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Choosing between going out for a run or staying slumped on your sofa can be tricky, but it turns out your eyes can reveal your decision before you have even made it.

When we do something that requires physical effort, our pupils can dilate and activity heightens in the prefrontal cortex, the part of the brain involved in cognition. Now, it seems that these two reactions may also guide our decisions about activities that we have yet to carry out.

To investigate this idea, Irma Kurniawan and her colleagues at the University of Zurich in Switzerland asked 49 people to choose between different tasks that varied by level of effort.

The researchers first trained participants to do hand squeezes using a handheld device at varying degrees of physical difficulty. Each person was then placed inside a functional MRI scanner to record their brain activity while an eye-tracker also monitored their pupil size.

While in the scanner, participants were asked to choose between doing more strenuous or effortless hand contractions later on, with a greater cash reward for choosing the more difficult exercises. Once outside, 30 minutes to an hour later, they completed a random selection of hand squeezes at their chosen levels of effort.

The team saw changes in pupil size and prefrontal cortex activity as people made their decision in the scanner.聽Because these changes occurred before doing the exercises, it suggests that they were anticipating the amount of effort that would be required.

What鈥檚 more, if someone chose the most difficult activity, this was revealed by specific pupil dilation and brain activity patterns. The team suggests these signals influence the outcome of people鈥檚 decisions, by helping to predict the amount of energy required, and this reveals whether they will end up doing a higher effort task.

鈥淚t鈥檚 a very interesting proposal,鈥� says Tobias Hauser at University College London, who wasn鈥檛 involved in the work. However, pupil size and the prefrontal cortex signal are known to reflect different things, he says. 鈥淸They] have been linked to different aspects of cognition, be it effort, be it surprise, be it difficulty, so it鈥檚 not a unitary thing.鈥�

As to whether pupil dilation and prefrontal cortex activity really play a role in our decision making about future exertion, Hauser says 鈥渋t鈥檚 a long shot, but long shots are worth pursuing鈥�. Follow-up studies would need to establish whether these two factors directly affect our behaviour and if they might act differently in people with low motivation, he says. If that holds true, Hauser thinks 鈥渋t鈥檚 definitely an interesting new perspective on effort and decision making which could in part reformulate what we understand鈥�.

Reference: bioRxiv,