WHY can鈥檛 teachers keep a secret? Because their pupils give them away. It turns out that when people make decisions, their pupils dilate, a subtle cue that could be used to predict a person鈥檚 intentions, or even converse with people with locked-in syndrome.
It is well known that pupils dilate as it gets darker, and in stressful situations as part of the 鈥渇ight-or-flight鈥� response. The reflex is mediated by the release of the hormone noradrenalin, which in animals has also been implicated in memory and decision-making. , a neuroscientist at the University of Melbourne in Australia, wondered if noradrenalin 鈥� and by implication pupil dilation 鈥� might also be linked to human cognition.
To investigate, her team asked volunteers to pick one of five random digits that displayed on a monitor one after another for 2 seconds each, and to press a button indicating their choice after the final number vanished. An eye scanner revealed that volunteers鈥� pupils were at their widest during the 2 seconds that corresponded to their eventual number choice. Carter鈥檚 team found that they could eventually predict with 62 to 100 per cent accuracy which of the five numbers people would choose, depending on the volunteer, based only on when their pupil dilated (Frontiers in Human Neuroscience, ).
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In 2008, Carter鈥檚 team found that people鈥檚 pupils dilate when they switch between two alternative ways of viewing an optical illusion, such as the video-based , in which the woman seems to switch direction but in fact does not. The effect results from the viewer swapping how they view her. Carter concludes that, rather than driving decisions, noradrenalin helps us to cement decisions we are veering towards anyway, and that pupil dilation is an outward sign of this. 鈥淥nce you鈥檝e made a decision, you may as well make it,鈥� she says. 鈥淵ou don鈥檛 want to dither.鈥�
鈥淣oradrenalin helps us to cement decisions, and pupil dilation is an outward sign of this鈥�
, a cognitive neuroscientist at the University of Oxford, isn鈥檛 convinced that the dilation 鈥� and therefore noradrenalin 鈥� is related to decision-making. Pupils dilate when people are excited, and Carter鈥檚 team could be capturing that, he suggests.
Carter points out that in some of her trials volunteers earned 10 cents when they chose certain numbers, while in others there was no option to win a prize. But in both scenarios, volunteers鈥� pupils tended to dilate when they made their choice, indicating that dilation isn鈥檛 related to excitement. Even if pupil dilation isn鈥檛 a sign of decision-making per se, pupils could still be used to deduce when someone is making a decision, or to help predict their choices, says Carter.
at the Bernstein Center for Computational Neuroscience in Berlin, Germany, has used brain activity picked up by an MRI scanner to determine when a decision is being made. He says pupil dilation may reveal a person鈥檚 concealed thoughts without having to put them into contact with a machine.
That might make for a low-tech way of reaching out to people who cannot communicate due to brain damage or paralysis, such as those with locked-in syndrome. at the University of Li猫ge in Belgium says that pupil dilation persists in locked-in syndrome. Responses to yes/no or multiple-choice questions could be inferred from pupil dilation if choices were read out or displayed on a monitor. Laureys鈥檚 team is using MRI to attempt communication with people with brain damage, but he says that eye-tracking gear is more portable and cheaper.
