By Emma Young
While there’s still a debate about whether we have free will or not, most researchers at least agree that we feel as if we do. That perception is often considered to have two elements: a sense of having decided to act – called “volition”; and feeling that that decision was our own – having “agency”.
Now in a paper in PNAS, Ryan Darby at Vanderbilt University Medical Center and colleagues have used a new technique – lesion network mapping – to identify for the first time the brain networks that underlie our feelings of volition and for agency. “Together, these networks may underlie our perception of free will, with implications for neuropsychiatric diseases in which these processes are impaired,” the researchers write.
Darby and his colleagues first mapped the locations of lesions (brain damage) in 28 people with impaired volition (including people with akinetic mutism, for example, who seem to have lost the motivation to move or speak) and in 50 people with an impaired sense of agency (such as people with so-called “alien-hand syndrome” who feel that they are not in control of their own limb movements).
Then they looked at which other regions in the brain these various damaged locations typically communicate with (for this they referred to a connectivity map based on scans of 1000 healthy people), to investigate whether or not they all form part of the same network.
For the patients with disordered volition, while their lesions were in different locations in the brain, all 28 were part of a single brain network that connected to the anterior cingulate cortex (ACC), a part of the brain already understood by neuroscientists to be involved in the motivation, planning and control of voluntary movements.
Meanwhile, for the people whose brain lesions were causing disordered agency, again, “while the lesions themselves were spatially diverse, mapping showed that… 90 per cent fell within a single brain network defined by functional connectivity to the precuneus cortex,” the researchers write. The precuneus has previously been shown to play a role in having a sense of agency.
The researchers also looked at lesion connectivity in people with partial paralysis but who retained a sense of volition, and people who made involuntary movements but who continued to feel that they were responsible for these movements (so who had a sense of agency). And they considered the results of studies that have found free will perception can be altered by either electrical or magnetic brain stimulation to particular regions. The results of both types of study supported their identification of the ACC-connected network and the precuneus-connected network as being related to volition and to agency, respectively.
Next, the researchers looked at brain imaging studies of people with psychiatric disorders that have been conceptualised – at least by some researchers – as disorders of free will (affecting volition, agency or both). This includes patients with catatonia, who don’t move or respond though they look to be awake, and cases of psychogenic non-epileptic seizures (PNES), which resemble epileptic seizures, but which do not involve the characteristic electrical discharges in the brain. Though these patients did not have specific brain lesions, the researchers looked for any brain regions where there was reduced function and in 85 per cent of cases these were part of the volition and/or agency networks.
So, perhaps damage to a node on the agency or volition network affects the functioning of the precuneus or the ACC, the researchers write – or perhaps volition and agency depend on all nodes in their networks working well. Either, or both, could be the case, and could help guide future investigations, diagnosis and therapeutic interventions.
There are some important caveats in relation to this research, however. One of the biggest questions has to be around the interpretation of some of the neuropsychiatric disorders. For instance, it’s unclear exactly what’s going wrong in people with akinetic mutism and catatonia. Have they really lost the will to move or speak – their “volition”, as the researchers interpreted it – or is it rather the case that they experience problems with initiating a movement that they actually want to make? In which case, perhaps what the researchers have identified as a “volition network” is actually more about movement execution than volition (this would be consistent with prior research that’s found the ACC – which remember is a key part of the purported “volition network” – is involved in the planning and control of movements as well as the motivation to make them).
Also, as the researchers note, even if they have identified a volition network, it may not relate to free will as it’s commonly understood – in terms of social, legal and moral responsibility for decisions. “It remains unknown whether the network of brain regions we identify as related to free will for movements is the same as that important for moral decision making, as prior studies have suggested important differences.”
Scholarly debate about free will has been raging for at least a few thousand years. It’s certainly not over yet.