"Our eyes only tell us part of what we need to be able to see. The other part is done by the brain, taking the input from the eyes and making guesses or inferences about what's out there in the enviro You can't always trust your eyes. nment. Usually these inferences are very accurate, but sometimes they lead us astray in the form of visual illusions," said Scott Murray, a UW assistant psychology professor and lead author of a study published in the current issue of Nature Neuroscience.
Murray and his Minnesota colleagues, Huseyin Boyaci and Daniel Kersten, used functional magnetic resonance imaging (fMRI) to see how the brain processes the size of objects when faced with an illusion such as the long-known moon illusion. For centuries it has been known that the moon, while rising, looks huge when it is near the horizon and smaller when high in the sky. It is in reality always the same size.
The researchers used a similar illusion, one that looked at the perceived difference in the size of an object at different distances. For their experiment they placed two identical spheres decorated with a checkerboard pattern in the front and rear of a receding brick hallway. In this kind of illusion, the more distant object appears to occupy a larger portion of the visual field.
Using fMRI, the researchers examined how the brains of five people with normal vision registered this difference in perceived size.
They found that the brain region known as the primary visual cortex, which is the first area in the cortex to receive input from the retina, showed a difference. Even though both spheres occupied exactly the same size on the retina, the rear sphere activated an approximat
Contact: Joel Schwarz
University of Washington