1993: von der Heydt

Professor Rudiger von der Heydt
Neuroscience
Johns Hopkins University

Now You See It – Maybe

Researchers from The Johns Hopkins Medical Institutions and the University Hospital of Zurich have pinpointed where and how the brain "sees" optical illusions. In doing so, they have developed a better understanding of how vision is processed in the brain and how the "mind" fills in missing information.

The work by Hopkins neurophysiologist Rudiger von der Heydt, Ph.D., and cm researchers Esther Peterhans, D.M.V., and Gunter Baumgartner, M.D., in Zurich, deals with a phenomenon called illusory contours - in which the brain fills in the outline of an object, even though there are genuine gaps in that outline. For his role, von der Heydt will receive the Berkeley-based Minerva Foundation's 1993 Golden Brain Award on Nov. 8, given annually for exceptional basic research on vision and the brain.

By showing monkeys optical illusions and recording the response of brain cells, the research team found that cerebral cells in an area at the back of the head called the prestriate cortex can fill in the gaps, anticipating a complete object.

These cells, which "fire" when the eye is shown a contrasting object at a certain orientation - a tilted white bar on a black background, for example, will fire also in the same way if there are gaps in the bar. Cells at a lower level in the visual system, however, did not fire.

"The brain devotes from a quarter to a third of the cerebrum to interpreting what we see, and the area where we found the illusory contour responses is one of the largest of the visual areas," says von der Heydt. "That's a large piece of the brain to use just for optical illusions, so there are certainly other or more important purposes."

One purpose, he explains, is that "the system wants to have something that is constant." If, for example, you look at a car driving away, there may be parts of the car's contour that don't stand out because of changes in lighting. But your perception tells you that the car is still intact. Constancy, von der Heydt says, keeps the world a reasonable place.

Another purpose, he says, is 3-D perception. "We see the world in three dimensions, although our eyes receive only two dimensional images. When this happens, information from near and distant objects gets jumbled in the brain at the lower levels of the visual system, closer to the eye where sensory information enters."

Higher centers in the brain sort out this jumble to produce a 3-D representation, he explains. The question has been how far up in the visual system this occurs.

Researchers once thought perception was strictly a higher brain function. Now it appears that certain perceptions, such as illusory contours, may be "wired" at a lower level. "These studies of perception are a gate to understanding brain function in general," says von der Heydt. "We get closer to understanding thought this way."

1993: Professor Rudiger von der Heydt