,
however, since the veto could also be initiated subconsciously long
before the act.
Another whole set of issues revolves around the problem of the timing
of subjective events. Consciousness can be deceptive, so is it
possible that our sense of W, of willing a movement, is incorrect in
regard to when it actually happened in the brain? A number of
experiments have explored this. The results show that, first, W is
not strongly linked to the time of movement onset, so whatever is
going on in the brain at time W cannot be responsible for movement
genesis.1 Moreover, the brain event of W may even be later than we
subjectively report. This should not be a complete surprise since
humans "live in the past"-certainly perception of a real-world event
has to be subsequent to its actual occurrence, since it takes time
(albeit very little time) for the brain to process sensory information
about the event. A recent experiment showed that it was possible to
manipulate the conscious awareness of willing a movement by delivering
a transcranial magnetic stimulus to the area of the brain just in
front of the supplementary motor area after the movement had already
occurred.3 This suggests that the brain events of W may occur even
after the movement.
If free will does not generate movement, what does? Movement
generation seems to come largely from the primary motor cortex, and
its input comes primarily from premotor cortices, parts of the frontal
lobe just in front of the primary motor cortex. The premotor
cortices receive input from most of the brain, especially the sensory
cortices (which process information from our senses), limbic cortices
(the emotional part of the brain), and the prefrontal cortex (which
handles many cognitive processes). If the inputs from various neurons
"compete," eventually one input wins, leading to a final behavior.
For example, take the case of saccadic eye movements, quick target-
directed eye movements. Adding even a small amount of electrical
stimulation in different small brain areas can lead to a monkey's
making eye movements in a different direction than might have been
expected on the basis of simultaneous visual cues.4 In general, the
more we know about the various influences on the motor cortex, the
better we can predict what a person will do.