J. P. Gerber, Ph.D.

The Shape of Traits

The Lie You Were Taught About Your Eyes

Our pupils can anticipate what we are going to see next

Posted Dec 30, 2017

127071 / pixabay / CC0
Source: 127071 / pixabay / CC0

Were you taught a lie about your eyes? I remember being a child and walking around exhibits at Taronga Zoo in the bright Sydney sun. When I entered the nocturnal animals exhibit, the only thing I immediately sensed was the dank smell because I couldn’t see anything. However, over what seemed like an eternity, the pupils in my eyes dilated to adjust for the darkness inside and then I could see the flying foxes, bats and echidnas.

My parents and teachers later explained that pupils react to the light in the environment. My pupils adjusted to the bright sun by being small, my pupils adjusted to the dark room by becoming large. I imagine you have heard the same story.

To the majority of perception researchers, the pupil is similarly reactive. For example, Firestone (2013, p. 455) writes “visual processing operates on little more than the stimulus information reaching the eyes and is stubbornly insulated against states such as the perceiver’s goals, beliefs, or action capabilities”. Under this view, pupils only react, they cannot anticipate. We cannot anticipate the upcoming light and pre-emptively correct my pupil’s response to what we are about to see.

However, recent research suggests that our pupils do more than react, they also anticipate. Mathôt & Van der Stigchel (2015) report on a line of research showing that, if we are expecting to look at a bright part of a computer screen, our pupils get ready to contract just before our eyes move, and that our pupils contract as we move our eyes, so that we don’t have to wait for our eyes to adjust to the new part of the screen. Our pupils don’t just react, they act. Our expectations prepare our pupils and change them.

Why is this important news? Because it removes that hard and fast line that our perceptions are hard-wired or “stubbornly insulated”. Other accounts of motivated perception (e.g. hills look steeper when you’re tired) are very debatable due to design flaws. In particular, they rely on participants verbalizing or actively responding to a situation, which can make them give the answer they think the experimenter wants. This new work on pupils, however, is hard to fake. How can participants fake a pupillary response?

So, in short, Mathôt & Van der Stigchel (2015) have removed that hard line in the ground. We, as people, are not impassionate perceivers. At the most basic level, at the point where the light enters the eye, the very first stage of visual perception, our expectations are already shaping our perceptions.

What does this mean for my visit to Taronga Zoo? What if a kid really loved the dark halls but also the Sydney sun? Would the time it takes for their pupils to adjust get just a little bit shorter each time they ran inside and then out again, excitedly viewing the animals? I wonder.


Firestone, C. (2013). How “paternalistic” is spatial perception? Why wearing a heavy backpack doesn’t­—and couldn’t—make hills look steeper. Perspectives on Psychological Science, 8, 455-473.

Mathôt, S. & Van der Stigchel, S. (2015). New light on the mind’s eye: the pupillary light response as active vision. Current Directions in Psychological Science, 24, 374-378.