Brain’s Image-Making Ability Probed : Scientists Refute Old Findings, Say Left Side Forms Pictures

Studies of brain-damaged and normal people suggest that the ability to form mental images is centered in the left side of the brain, although the right side helps people to recognize what they see, a psychologist says.

“A lot of people have assumed mental imagery is carried out by the (brain’s) right hemisphere because the right hemisphere is important for recognizing complex visual patterns,” said Martha J. Farah of Carnegie-Mellon University.

Not Based on Science

But her research shows that “the process of forming images from memory seems to be carried out in the left hemisphere of the brain.”

The studies, carried out by Farah and her colleagues at Cornell Medical College and Harvard University, were presented to the Cognitive Science Society’s annual meeting.

Rick Granger, the society’s program chairman and a psychologist-computer scientist at the University of California, Irvine, said, “Popular left brain-right brain kind of books (which advocate the theory that mental images are formed in the brain’s right hemisphere) for the most part are not based on recent scientific work.

“Some of the crucial pieces of a mental image may well be on the left side of the brain and there also may be crucial pieces in other localities,” he said.

Scientists previously found the left side of the brain tends to be associated with language abilities and certain motor skills, although some speech-related functions are also found in the right hemisphere. They believe the right side is linked with visual and spatial abilities, such as recognizing faces or understanding maps.

Brain-Damage Cases

In one study, Farah reviewed medical literature for reports on neurological problems in people who suffered brain damage through strokes or direct injury.

She found 12 reports of patients who couldn’t conjure up mental images of objects, even though they could recognize the objects when they saw them. In all 12 patients, most of the damage was to the left, rear portion of the brain, suggesting that is the place where mental images are formed.

Her other studies involved 20 normal men and two epileptics whose left-right brain connection had been surgically severed to prevent extreme seizures.

They sat in front of computer screens, focusing their eyes on a central dot as pictures of letters or symbols were flashed on one side of the screen or the other. The men had to perform tasks that involved forming mental images of the letters and symbols to help sort the pictures on the screen into groups.

The people were fastest and most accurate when the symbols were fed to the left brains, suggesting that the mental images formed there, Farah said.

She said she believes the normal people were slower and made more errors when the symbols were fed to the right brain because that side had to communicate with the image-making process on the left before the men could perform the task. The epileptics had great trouble when pictures were fed to their right hemispheres, since the separated halves of their brains couldn’t communicate well.