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?