Could a poker player draw conclusions about their opponents by looking at the size of their pupils? A recent German study has shed new light on the relationship between uncertainty, decision-making and dilated pupils.

Together with two colleagues, Dr Tobias Donner, a professor of neuroscience at Hamburg-Eppendorf University Medical Centre in Germany, asked 27 men and women to perform difficult decision-making tasks.

Random dot patterns moved across a screen in a series of two successive displays, and the participants had to say which of the two displays was stronger.

Meanwhile, the scientists measured the diameter of their pupils with a video camera before giving them feedback about the correctness of their choice.

Pupil diameter – “a proxy for central arousal state” – increased most strongly when the participants were uncertain about their decision, the scientists wrote in the London-based journal Nature.

Dilated pupils also increased the probability of participants making a different decision during the next part of the trial.

As Donner explains, pupil diameter is closely associated with activity in the brainstem, the stalk-like posterior part of the brain that is linked to the spinal cord.

This part of the brain, he says, is activated when we unconsciously adapt our behaviour because we are unsure about a decision – by paying closer attention to the facts the next time, for example.

“It was already known that pupil diameter reflects a general state of arousal,” Donner says. “What’s new is that this was coupled with a very precise mathematical measure of decision uncertainty.”

The pupils are an indirect indicator of what’s happening in the brainstem, he says – showing us that it’s not only central regions of the brain that are involved in decision-making.

Donner hopes his findings could help in the treatment of depression and other mental illnesses in which brainstem centres appear to be disrupted.


A person’s pupil diameter is closely associated with activity in the brainstem, this means emotions can cause the pupils to react.

It is all in the eyes

The pupil – the circular opening in the centre of the iris of the eye – controls the amount of light allowed to pass to the retina. For centuries, it has been known that the intensity of light falling on the eye isn’t the only factor causing the pupil to dilate or constrict.

Wolfgang Einhaeuser-Treyer, a neurophysicist at the Chemnitz University of Technology in Germany, points out that deadly nightshade, a perennial herbaceous plant known scientifically as Atropa belladonna (“bella donna” means “beautiful lady” in Italian), got its name because women used the herb in eyedrops to dilate their pupils and make themselves more alluring.

Since the 1960s it’s been known that pupils also dilate during decision-making, partly due to uncertainty.

According to Einhaeuser-Treyer, what makes Donner’s study noteworthy is the experimental set-up and the fact that it focuses on the correlation between successive decisions.

“The correlation seems to me to be quite elegant and could be an important step forward in the field,” he says, noting that understanding decision-making processes is of fundamental importance.

“A better understanding of the principles of decision-making can ultimately help us to better understand wrong decisions and systematic fallacies – and perhaps to avoid them.”

So could a pupil-measuring device – if it were somehow practicable – help a poker player, or even a criminal investigator during an interrogation of a suspect?

Donner cautions against the use of such a device, pointing out that the best lie detectors already claim to be almost 100 per cent accurate using just a single measurement.

“But I’d definitely recommend that pupil diameter be measured as well,” he says. – dpa/Bernhard Sprengel