In a new book "Perceiving Geometry: Geometric Illusions Explained by Natural Scene Statistics" (Springer), Purves and colleague Catherine Howe explore why the brain generates geometric illusions.
Visual perception is a daunting task for the brain, explains Purves, because light streaming into the eye carries only ambiguous information about the environment.
"The basic problem, recognized for several centuries, is that the image on our retinas can't specify what's out there in the world," said Purves. "The light received by our retinal receptors tangles up illumination, reflectance, transmittance, size, distance and orientation," said Purves. "This means that there's no logical way to get back from the retinal image to what's actually out there in the world."
Nevertheless, many neurobiologists have attempted to explain vision by postulating that the brain's neural wiring can definitively "calculate" the features of a visual scene, despite the visual world's inevitable ambiguity. Such "rule-based" theories, said Purves, have arisen because neurobiologists have concentrated on understanding how neurons in the brain's visual region extract and recognize specific features such as edges in a visual scene.
"Because of the enormous power and success of modern neurophysiology and neuroanatomy, there just didn't seem to be any reason to think much about this issue," said Purves. "However, we began worrying about it seven or eight years ago because the physiology and anatomy people had described didn't explain what we end up seeing. There was no instance, even in the simplest aspects of vision, where the properties of vi
Contact: Dennis Meredith
Duke University Medical Center