When we’re shown two options, our eyes tend to flick back and forth several times as we deliberate the pros and cons of each.
Researchers from Johns Hopkins University in the United States have found that the speed at which our eyes switch between options reveals our true preference and predicts the final decision we will make.
This rapid eye movement – called a saccade – is what allows you to read; your attention shifts sharply from word to word, briefly fixing on certain words before moving on to constructing meaning from a block of text.
Saccades, which happen over a few hundred milliseconds, are also what happen during rapid eye movement (REM) sleep.
“Unlike your arms or legs, the speed of eye movements is almost completely involuntary,” says Colin Korbisch, a mechanical engineer at the University of Colorado at Boulder and lead author of the study. “It’s a much more direct measure of these unconscious processes happening in your brain.”
To test whether our eyes are a window to our minds, researchers recruited 22 study participants and had them walk on a treadmill. They were presented with a choice of two treadmill settings: a short walk on a steep incline or a longer walk on the flat.
They tracked eye movements using a high-speed camera as participants took a few seconds to consider symbolic representations of their options.
“Initially, the jerks between the two options were equally vigorous,” says Alaa Ahmed, a mechanical engineer and lead author of the study. “Then over time that vigor grew, and it grew even faster for the option they ultimately chose.”
When researchers offered a choice of two treadmill options where one required significantly more effort than the other, participants’ eye speeds tended to increase when thrown in the direction of the option. the easiest, accentuating their degree of preference.
“We’ve discovered an accessible metric that will tell you, in just seconds, not just what you prefer but how much you prefer it,” says Ahmed.
Those who took less time to decide tended to have faster eye movements. These are perhaps the most impulsive people that researchers suspect.
The rapid eye movements stopped once the participants made their final decision, suggesting that the eyes were busy collecting information in order to decide between two alternatives.
Neurons in the frontal eye field and parietal cortex, which receives sensory information in the brain, ramp up activity when people are engaged in decision-making activities. When a preference has been established in our brain, it seems to lift the suppression of a region that controls eye movements, called the superior colliculus. This helps increase eye speed, the researchers speculate.
“Real-time readouts of this decision-making process typically require invasive electrodes placed in the brain. Having this variable more easily measured opens up many possibilities,” Korbisch said.
This article was published in Current biology.