When you move your body, how do you know that it was “you” who chose to move it? One answer comes from a computational perspective. Your brain builds expectations (known as a “forward model”) about the outcomes of your planned movements, and when sensory information matches these predictions, this suggests your movement was internally generated. But that still leaves the mystery of how you acquire a feeling of subjective ownership. How do you know you willed the movement to happen?
A theory with roots in early nineteenth-century philosophy states that the subjective feeling of effort is crucial to this sense of agency. Toil makes a movement our own. Now a team of experimental psychologists have used modern methods to put this classic idea to the test.
Led by Jelle Demanet and Paul Muhle-Karbe at Ghent University, the clever study took advantage of an implicit marker of volition called “intentional binding”. This is the way the sensory consequences of our voluntary actions are perceived as having occurred closer in time to our action than they really did. If you deliberately flick a switch to make a light come on, intentional binding reduces the delay you perceive between your action and the light onset. Importantly for this study, increased intentional binding would be an indication that a movement felt more volitional.
Thirty-six undergrads (8 men) took part. On each trial they watched a second hand rotating on a clock and pressed the space bar on a keyboard at a time of their choosing. A quarter of a second later, a beep sounded. Depending on the experimental condition, the students’ task was to say what time was on the clock when they pressed the bar or when they heard the beep. Crucially, the students did all this while pulling with their other hand on either a high or low-resistance latex band of the kind that are used for gym work.
Here’s the key finding: When the band was higher resistance – i.e. it took more effort to keep pulling on it – the students showed more evidence of intentional binding. That is, they either perceived the timing of their button clicks as later, or they perceived the beeps as having occurred earlier, as compared with when the band was low resistance. Both these perceptions are signs of increased intentional binding – the sense that the action and its sensory consequence occurred closer together in time. This implies that when the students were exerting more physical effort, they perceived greater wilful control over their button presses, which manifested as increased intentional binding. This is despite the fact that the extra effort was being spent on a different, task-irrelevant movement.
In contrast, the strength of the resistance bands made no difference to students’ judgments about the timing of a button press or a beep when these events occurred on their own (as they did in a number of control trials). This shows the perceptual consequences of greater effort were specific to the intentional binding effect, they didn’t just reduce the students’ all-round accuracy.
The results provide experimental support for the nineteenth-century idea that feelings of effort fuel our sense of agency. Demanet and his colleagues said their findings also build on past research showing that our thoughts too feel more intentional when they require more effort. “In addition, this finding demonstrates the potential utility of using effort manipulations to study abnormal agency attributions in patients with schizophrenia and Parkinson’s disease,” they said.
Demanet J, Muhle-Karbe PS, Lynn MT, Blotenberg I, and Brass M (2013). Power to the will: How exerting physical effort boosts the sense of agency. Cognition, 129 (3), 574-8 PMID: 24060604