Neural Determinism

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How I bend my finger
Psychological Level

Intercellular Level
  • An upper motor neuron runs from the primary motor cortex of the brain to the spinal column, where it synapses on a lower motor neuron, which runs from the spinal column to the hand, where it synapses on the muscle controlling bending the finger.
  • When I bend my finger …
    • The upper motor neuron fires
    • Which causes the lower motor neuron to fire
    • Which causes the lower motor neuron to release the neurotransmitter acetylcholine across the synapse with the muscle.
    • Which causes the muscle to contract
    • Which causes the finger to bend
Primary Motor Area
    • “One important structure in the frontal lobe is the precentral gyrus, which constitutes the primary motor region of the brain. When parts of the gyrus are electrically stimulated in conscious patients (under local anesthesia), they produce localized movements on the opposite side of the body that are interpreted by the patients as voluntary.”
Neural Level
  • Neuron Components
    • Cell body
    • Dendrites (for input)
    • Axon
    • Axon Terminals (for output)
    • Neurotransmitter
    • Synapse
  • The mission of the neuron is to fire, i.e. to generate an electrical signal that travels from the cell body, down the axon, to the axon terminals.
    • The cause of the nerve impulse is the influx of neurotransmitters at the neuron’s dendrites from upstream neurons.
    • The result of the nerve impulse is the release of neurotransmitters at the axon terminals to downstream neurons.
      • Neurotransmitters are either excitatory (meaning they tend to make the neuron fire) or or inhibitory (meaning they tend to prevent the neuron from firing)

Molecular Level

The key variable in the operation of a neuron is the voltage difference between inside and outside the cell, like the voltage difference between the poles of a battery. The voltage difference is determined by different kinds of channels in the cell membrane that control the flow of charged particles into and out of the cell.

Resting Potential
Action Potential
Neurotransmitter Operation
Ion Channels
A Neuron fires because of a voltage difference
  • A neuron fires because the difference in electric charge between inside and outside the neuron reaches a certain threshold (about -65 mV) due to the influx of neurotransmitters at its dendrites. When that happens the neuron fires. (mV = millivolt =1/1,000 of a volt)
  • Suppose, in particular, that:
    • Neurons A, B, and C all synapse on the dendrites of an upper motor neuron (UMN)
    • No other neurons synapse on UMN’s dendrites
    • Neurons A and B release excitatory neurotransmitters, tending to make UMN fire, while C releases inhibitory neurotransmitter, tending to keep UMN from firing. So:
      • If A and B fire simultaneously and C doesn’t fire, UMN fires.
      • If A, B, and C all fire, UMN doesn’t fire because firing is inhibited by C
      • If A fires and B and C don’t, UMN doesn’t fire because A by itself doesn’t supply enough neurotransmitter to make UMN fire
      • If A and C fire and B doesn’t, UMN doesn’t fire because A and C cancel each other.
Neural Determinism
  • A person has free will to bend or not bend their finger only if they control whether certain neurons fire, e.g. whether UMN fires or not.
  • But just before UMN fires, they have no control over whether A, B and C fire, since those events have already occurred. They thus have no control over whether UMN fires and therefore no control over whether their finger bends.
Deterministic Network of Neurons
  • Nodes are neurons
  • Arrow means synapses on
  • Root node (with no arrow pointing away) fires because of its three presynaptic neurons.
  • Those neurons fire because of their presynaptic neurons. And so on.
  • Leaf nodes (with no arrows pointing to them) fire because of sensory organs

Predicting Voluntary Actions
  • One kind of support for Neural Determinism would be experiments in which scientists predict voluntary actions based on prior brain activity.
  • Several academic papers have reported such predictions
    • “Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act,” Brain 106, Sept 1983, by Libet B, Gleason CA, Wright EW, Pearl DK
    • “Unconscious determinants of free decisions in the human brain,” Nature Neuroscience, April 2008, by Chun Siong Soon, Marcel Brass, Hans-Jochen Heinze, John-Dylan Haynes 
    • “Internally generated preactivation of single neurons in human medial frontal cortex predicts volition,” Neuron, Feb 2011, by Fried I1, Mukamel R, Kreiman G.
  • Unconscious determinants of free decisions in the human brain, Nature Neuroscience, April 2008, by Chun Siong Soon, Marcel Brass, Hans-Jochen Heinze, John-Dylan Haynes
    • There has been a long controversy as to whether subjectively ‘free’ decisions are determined by brain activity ahead of time. We found that the outcome of a decision can be encoded in brain activity of prefrontal and parietal cortex up to 10 s before it enters awareness. This delay presumably reflects the operation of a network of high-level control areas that begin to prepare an upcoming decision long before it enters awareness.
    • The conscious decision to push the button was made about a second before the actual act, but the team discovered that a pattern of brain activity seemed to predict that decision by as many as seven seconds. Long before the subjects were even aware of making a choice, it seems, their brains had already decided.
    • “The will has also recently become a target of empirical study in neuroscience and cognitive psychology. Benjamin Libet (2002) conducted experiments designed to determine the timing of conscious willings or decisions to act in relation to brain activity associated with the physical initiation of behavior. Interpretation of the results is highly controversial. Libet himself concludes that the studies provide strong evidence that actions are already underway shortly before the agent wills to do it. As a result, we do not consciously initiate our actions, though he suggests that we might nonetheless retain the ability to veto actions that are initiated by unconscious psychological structures. Wegner (2002) amasses a range of studies (including those of Libet) to argue that the notion that human actions are ever initiated by their own conscious willings is simply a deeply-entrenched illusion and proceeds to offer an hypothesis concerning the reason this illusion is generated within our cognitive systems. Mele (2009) and O’Connor (2009b) argue that the data adduced by Libet, Wegner, and others wholly fail to support their revisionary conclusions.”