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Readiness Potential (RP) is a measure of activity in the motor cortex and supplementary motor area of the brain, leading up to voluntary muscle movement, which was first recorded and reported in 1964 by Hans Helmut Kornhuber and Lüder Deecke.

Soon et al. (2008) performed a similar experiment using fMRI which showed that the conscious decision a person makes between pressing a left or a right button is encoded in brain activity up to 10 seconds before that decision

Only 100 ms is available in which the conscious function might affect the final outcome of the volitional process (the primary motor cortex needs some time to the spinal motor nerve cells). This is enough time for stopping or voting the final progress of the volitional process.

Subjects in Libet's experiment reported that in some cases a conscious wish appeared but that they suppressed or vetoed that. Libet have shown that in such cases a large RP proceeded the veto, signifying that the subject was indeed preparing the act, even thought the action was aborted by the subject.

And what if the conscious veto have a preceding unconscious origin? In this case we cannot consider the veto as action of free will. Libet proposes, instead, that the conscious veto may not require preceding unconscious processes.

Psychological implication: Consciousness is not a high level authority that gives orders to subordinated instances. Instead, it's main role is a selective one: make a decision between the bulk of possibilities that are proposed by unconscious processes.

Criticism of Libet's Experiment

Another claim is that Libet’s experiment involves an attention shift from the participants’ subjective intention to the clock, which may have introduced temporal mismatches between the felt experience of will and the perceived position of the clock hand.

soon et al. (remember?) found that "decision can be encoded in brain activity of prefrontal and parietal cortex almost 10 seconds before it enters awareness". That's makes the debate about few hundreds of ms a bit silly...

But... the activity of prefrontal and parietal cortex was correlated with the decision (to use the left or right hand) with 60% prediction accuracy. If the activity indeed reflects the decision, the accuracy should be 100%!

Libet's claim ("our brains initiate conscious voluntary movements as well as the will to move before we are consciously aware of the will to move") assumes:

(1) the RP reflect a neural ‘decision’ to move.

(2) the neural activity causes both (a) the will to move and (b) the movement itself.

But...

Even if (2) is true, (1) requires further explanation:

the decision must be caused by a chain of preceding neural events, and the RP might reflect some of these, rather then the actual neural decision.

The timing argument focuses on the RP’s onset (the earliest time of RP), which originates in the SMA (Supplementary Motor Area).

The SMA has been known to be strongly activated when subjects imagine a complex movement without actually performing it, so it cannot be easily assumed that the earliest part of the RP necessarily reflects neural processes underlying a decision to move.

Judging the time of awareness

http://tinyurl.com/libetclock

TIME OF CONSCIOUS INTENTION TO ACT IN RELATION TO ONSET OF CEREBRAL ACTIVITY (READINESS - POTENTIAL)

What does RP really reflect?

Libet, B., Gleason, C. A., Wright, E. W., & Pearl, D. K. (1983)

Brain, 106(3), 623-642.

Generally speaking

And some more details...

Libet's Conclusion regarding Veto Effect

The Readiness Potential

RP preceded the conscious intention to act by 350, and the actual movement by 500 milliseconds

Subjects made a quick flexion of the fingers and/or wrist of their right hand whenever they felt the internal intention to do so.

Slow negative wave that precedes a voluntary movement

The Readiness Potential as an indicator of the neural preparation for movement.

Begins 1-1.5s before movement

Maximally negative at vertex (Cz)

conscious intention

neural processes

Let's talk Philosophy

Conscious will ≠ Free will

Usually when we use the term free will we refer to the choices among a variety of options, often with moral implications, and this may require careful deliberation over a period of time.

That's not what happens in Libet's Experiment!

  • No moral decision.
  • No decision whether to move, but only when.
  • The subjects were specifically instructed not to deliberate but to act spontaneously.

Thank you for your attention!

The big question:

If so,

Do we have free will?

It's not easy to judge the precise time of the decision to move the finger/wrist.

Try it

Yourselves

Therefore, researchers have instead measured the timing judgements for perceptual events (visual/auditory stimulus), which is easier to do (Danquah et al.). Results have been variable, but several groups found serious biases.

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