Benjamin Libet’s experiment on the neuroscience of free will needs little introduction. (If you do need an introduction, it’s the topic of my latest column for BBC Future). His reports that the subjective feeling of making a choice only come after the brain signals indicating a choice has been made are famous, and have produced controversy ever since they were published in the 1980s.
For a simple experiment, Libet’s paradigm admits to a large number of interpretations, which I think is an important lesson. Here are some common, and less common, critiques of the experiment:
The Disconnect Criticism
The choice required from Libet’s participants was trivial and inconsequential. Moreover, they were specifically told to make the choice without any reason (“let the urge [to move] appear on its own at any time without any pre-planning or concentration on when to act”). A common criticism is that this kind of choice has little to tell us about everyday choices which are considered, consequential or which are actively trying to involve ourselves in.
The timing criticism(s)
Dennett discusses how the original interpretation of the experiment assumes that the choosing self exists at a particular point and at particular time – so, for example, maybe in some central ‘Cartesian Theatre’ in which information from motor cortex and visual cortex come together, but crucially, does not have direct report of (say) the information about timing gathered by the visual cortex. Even in a freely choosing self, there will be timing delays as information on the clock time is ‘connected up’ with information on when the movement decision was made. These, Dennett argues, could produce the result Libet saw without indicating a fatal compromise for free choice.
My spin on this is that the Libet result shows, minimally, that we don’t accurately know the timing of our decisions, but inaccurate judgements about the timing of decisions doesn’t mean that we don’t actually make the decisions themselves that are consequential.
Aaron Schurger and colleagues have a nice paper in which they argue that Libet’s results can be explained by variations in spontaneous activity before actions are taken. They argue that the movement system is constantly experiencing sub-threshold variation in activity, so that at any particular point in time you are more or less close to performing any particular act. Participants in the Libet paradigm, asked to make a spontaneous act, take advantage of this variability – effectively lowering their threshold for action and waiting until the covert fluctuations are large enough to trigger a movement. Importantly, this reading weakens the link between the ‘onset’ of movements and the delayed subjective experience of making a movement. If the movement is triggered by random fluctuations (observable in the rise of the electrode signal) then there isn’t a distinct ‘decision to act’ in the motor system, so we can’t say that the subjective decision to act reliably comes afterwards.
The ‘only deterministic on average’ criticism
The specific electrode signal which is used to time the decision to move in the brain is called the readiness potential (RP). Electrode readings are highly variable, so the onset of the RP is a statistical artefact, produced by averaging over many trials (40 in Libet’s case). This means we lose the ability to detect, trial-by-trial, the relation between the brain activity related to movement and the subjective experience. Libet reports this in his original paper  (‘only the average RP for the whole series could be meaningfully recorded’, p634). On occasion the subjective decision time (which Libet calls W) comes before the time of even the average RP, not after (p635: “instances in which individual W time preceded onset time of averaged RP numbered zero in 26 series [out of 36] – which means that 28% of series saw at least one instance of W occurring before the RP).
The experiment showed strong reliability, but not complete reliability (the difference is described by Libet as ‘generally’ occurring and as being ‘fairly consistent’, p636). What happened next to Libet’s result is a common trick of psychologists. A statistical pattern is discovered and then reality is described as if the pattern is the complete description: “The brain change occurs before the choice”.
Although such generalities are very useful, they are misleading if we forget that they are only averagely true, not always true. I don’t think Libet’s experiment would have the imaginative hold if the result was summarised as “The brain change usually occurs before the choice”.
A consistent, but not universal, pattern in the brain before a choice has the flavour of a prediction, rather than a compulsion. Sure, before we make a choice there are antecedents in the brain – it would be weird if there weren’t – but since these don’t have any necessary consequence for what we choose, so what?
To my mind the demonstration that you can use fMRI to reproduce the Libet effect but with brain signals changing up to 10 seconds before the movement (and an above chance accuracy at predicting the movement made), only reinforces this point. We all believe that the mind has something to do with the brain, so finding patterns in the brain at one point which predict actions in the mind at a later point isn’t surprising. The fMRI result, and perhaps Libet’s experiment, rely as much on our false intuition about dualism as conclusively demonstrating anything new about freewill.
Link: my column Why do we intuitively believe we have free will?
5 thoughts on “Critical strategies for free will experiments”
Suppose I take training that enables me to react as fast as possible if anybody moves aggressively in my vicinity, and kill them. (Some combat schools claim to achieve this: assume for a moment that this is true.)
Somebody approaches fast, and without pause for analysis, I destroy them. Regardless of neural pathways, the choice that killed them was my choice to enter the training. Freedom at the exact moment of action is no more philosophically interesting than my inability to keep my sphincters closed a few hours after a large meal.
I don’t know what it means for choices to be ‘free’, but the existence of a choice process is clear even in small-animal studies. With humans, we can influence the choices they make. Philosophical determinism does not help us do that better.
In my monograph Free Will: A Response to Sam Harris (http://www.amazon.com/Free-Will-Response-Sam-Harris-ebook/dp/B00869S35Q/ref=sr_1_2?ie=UTF8&qid=1438962427&sr=8-2&keywords=keefner) I make one of the same points that Tom makes in this post: Libet asked his subjects to generate random actions, so it’s not surprising that the RP showed up well before the impulse to act.
I also make the point that Tim Poston makes in his comment: The actual decision in the Libet experiment takes place when the subject decides to follow the experimenter’s directions, not when the RP appears or when the subject jerks his wrist.
In addition, the usual interpretation of Libet’s experiment depends on a model of the human person in which consciousness is “sliced off” from the rest of the organism (in a rather Cartesian way) and expected to act on its own, rather than as an aspect of an integrated organism that functions non-consciously and consciously in the same action. In other words, a conscious action is also a brain action and it is artificial to separate them into time slices: Perhaps the “present” for human action lasts for more than 300 milliseconds.
To my mind the determinist interpretation of Libet depends on a Descartes-like disintegration of mind and brain in which consciousness is regarded as an independent entity (albeit a helpless one) rather than as an activity of the organism.
My opinion on the subject:
My website offers a longish essay titled “How to solve ‘free will’ puzzles and overcome limitations of platonic science.” My proposed solution incorporates criticisms stated in the posting above but with additional novel features. The summary of the essay states:
“Free will” puzzles are failed attempts to make freedom fit into forms of science. The failures
seem puzzling because of widespread beliefs that forms of science describe and control
everything. Errors in such beliefs are shown by reconstruction of forms of “platonic science”
that were invented in ancient Greece and that have developed into modern physics. Like Plato’s
Ideas, modern Laws of Physics are said to exercise hegemonic control by means of universal and
invariant principles. Linear expressions and rigid symmetries are abstracted from geometry and
indifference. Processes tied to equilibrium require static surroundings and confine changes to
series of increments. Such forms, based on empty space, fail to describe actual material
transformations that occur during the making of steel or the production of snowflakes. They also
fail to describe muscular movements and related bodily feelings of persons and other animals
that have actual life. Limitations of platonic science are overcome by means of new forms with
the character of time, beginning with “beats” and saccadic, jumpy forms. New technologies of
action and freedom generate and control such temporal forms in proposed device models of
muscles and brains. Development leads to episodic balancing forms, which pass through critical
moments of transformation, resembling those that occur when persons exercise freedom, e.g.,
during a moment of overtaking in a footrace or during a moment of decision by a courtroom jury.
Is there not a problem in applying funny notions to brain research?
‘Free will’ is originally a religious concept, since modified by philosophers to become the opposite of ‘deterministic causation’. Is any scientist ever likely to find that deterministic causation is not the reason for any occurrence? Causation is an assumption of science; and to the extent that chains of causation are not deterministic, they are conceived as random (as in quantum physics).
The Libet experiments seem to me to be somewhere around the beginning of proper research into how consciousness, not ‘free will’, is involved in decision-making. The result that we are on average not conscious of our decisions at the time when they are made would be interesting — though, as some of your criticisms make clear, that result is not necessarily established by these few experiments. Talk of the implications of these experiments for ‘free will’, however, seems to me to be simply confused.