Neuroscientists have long believed that evolution hardwired the brain to amplify slight differences in shading, making it easier to perceive subtle details like a green snake in a green tree. Thus objects on dark backgrounds appear lighter than they are, and those on bright backgrounds appear darker. But science advances by replacing approximate truths with more precise ones, and new research suggests that this scientific “truth” is, at best, incomplete. The two experiments that follow help show why the thinking on this subject is changing.
Courtesy of Dr. R. Beau Lotto in collaboration with Dr. Dale Purves
What are the colors of the squares indicated by the arrows in the two figures at right? For most observers, the one on the top looks blue and the one on the bottom looks yellow. But the two squares are actually an identical shade of gray. One possible explanation for this illusion is simultaneous contrast, a process by which your brain makes foreground objects take on the opposite hue of their backgrounds in order to improve your discrimination of subtle color differences. According to this theory, the top square appears blue because the figure is on a mostly yellowish background, while the bottom square looks yellow because it’s set against a predominately bluish background.
The question to ask in this exercise is why the two gray squares at right look so much more like each other than do the squares in the cubes at left (to which they are identical in hue). The backgrounds here are more blue and yellow than those in Experiment 1, so one would expect the brain’s contrast-enhancing neural circuits to make these gray squares appear more, not less, different.
Vision researchers Dale Purves and R. Beau Lotto, who created the cubes in Experiment 1, believe the explanation for this inconsistency is that there’s more going on with color perception than simple contrast enhancement. They propose that the brain has evolved sensory “reflexes” that enable it to resolve stimulus ambiguities that might be confusing. For example, when you see the color gray, you could be looking at a truly gray object as it would appear under white light, a blue surface under yellow light (which would turn blue into gray), or a yellow surface under blue light. Faced with such ambiguities, say Purves and Lotto, the brain chooses the most probable scenarios based on previous experiences and those of our ancestors.
Because it is most likely that the two cubes in Experiment 1 were illuminated by yellow and blue lights, respectively, your brain automatically adjusted the gray surfaces on the cubes to assume the hues they would have had in normal (white) light.
It may seem troubling that the brain can make snap decisions that are wrong. Yet if your distant ancestors had made a habit of dawdling while deciding whether objects were of one type or another, dinosaurs that didn’t dawdle would have devoured them before the DNA that defines you could be disseminated.