I’ve written quite a bit on experiments showing that one can, using brain scans, predict “decisions” before the human subject is conscious of having made them. These decisions include either things like deciding when to press a button, or “choice” experiments in which you decide to add or subtract, or to press a button with your right or left hand. In the “choice” cases, brain scans (EEGs or fMRIs) can predict which choice will be made with significant but not perfect accuracy (about 60-70%), but in some cases those predictions can be made up to 7 seconds before the subject is conscious of having made a decision.
The first one of these studies was published in 1985 by Benjamin Libet, who showed that a “readiness potential” (RP) for pressing a button could be seen in the brain about a third of a second (300 milliseconds) before the subjects were conscious of having decided to do the press. Since then the decisions have become more complex, the brain scans more refined, and the time of “readiness potential” pushed farther and farther back.
These results won’t surprise any determinists or even free-will compatibilists, who all agree that our decisions are made not by some spooky “will,” but by the laws of physics. And of course we all know of “decisions” we make that appear to derive from our unconscious (e.g., driving a well-travelled route, where you don’t think to yourself “turn here”, but where you seem to be operating on autopilot). But these brain-scan results are distressing to dualists and to those who believe in the religious (libertarian) form of free will, in which decisions are made by something detached from the physical brain.
The implications of these studies—that decisions can precede consciousness of having made them—even disturbed Libet, who, though admitting that his studies did cast doubt on “free will”, still opted for something dualistic: “free won’t.” That is, although one’s decision to do something might be decided in the brain before coming to consciousness, there was still a form of dualism in the decision to cancel or override one’s action.
That doesn’t make much sense, since cancellation is still something that takes place in the brain. If you think about it for a minute, you can see that canceling or overriding a decision can in fact derive from simialar physical and neural antecedents as making a decision itself. That is, there’s no substantive difference between deciding to do something and then deciding not to do it. After all, both are decisions, and both might be predictable in advance by brain scans. I find this whole area of research fascinating because of its implications for how we make “decisions.”
A new paper in Proc. Nat. Acad. Sci.by Matthias Schultze-Kraft et al. (free download, full reference at bottom) investigates how the brain works when it cancels a decision. Although it doesn’t show the neural basis for canceling decisions, it does show that if you’ve made a decision to do something, and then are asked to cancel it, there’s a “point of no return” after which you simply can’t cancel it.
The experiment is a very complicated one, with lots of controls, training of computers and investigators, and analyses, so I’ll briefly describe the salient results. My apologies if I get some of this wrong.
Subjects were shown a green light, and then asked to push a button with their foot after counting (to themselves) two seconds after they saw the green light. An electromyogram (EMG) was connected to the button-pushing leg to detect when movement began. And an electroencephalogram (EEG) was connected to the head to monitor brain activity. (The average time to press the button after the green light went on was 5.4 seconds.)
After some human examination of the EEG’s, these brain readouts were analyzed and then programmed so that the scans themselves would flash a red light when the computer detected that the subject had started the “readiness potential” in the brain to push the button. The subject would then get “points” (towards a reward, I presume) if, after seeing the red light, they managed to NOT press the button. In other words, the subjects were asked to cancel a movement whose processing had already begun in the brain, but which had not yet produced a movement.
The readiness potential in the brain began about one second before the muscles gave an EMG reading from the leg muscle, and there was another 0.3 seconds before the button was actually pressed. The computer was trained for each subject based on their observed RPs, and when the RP crossed a threshold, the computer program turned on the red light, telling the subject “DO NOT PRESS BUTTON!”
Because of variations in threshold crossing and onset of an individual’s RP, the light went on at various times before the EMG lit up and before the subject pressed the button. Sometimes the red light didn’t go on, and subjects pressed the button. But sometimes the red light did go on but they still pressed the button, giving us the Big Result:
If the red light went on 200 milliseconds or less before movement began, subjects could not help starting their move toward pressing the button.
In other words, there’s a “point of no return” that occurs about 0.8 sec after the RP has started (but before the muscles move), after which—even if the subject sees the red light—he/she cannot help but move. Now sometimes they can still avoid pressing the button itself, but their leg is still moving towards it.
What does this mean? Well, it doesn’t show that there’s “free won’t”. After all, the subjects are cancelling their movement (the “won’t”) as a reaction to seeing a light: an environmental stimulus rather than some conscious “decision”. What it does say is that there appear to be physical constraints in cancelling a decision, so that even if you “want” to to get your reward, you can’t. Now the constraint, I think, is likely to be the reaction time to the red light: that is, there’s a certain time you need to see the light, process the information in your brain, and then use it to send a signal to your leg to stop moving; and that time is about 200 milliseconds. In other words, you could still have “free won’t,” but this experiment says little about it. In fact, I’m not sure that this experiment CAN say anything about “free won’t”, since you are not making a “conscious” cancellation but are told to cancel in response to a light. But what it does show is that what is determined by unconscious brain activity is reversible by an external stimulus.
What would truly refute the notion of “free won’t” is the demonstration that cancellation of a movement itself previously decided and predicted by brain activity can show up as a brain signal (i.e., the cancellation can be predicted) before you’re conscious of it. The authors report two studies of “spontaneous self-cancellation”, and one of them (Brass et al., J. Neurosci. 27:9141 [2007]) might indeed give evidence against “free won’t”, but I haven’t read it. Perhaps readers can and report back. But since cancellation is a brain output qualitatively similar to an “action” decision, I can’t imagine why there wouldn’t be libertarian free will but could be libertarian “free won’t”.
The authors of this paper themselves don’t appear to accept dualistic free will or free won’t (“free won’t”, of course, is just a form of free will), as is clear from their discussion. As you see below, they discuss their results in terms of naturalistic, materialistic brain phenomena, with cancellation associated with specific brain regions. Here’s an excerpt from the paper (I’ve left out the references, but you can see them in the paper). Note how they avoid discussing “free will”, though the senior author, John-Dylan Haynes, said in an interview that he doesn’t think any of these experiments support the idea of free will.
It has been previously reported that subjects are able to spontaneously cancel self-initiated movements. This has been referred to as a “veto”. The possibility of a veto has played an important role in the debate about free will, which will not be discussed further here. Note that the original interpretation of the veto was dualistic, whereas in our case veto is meant akin to “cancellation.” Our study did not directly address the question of which cortical regions mediate the cancellation of a prepared movement. However, many previous studies have investigated the neural mechanisms that underlie inhibition of responses based on externally presented stop signals. Please note that, in contrast to stop signal studies, in our case the initial decision to move was not externally but internally triggered. Conceptually, this could be compared with a race between an internal go signal and an external stop signal. Many stop signal studies have reported that inhibition of a planned movement is accompanied by neural activity in multiple prefrontal regions, predominantly in right inferior PFC . It has been proposed that right inferior PFC [pre-frontal cortex] acts like a brake that can inhibit movements both based on external stimuli or on internal processes such as goals. Another region that has been proposed to be involved in movement inhibition is medial PFC; however, its role is more controversial. On the one hand, stop signal studies show that activity in medial PFC might not directly reflect inhibition. However, it seems to be involved in cancelling movements based on spontaneous and endogenous decisions rather than based on external stop signals.
At least one article (in Gizmodo) has suggested that this study gives some evidence for dualistic, libertarian free will, arguing that “the ‘readiness potential’ doesn’t govern our brain.” But I don’t think this study gives any solace to advocates of libertarian free will. All it shows is that a decision made by the brain, and later arriving at consciousness, can be halted by an external stimulus that also impinges on the brain. That’s exactly what we predict from the notion that the brain is a computer, that consciousness is an epiphenomenon that often follows a brain’s “decision”, and that we can affect the working of the brain by changing the environment of the brain-owner.
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Schultze-Kraft, M. D. Birman, M. Rusconi, C. Allefeld, K. Görgen, S. Dähne, B. Blankertz, and J.-D. Haynes. 2015. The point of no return in vetoing self-initiated movements. Proceedings of the National Academy of Sciences., early edition.
