03 January 2014

Eye Pupils

Look into my eyes. Tell me what you see. Well, for most people you’ll see the outer white sclera (it might be a bit bloodshot); the colored circular iris; and the small dark hole, the pupil, at the center of the iris. There’s a cornea in front of everything, but it’s transparent.
A young girl’s eye.
(Yes, it’s Rachel!)

Welcome back. A research paper caught my eye and led me to search: Why does the pupil change size? Ask.com blamed it on the amount of light entering the eye, giving a short description of the eye mechanism.  

Even without the research paper I knew that answer shortchanged the question. It was a good start, yet it’s hardly news that pupil size also responds to mental and emotional states and, as the nice policeman will tell you, to alcohol and drugs.

What is news or at least what hadn’t been shown before this recent study is that pupils also respond to imagined changes in light.

Effect of Imagined Light

Researchers from Norway’s University of Oslo monitored the eye pupil size of human test participants. The participants first watched a screen that showed shapes that varied in brightness and complexity. The individuals were then instructed to generate mental images of what they’d seen while looking at the same neutral screen.

The pupils’ size responded in the same manner to both the observed and imagined dark and bright shapes. Shape complexity increased the mental effort and pupil size independently of shape brightness.

The test participants were also asked to imagine familiar scenes, such as a sunny sky, dark room or face in the sun or shade. For these, too, the eye pupils’ size responded to the different level of imagined brightness.


At this point, you may be thinking, Yes, Warren, it’s cool, but is this just some scientists playing in their research sandbox? Good question. It was around 1997 that I first heard people saying “Good question” regardless of the quality of the question. In this case, however, I had the same question so I dug deeper.

I found studies that linked measurement of pupil size (pupillometry) with the autonomic nerve system--the nerves that control “automatic” things like blood pressure and heart rate.

One very recent study from the University of Texas Southwestern Medical Center determined values for eight pupillometry indices through measurements on healthy individuals. The investigators found the values of those indices were significantly different for patients with moderate to severe autonomic dysfunction.

Wrap Up

I wonder if a tested individual could imagine light changes and throw off the pupillary measurements in the University of Texas study. Under the test constraints and stress of being tested that probably wouldn’t happen.

I also wonder if imagining pupil-affecting causes besides light would affect pupil size. Would pupil size change if you imagined that you were thinking hard, in pain, drugged, disgusted or sexually aroused? That answer’s probably yes, but maybe I’m in the sandbox now. Would you care to join me? Thanks for stopping by.


- University of Oslo study in Psychological Science Journal:
- Articles on University of Oslo study:
- University of Texas study in Clinical Autonomic Research Journal:
- Articles on how pupil size responds to more than changes in light:

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