Ever had the question, “If you don’t believe in God, how do you explain ‘free will’?” The question itself presupposes that the asker actually knows what ‘free will’ is. Indeed, just as we have a strong perception of the dual nature of humanity, we have this strong perception that we have ‘free will’. Long considered the domain of theology (and later philosophy), science has been reluctant to answer questions regarding ‘free will’. I’m not one to take philosophy in the absence of evidence in support seriously at all. My mantra has always been “Show me the evidence, or go home!” I don’t give philosophy alone the time of day. Some, like William Lane Craig, consider philosophical arguments as the gold standard and that it is up to others to prove his philosophy wrong. Phah! That’s just a burden of proof shift. It’s up to him to provide evidentiary support for his arguments. Craig is a good debater, but as a philosopher he’s a failure. How much of a failure? When I mentioned his name to a colleague at a CFI meeting, a member of the faculty of philosophy at the local U, her response was delicious: “Who?” (I had to put a dig in there on Craig. Anyone who can justify genocide is going to get hounded for life from me.)
But perceptions can be deceiving, and the perception of ‘free will’ is no exception. We have this belief that we control our actions which are initiated by conscious decision making processes. But, at the risk of lapsing into philosophy, consider what would happen without a huge amount of background processing by the brain. We would all be paralyzed into inaction by the sheer number of choices we would make in actions we don’t even consciously think about. The act of walking in bipedal fashion is a marvel of real-time sensory feedback processing only now being duplicated in robotics (and not all that well, either). Why on Earth would we be so arrogant as to believe that decision making isn’t just as automated?
So, it’s no surprise that the neurosciences are showing us that things aren’t as they seem. Especially when we trust our perceptions and draw conclusions from them alone. (Alvin Plantinga (unlike Craig, a real philosopher….) seems to miss this point in his Evolutionary Argument Against Naturalism, to which I will get to at a later date, misses this point entirely.) The ‘free will’ concept seems to be strongly tied to mind-brain dualism, a concept I give short shrift to in another blog post. The evidence not only fails to support dualism, it screams that it is just flat wrong.
Volition, which is what we are really talking about when discussing ‘free will’ (and hence, I will be using this term), lies on a continuum with voluntary action at one end of the spectrum and reflex action at the other. Reflexes are immediate motor responses, while voluntary action is defined as ‘freedom from immediacy’1. It is this independence from stimulus that makes volition a difficult nut to crack experimentally.
One of the earliest experiments in volition showed some surprising things about volition that don’t jibe with the classical concept of ‘free will’. Libet2 had subjects (wired with electroencephalography (EEG) electrodes on the scalp to measure brain electrical activity) watch a clock and at a time of their own choosing move their right hand, reporting at what position the clock hand was when they first ‘felt the urge’ to move their hand. This was then compared with the results from EEG measurements. The ‘urge’ to move their hand occurred about 200 ms prior to muscle movement. But the electrical activity in certain brain regions began at least one second before hand movement! Thus, brain activity is the cause of conscious intention and not the other way round as one would expect from classical concepts of ‘free will’. This discrepancy in the timing of electrical activity Libet called the ‘readiness potential’.
There have been criticisms of this study (the results of which have been replicated, like all good science), but none are all that convincing. Some I’m sure are from philosophers. For instance, one criticism is that the ‘true’ voluntary act in this experiment was to be a participant in the first place. Bah! All voluntary actions are contingent upon previous decisions, so there is no way to avoid an infinite regress. That a decision you voluntarily make now is the result of a decision you made previously does not at all change the fact that they were all decisions. Regardless, this experiment was a watershed in the study of decision making and voluntary action.
So, how does one go about studying volition experimentally? Typically, the experimenter gives the subject a stimulus and the response is measured. But this isn’t a very good paradigm since true volition is independent of stimulus. To get round this issue, experimenters provide a stimulus or instruction which only partly determines what the subject should do in one of the following manners:
- the subject performs a fixed action, but chooses when to do so;
- the subject performs an action at a specific time but chooses amongst several possibilities;
- the subject chooses whether or not to perform an action.
But such experimental designs fail in their ability to study the context of decision making, so these paradigms greatly limit our ability to understand volition. In none of these designs is there any value which motivates the subject to make their choices. But they do tell us something about the computation of decision, the generation of the information needed to perform the action by the subjects themselves.
Where do we go from here? Looking at the difference between voluntary and reflex actions, the contrast can point us in experimental directions. Voluntary actions primarily involve the cerebral cortex; reflexes the spinal cord. Volition matures late in development; reflexes before birth. Volition induces the perceptions of ‘intention’ and ‘agency’. The former is the feeling that we have planned to perform an action and the latter is the feeling that the action performed causes an external event. Neither perception is produced by reflex action.
To see how this might be useful, I’ll use the same example as that in reference 3. A subject may experience the feeling of intent in flipping a light switch. When the light turns on (the external event), the subject attributes this to the action of flipping the switch. In a reflex action, the jerk caused by a doctor using a mallet on your knee during a physical, the feelings of intent and agency are totally absent. It isn’t hard to see that most actions are a mixture of the two. For instance, we easily learn to associate motor responses to various stimuli. Crossing the street at a traffic light, for instance. We go when the light turns green. But crossing the road depends on whether we want to do so or if we have a reason to do so, and so even this situation has a variety of voluntary/reflex action mixes.
So, where are the brain regions responsible for voluntary actions? Several regions of the cerebral cortex have been identified which provide the circuits for voluntary action. These include (figure from reference 4):
- the premotor cortex, which prepares commands for voluntary actions;
- the presupplementary motor area (preSMA), which prepares commands for internally generated “intentional” actions;
- the primary motor cortex (M1), which executes motor commands from the premotor cortex and presupplementary motor area (as well as the basal ganglia and prefrontal cortex) by sending signals to the spinal cord and muscles. As such, it is considered the final common path for voluntary action;
- the parietal cortex, which is given copies of commands prepared by the premotor cortex and presupplementary motor area and computes predicted sensory consequences of the action;
- the inferior part of the posterior parietal cortex, which generates the sensory representations of the predicted consequences of the action.
Both functional magnetic resonance imaging (fMRI) and EEG have demonstrated that neuronal activation for manual actions performed at a time of the subject’s own choosing and in reaction to an external stimulus are quite similar, but the former situation demonstrated a stronger activation in the preSMA. The readiness potential that Libet observed has been located and is indeed due to activity in the preSMA. The view is that the readiness potential initiates a cascade of neuronal activity that moves from the preSMA back to the SMA and on to M1, resulting in movement.
Is this one second or so of readiness potential the earliest ? Not necessarily. Electrodes implanted in the basal ganglia demonstrate that a voluntary action can be predicted up to 2 seconds prior to movement5. One fMRI study had subjects chose between a right and left hand action during scanning. Areas of the prefrontal cortex showing activation could be used to predict which hand would be chosen up to 8 seconds prior to the action6.
In this first installment of the science of ‘free will’ I have shown that there is a definite disconnect between the usual idea of how ‘free will’ works and what neuroscience is telling us. At the very least the concept of ‘free will’ bound with mind-brain duality must be abandoned. I always view things like ‘free will’, ‘soul’, ‘mind-separate-from-brain’, etc. as early explanations for phenomena related to consciousness. As such, they are early science. While fact that they are old ideas in and of itself does not invalidate them, new data contradicting them does. As a scientist I must discard them and search for new explanations which do fit the data. In other installments I will be discussing voluntary action in terms of decision making and what neuroimaging (fMRI in particular) has to say. A further installment will discuss how this relates to the computational model of the brain so ably discussed by Stephen Pinker in his book How the Mind Works7.
- Shadlen MN, Gold JI. In The Cognitive Neurosciences, 3rd edition, Gazzaniga MS ed. MIT Press. 1229-41 (2004)
- Libet B, Gleason CA, Wright EW, Pearl DK. “Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act. Brain 106:623-42 (1983)
- Haggard P. “Human volition: towards a neuroscience of will.” Nat Rev Neurosci 9:934-46 (2008)
- Haggard P. “The sources of human volition.” Science 324:731-33 (2009)
- Loukas C, Brown P. “Online prediction of self-paced hand movements from subthalamic activity using neural networks in Parkinson’s disease.” J Neurosci Methods 137:193-205 (2004)
- Soon CS, Brass M, Heinze HJ, Haynes JD. “Unconscious determinants of free decisions in the human brain.” Nature Neurosci 11:543-45 (2008)
- Pinker, S. In How the Mind Works, reissue edition. W.W. Norton & Co. (2009)