Brain sets sights on mind's eye.

Like a private movie screen showing personalized films, each person's mind's eye generates a flow of imaginary visual scenes. These internal performances are inspired by a variety of sources, from a riveting mystery novel to memories of last summer's Olympics.

A new study by neuroscientists lends support to the notion that at least some of the brain regions involved in visual perception of the external world animate the mind's eye as well.

California researchers probed the neural roots of the mind's eye in four men and five women suffering from severe, uncontrollable brain seizures. So that the scientists could identify the neural epicenter of the seizures for possible surgical removal, each patient had electrodes implanted in the same general area of his or her brain for 1 to 2 weeks.

During that time, volunteers viewed pairs of pictures that included faces, objects, and scenes. After looking at two images, they closed their eyes and imagined one of them, as specified by the experimenters. As the trial proceeded for each participant, the implanted electrodes recorded the rate of electrical bursts from nearly 300 individual nerve cells, or neurons.

Fourteen of these neurons exhibited matching rises and falls in electrical output when volunteers viewed a picture and imagined the same picture. Some of these cells were in a visual-perception area of the brain's outer layer, or cortex, and others in inner structures linked to memory and emotion. Gabriel Kreiman and Christof Koch of the California Institute of Technology in Pasadena and Itzhak Fried of the University of California, Los Angeles School of Medicine report their results in the Nov. 16 NATURE.

They theorize that these brain cells belong to a neural system that orchestrates both the storage of visual information and its recall during imagination. Earlier brain-scan studies of healthy adults support this possibility (SN: 3/9/96, p. 155).

Another 28 neurons boosted their activity only when patients viewed pictures, and 7 neurons responded only when patients imagined pictures. This fits with evidence that some forms of brain damage impair the ability either to identify visually perceived objects or to form mental images of them.