In their experiment, the researchers presented six volunteers with four types of ambiguous stimuli. The volunteers viewed or listened to the stimuli and pressed a key on a keyboard when a perceptual shift occurred. At the same time, infrared eye-tracking software measured the diameter of the subjects’ pupils.
The scientists found a significant increase in the diameter of the pupil at the instant preceding the perceptual switch. The pupil, which is about 2 mm wide in bright light, dilated by as much as 1 mm at that moment–a change that, in theory, could be noticeable to a casual observer. Koch and his colleagues also found that the more the pupil dilated, the longer the period of time before the switch from one interpretation to the other.
“The pupil is not only there to regulate light, but is linked to our emotional state. This may have evolved for us to monitor the emotional state of others, and may offer a very simple way to track decision-making in general,” says Koch.
Perhaps this is why so many poker players on the pro circuit wear dark sunglasses.