Thursday, 29 May 2008

Forget the hands, the future is in your cerebellum

The brain is a masterpiece of neurological wiring, a mysterious, convoluted mass of matter that manages to somehow think, recall and control the human body. In many areas, the function of the brain remains a mystery. Now scientists have found the part of the brain that can predict your future.

Imagine part of your body is constantly moving. The countless receptors that contribute to your senses can detect where you are, but as clever as your brain is, this information will take a small amount of time to be computed. It has to be encoded so that it can be transmitted from the sensor along a nerve to the brain, which might be a good metre or more away, and then decoded. By the time you know where you are, your motion has taken you further, so you are no longer where you think you are.

To compensate, scientists have predicted that part of the brain can integrate up-to-date positional information with the motion commands it gave to that body part, prior to receiving ‘afferent’ information from sense receptors. In other words, by adding together where you were with what direction you are going, it can compute where you are, before it knows for sure.

This ability is known as state estimation. It is what stops you from knocking over your mug of coffee in the morning — when reaching for your daily dose of caffeine you want to stop just before you reach the handle, rather than waiting for your fingers to say that you’ve arrived. Otherwise, that’s today’s crossword ruined.

Until recently, there had been no direct experimental evidence to pinpoint the part of the brain that is responsible. Indirect evidence, as determined from patients with lesions in particular parts of the brain, suggested that it was the role of the cerebellum, a brain region already known to play a part in sensory perception and motor control.

To test this, Chris Miall of the School of Psychology at the University of Birmingham, and colleagues, designed an experiment to directly manipulate the cerebellum during a simple movement task. To describe it sounds like a horror scene from a science-fiction novel: participants have a transcranial magnetic stimulation (TMS) device — in essence a very large electromagnet — strapped to their scalp, and their head is trapped between a chin rest and a rigid metal frame. Wearing goggles and with a computer sensor strapped to their finger, the subject begins to resemble a cyborg, the inner workings of their brain subject to manipulation from the machine.

The actual explanation of the study is somewhat less melodramatic but equally as enthralling. TMS creates rapidly changing magnetic fields which are directed to a focal point — in this case the cerebellum — to depolarize brain neurons and consequently induce electric currents.

Participants were asked to move their hand in a straight line until a random cue signified that they should make a rapid movement up and left to point at a predetermined virtual target, which was created by mirrors reflecting a high-tech yellow and pink earplug. After a few practice runs, the goggles went opaque and the trials in essence became blind.

In half of the trials, three pulses of TMS were triggered shortly after the ‘go’ cue. Each pulse lasted 50 milliseconds — that’s one twentieth of a second.

Miall’s team believe that by using this technique, they disrupt state estimation by the cerebellum. Consequently, when the participant reaches for the target their motion is based on out-of-date information.

“By zapping the cerebellum with TMS we recorded systematic deviations in subjects' trajectories,” says Owen Cain, one of Miall’s team. “This effect was localized to the cerebellum: stimulation of other brain areas (such as the motor cortex) gave much smaller deviations. These deviations were not random: subjects moved their hands as if they were where they had been 100 milliseconds in the past. In other words, they were planning their trajectories based on out-of-date information.”

Their study implies that TMS might be highly useful in future experiments to directly test brain function. “The thing about TMS is you can use the subject as their own control: when TMS is on, the part of the brain that it is stimulating is off, and vice versa. That's very useful when you want to know what that part of brain is doing.” The functions of the brain might not remain mysterious for long.




Miall, R. C., Christensen L. O. D., Cain O. & Stanley J. Disruption of State Estimation in the Human Lateral Cerebellum. PLoS Biol 5 (11), e316 (2007). doi:10.1371/journal.pbio.0050316
This is available to download for free from http://www.plos.org


Quotes come from conversations with Owen Cain, and detailed descriptions of the experimental set-up come from first-hand experience: I am one of the forty-five participants of the trials.

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