Correlates of volition

Let’s go through this again. A Patrick Haggard paper gives us the bare list. The Libet paper from 1983 was the first evidence that decisions are not made consciously.

In this experiment, participants are asked to make a simple voluntary action, such as a key press, whenever they feel like it. Brain activity is measured throughout, originally using EEG (Lau repeated using fMRI). At the same time, they observe a rotating clock hand and are asked to note the position of the clock when they first experience the conscious intention, or ‘‘feel the urge,’’ to press the key. This hotly debated marker of volition is referred to as W (judgment of will, following Libet’s terminology). EEG activity over frontal motor areas began 1 s or more before movement (the so-called ‘‘readiness potential’’), while W occurred much later, a few hundred ms before movement itself. Although the Libet experiment was published almost 30 years ago, it is still serves as a nexus in the neuroscience of volition.

 

EEG blurs space and fMRI blurs time at some extent. Recording from single neurons is very precise in both time and space. This technique has been used on primates and on humans with implanted electrodes in preparation for epilepsy surgery. Fried in 2011 repeated the Libet experiment while recording from electrodes implanted in the medial frontal lobes of 12 epileptic patients.

These areas generate the scalp readiness potentials recorded prior to voluntary movement. Moreover, stimulation of these areas has been reported to generate a feeling of urge to move a particular body part, without necessarily causing actual movement . Therefore, direct recordings from medial frontal neurons are an important part of the puzzle of the neuroscience of will. .. A relatively small subset of medial frontal neurons showed a gradual ramp-like increase in firing rate before movement that recalls both EEG readiness potentials … The time of conscious intention could be predicted from small subpopulations of these neurons, using an integrate-and-fire model, well before the time that participants reported the experience of volition. …these data give the impression that conscious intention is just a subjective corollary of an action being about to occur. Such models agree with previous accounts that voluntary actions begin unconsciously and enter into our conscious experience only when medial frontal activity has reached a given threshold level of activity… the current work is in broad agreement with a general trend in neuroscience of volition: although we may experience that our conscious decisions and thoughts cause our actions, these experiences are in fact based on readouts of brain activity in a network of brain areas that control voluntary action.

 

Recording from the supplementary and the pre-supplementary motor area has added to the picture in a surprising way.

SMA proper contained relatively more neurons active before W than did the pre-SMA. In contrast, rather few SMA proper neurons were active in the brief interval between W and movement onset relative to the pre-SMA… This finding suggests a revision of how we interpret the W judgment. It is clearly wrong to think of W as a prior intention, located at the very earliest moment of decision in an extended action chain. Rather, W seems to mark an intention-in-action, quite closely linked to action execution. The experience of conscious intention may correspond to the point at which the brain transforms a prior plan into a motor act through changes in activity of SMA proper.

 

Another surprise was that some medial frontal neurons decrease firing leading up to W. They seem to be holding back action until the right action is prepared and the moment is right. This weakens any proposal of a conscious ‘free-wont’.

…there are interesting differences between the areas recorded, with decreasing neurons being more common than increasing neurons in the rostral anterior cingulate and also in the pre-SMA. The function of decreasing neurons remains unclear. …it is tempting to take decreasing neurons as evidence for an intrinsically inhibitory component of volition. Several classes of evidence suggest that suppression of action and voluntary initiation are profoundly linked in the medial frontal cortex…Decreasing neurons might withhold actions until they become appropriate through tonic inhibition and then help to trigger voluntary actions by gradually removing this tonic inhibition. Competitive inhibitory interaction between decreasing and increasing neurons could then provide a circuit for resolving whether to act or withhold action. …A similar model has already been proposed for decisions between alternative stimulus-driven actions in lateral premotor cortex (Cisek 2007).

 

This is far from a complete picture of how we act. It will take many more experiments to work out a proper model. But there is clear evidence in favour of the idea that decisions are not taken consciously but rather enter our awareness already taken. But this does not mean that we do not make decisions, we do, but just not make them consciously. If I say that I make decisions, how is this to be interpreted? Who is the ‘I’? It would be simpler if there was agreement on this. If ‘I’ is connected only to consciousness, then ‘I’ do not make decisions or much of anything for that matter. If ‘I’ is a whole unified brain then ‘I’ do make decisions. And this I, me, your truly, this I makes decisions with a unified brain.

 

ResearchBlogging.org

Patrick Haggard (2011). Decision Time for Free Will Neuron DOI: 10.1016/j.neuron.2011.01.028

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