(Medical Xpress) — We all know that our pupils contract when our eyes are exposed to increases in the brightness of light. The reason is to both protect the delicate inner workings of our eyes and to help provide for optimum viewing based on available light. But we also know that our pupils dilate, or become larger when we are aroused, regardless of the reason, which means that pupil size is not always just a reaction to lighting conditions. Now, new research by Bruno Laeng and Tor Endestad from the Department of Psychology at the University of Oslo, shows that our pupils also react based on what we think we see sometimes, rather than what is actually there. In their paper, published in the Proceedings of the National Academy of Sciences, the two show that pupil constriction occurs when viewing an optical illusion that at first makes us believe one image is brighter than another, when in reality, they are the same.