Field of Science

Neuroimaging study does not disprove free will

An excellent new paper in Nature Neuroscience made a big splash last week by purporting to show activity in the brain related to muscle movement starts up to ten seconds before the person is consciously aware of having made a decision to move. This study is in fact a replication and extension of previous research that had suggested that related brain activity starts at least 300 ms before the conscious decision. The big news presumably is that new technology (specifically pattern recognition algorithms) allowed the researchers to push back this time window (which really is big news and an excellent application of this technology).

The reason I am using words like "purported" is that there are some important methodological assumptions buried into this experiment (to be sure the authors did not mention "free will" in the actual paper, though I imagine they were aware of the implications). In this particular version of the experiment

The subjects were asked to relax while fixating on the center of the screen where a stream of letters was presented. At some point, when they felt the urge to do so, they were to freely decide between one of two buttons, operated by the left and right index fingers, and press it immediately. In parallel, they should remember the letter presented when their motor decision was consciously made.
Afterwards, they reported which letter had been on the screen when they made their decision. The assumption is that participants are reporting the letter correctly. We already know that conscious perception is a distortion of reality (in fact, this study is a demonstration of that fact), so this may not be a fair assumption.

This case was made some time ago by the philosopher Daniel Dennett in his excellent Freedom Evolves. The argument is somewhat long, but it goes like this. First, we have to assume that participants weren't deciding to press a particular button as soon as the next letter popped up; if they were doing that, they would have already made the decision before that letter appeared, throwing off the scientists' measurements. But even if we assume that is not the case, there is a bigger confound:
If we monitor your brain with an array of surface electrodes ... we will find that the brain activity leading up to [a hand movement] has a definite and repeatable time course, and a shape. It lasts the better part of a second ... ending when your wrist actually moves.
Dennett points out that we aren't aware that it takes our brains a good second to plan, coordinate and execute a simple motor movement.
When we perform an intentional action, we normally monitor it visually (and by hearing and touch, of course) to make sure it is coming off as intended. Hand-eye coordination is accomplished by a tightly interwoven system of sensory and motor systems. Suppose I am intentionally typing hte words "flick the wrist" and wish to monitor my output for typographical errors. Since the motor commands take some time to execute, my brain should not compare the current motor command with the current visual feedback, since by the time I see the word "flick" on the screen, my brain is already sending the command type "wrist" to my muscles.
The effect, though Dennett doesn't put it this way, of actually being aware of the time it takes for your conscious decision to be converted into muscle movement would create a bewildering sense of out-of-sync-ness, something like being drunk or watching a baseball game at a far distance, where the crack of the bat reaches your ears the same time the image of the runner reaches first base.

Dennett formulated this argument to explain the 300ms difference between conscious decision making and the related brain activity found in previous experiments. However, it certainly can be extended to the new study. If it turns out that it takes 10 seconds from the beginning of a decision to move until the actual movement is carried out, then we most definitely do not want to be aware of it. Much better if our minds trick us into thinking movement follows thought instantaneously.

This argument does require some mental time distortion: just because we think two things are happening simultaneously does not mean that they are. But why should they be? If we have learned anything about the brain in the last couple centuries, it is that perception is useful, not accurate.

Soon, C.S., Brass, M., Heinze, H.J., Haynes, J.D. (2008). Unconscious determinants of free decisions in the human brain. Nature Neuroscience

2 comments:

outerhoard said...

I've often wondered, when I've read of these sorts of experiments in the past, what would happen if you had an electronic mechanism that could instantaneously detect when a person had made a "decision" to move, and then used this mechanism to perform the following experiment.

Subject is given a device with a light and a button, and gains a point for pressing the button while the light is green, and loses points for pressing the button while the light is red. But the device is hooked up to the other gadgetry in such a way that the light always turns red when it detects that the subject's brain has made a "decision" to press the button.

Given this visual feedback, can the client avoid pressing the button once a "decision" to do so is detected? If not, what does the subject perceive to have taken place during the experiment?

As for free will in general, I think the belief that they have free will in the matter of inconsequential button-pushing is less important to most people as the belief that they have free will in the context of big decisions involving much agonising over ethics. Even if free will could be disproved in one case, philosophical debate would remain in the other.

Andy said...

I'm still surprised anybody believes they can tackle free will with an experiment.

You can show that decisions are made outside of the reportable bit of awareness. Well what someone claims they experience. And sure, this is interesting. But it's not tackling free will.