Alien limb syndrome isn’t as extraterrestrial as it sounds—but it’s still pretty freaky. Patients complain that one of their hands has gone “rogue,” reaching for things without their knowledge.
“They sit on their hand trying to get it not to move,” says Ryan Darby, a neurologist and neuroscientist at Vanderbilt University in Nashville. “They’re not crazy. They know there’s not something controlling their arm, that it’s not possessed. But they really feel like they don’t have control.”
Now, a study analyzing the locations of brain lesions in these patients—and those who have akinetic mutism, in which people can scratch an itch and chew food placed into their mouths without being aware they’ve initiated these movements—are shedding light on how our brains know what’s going on with our bodies.
The work shows how neuroscience is beginning to approach elements of the biological nature of free will.
“I think it's really nice work, carefully done and thoughtfully presented,” says Kevin Mitchell, a neurogeneticist at Trinity College in Dublin who studies perception and who wasn’t involved in the study.
Philosophers have wrestled with questions of free will—that is, whether we are active drivers or passive observers of our decisions—for millennia. Neuroscientists tap-dance around it, asking instead why most of us feel like we have free will. They do this by looking at rare cases in which people seem to have lost it.
Patients with both alien limb syndrome and akinetic mutism have lesions in their brains, but there doesn’t seem to be a consistent pattern. So Darby and his colleagues turned to a relatively new technique known as lesion network mapping.
They combed the literature for brain imaging studies of both types of patients and mapped out all of their reported brain lesions. Then they plotted those lesions onto maps of brain regions that reliably activate together at the same time, better known as brain networks. Although the individual lesions in patients with the rare movement disorders appeared to occur without rhyme or reason, the team found, those seemingly arbitrary locations fell within distinct brain networks.
The researchers compared their results with those from people who lost some voluntary movement after receiving temporary brain stimulation, which uses low-voltage electrodes or targeted magnetic fields to temporarily “knock offline” brain regions.
The networks that caused loss of voluntary movement or agency in those studies matched Darby and colleagues’ new lesion networks. This suggests these networks are involved in voluntary movement and the perception that we’re in control of, and responsible for, our actions, the researchers report today in the Proceedings of the National Academy of Sciences.
In the patients with akinetic mutism, overlaps between their damaged networks “peaked” in a region of the brain called the anterior cingulate cortex, which is involved in voluntary movement. In patients with alien limb syndrome, the team found some overlap within the temporoparietal junction, a brain region strongly implicated in self-awareness and agency. But the peak overlap actually fell in a little-studied region called the precuneus, which has also been linked to self-awareness.
Thus, regardless of whether we actually have free will, researchers are beginning to localize the areas that make us feel like we do.
Mitchell says the findings are intriguing, but cautions that just because lesions can reveal the brain networks implicated in cognitive functions such as volition or agency, it doesn’t mean researchers understand what’s truly necessary for those functions and what isn’t. “If you remove the steering wheel of your car, you're going to have trouble steering it, obviously,” he says. “But if you just have a steering wheel, you're not going to have much luck steering either.”
Darby is careful to note that although volition and agency are elements related to free will, the new study doesn’t attempt to answer the overarching question of whether the human brain is independently responsible for its decisions and actions.
Yet it’s still “a very welcome paper,” says Patrick Haggard, a neuroscientist at University College London who has grappled with questions of free will. “The ability to decide and take action is fundamental to who a person is and how our society works,” he says. “For a long time, that was thought to be inaccessible to neuroscience. … This study is a good example that shows neuroscience is beginning to tackle how we act.”