Free Will

Psy391D - Class 10

(This page last updated 13 October, 2004.)

• What is free will?
• What does it mean to have free will?
• Do we have it, or is it just an illusion?

Libet's Experiment on Timing the Decision to Act

Electroencephalograph (EEG)

• When a neuron fires, it produces changes in electrical potential.
• When many neurons fire together, the changes are large enough that they can be measured outside the skull.
• Electrodes placed on surface of scalp.
• EEG provides good temporal resolution (When things happen).
• Not so good for spatial resolution (Where things happen).

Event Relate Potential (ERPs)

• One way to use EEG experimentally is to measure brain potentials in relation to a specific event.
  • Presentation of a stimulus
  • Motor response
• EEG waveforms from many different trials can be averaged together to produce a clearer signal.

Readiness Potential (RP)

• Rise in negative potential over motor cortex before a movement.
• Begins ~800 msec before movement begins.

Which happens first, the readiness potential, or the will to move?

Timing the Decision to Move

• Dot moves around clock about once every 2 seconds.
• Subject moves whenever they want.
• They not the position of dot at the instant they decide to move.

Measurements taken during Libet's experiment.

• EEG measures readiness potential
• EMG measures muscle activity
• Subject uses clock to time decision to move.

Results

• On average, the RP appears 550 msec before the movement begins.
  • If the subject is contemplating the move long before executing it, the RP may be 100 msec before movement.
• The decision to move, as timed with the moving-dot clock, is only 200 msec before the movement.
• The RP begins well before the subjects report deciding to move.

Three Alternative Explanations

• The conscious decision to move is an illusion.
  • By the time you are aware of the decision, the mental processes for movement are well underway.
• We have conscious veto over decisions.
  • The plan to move begins outside of awareness.
  • The act cannot be carried out unless you consciously decide to let it proceed.
• Dennett's Alternative: There is no Cartesian Theater
  • Thus, there is no one moment at which the decision to move enters awareness.
  • Fruitless to try to measure the specific time at which awareness of an event occurs.

Relevant Websites

website in Germany describing Libet's experiments

The Julian Jaynes Society

Next class: Evolution

Psych 391D: Consciousness Kyle Cave Psychology Dept. U. Mass.