The philosopher Thomas Hobbes described free will (or "liberty") as "the absence of all the impediments to action that are not contained in the nature and intrinsical quality of the agent" – which, in plain English, is the ability to act without outside constraints whether that is an overbearing partner or something altogether more whimsical, like fate. 

The scientific definition, however, is far more specific. Essentially, it comes down to two cognitive processes: volition and agency.

Now, thanks to a study recently published in the Proceedings of the National Academy of Sciences, scientists have identified the exact location in the brain responsible for these processes and, therefore, our perception of free will.

Volition is "the desire to act", whereas agency is "the sense of responsibility for our actions". To move or speak we need both and damage to either can cause people to lose their motivation to act, or make them feel as though their behavior is not their own. 

Researchers from Beth Israel Deaconess Medical Center (BIDMC) studied 28 cases where brain injury had affected a patient's volition and left them with no desire to move or speak (akinetic mutism). They examined a further 50 cases where it had damaged agency so that the patients were left feeling as though their movements weren't their own (alien limb syndrome). To find out what exactly was going on, the team used a method called lesion network mapping.

"Lesion network mapping is a recently validated technique that allows scientists to map symptoms caused by brain injury to specific brain networks," Michael Fox, Director of the Laboratory for Brain Network Imaging and Modulation (BIDMC) and an associate professor of neurology at Harvard Medical School, said in a statement. 

Lesion network mapping has been used by scientists in the past to study various types of neurological processes, from abnormal movements to delusions, loss of consciousness, and even criminal behavior.

"In this study, we used this network localization approach to determine the neuroanatomical basis for disordered free will perception," Fox added.

The mapping revealed a diverse range of injury locations but interestingly, all lesions appeared within one of two networks. The injuries related to akinetic mutism were connected to the brain's anterior cingulate cortex (responsible for motivation and planning), whereas almost all injuries (90 percent) related to alien limb syndrome were connected to the brain's precuneus cortex, an area associated with agency. 

To confirm that these two networks are indeed the two areas responsible for free will (as defined by science, not Hobbes), the researchers examined the effect of brain stimulation on free will perception in healthy volunteers and looked at images of the brains of psychiatric patients with abnormal free will perception. Both revealed alterations in the networks that fit with the hypothesis.

While the information is not going to stop moral philosophers and sociologists debating the meaning (and very existence) of free will, the researchers hope it will prove useful when it comes to helping patients with volition and agency-inhibiting injuries.