Article last updated: Thursday, November 16, 2000   5:06 AM MST

By Usha Sutliff
Staff Writer

PASADENA - Imagine this.

You're seated comfortably in a chair, recalling the paintings and sculptures you saw during a recent trip to your favorite museum. With your eyes closed, you visualize the colors and shapes in vivid detail.

It's a simple task. But our ability to conjure up mental images is one scientists say we take for granted and yet have long sought to understand. A recent Caltech-UCLA School of Medicine study may be a step toward figuring out how the "mind's eye" works.

Researchers studied nine epilepsy patients who had tiny electrodes implanted in their brains so doctors could figure out which areas were causing seizures.

It was a rare opportunity to take an up-close look at individual neurons cells in the brain and their role in calling specific visual images to mind.

As the patients were shown pictures of famous people, animals, drawings and other images, neuroscientists recorded the electrical activity in various parts of their brains.

The patients were then asked to close their eyes and imagine what they had just seen. Researchers again recorded the impulses and found that the firing rate of the brain cells was almost as high as when the subjects were looking at the photos.

They also noted that single neurons in certain areas including the hippocampus selectively altered their firing rates depending on the stimulus the subjects imagined. For example, some neurons responded when the subject was looking at a picture, but not when he or she was asked to recall it.

The study helps settle long-standing questions about the nature of human imagery and sheds light on how the "mind's eye" works, said Christof Koch, a Caltech professor of computation and neural systems.

"If you try to recall how many sunflowers there are in the van Gogh painting, there is something that goes on in your head that gives rise to this visual image," Koch said. "There has been an ongoing debate about whether the brain areas involved in perception during 'vision with your eyes' are the same ones used during visual imagery."

Dr. Itzhak Fried, a UCLA neurosurgeon, said the study provided a rare opportunity to directly observe the activity of the human brain in a "pure mental state" without visual stimulation from the outside world.

"Obviously, the brain listens to the input of millions of neurons during the process of cognition, memory and behavior," Fried said. "Understanding how this happens is the ultimate goal of brain researchers everywhere. This study is another step in that direction."

The study is published in today's edition of the journal Nature.