Map unfolds for brain's vision areas.

Thanks to advances in brain-imaging technology, scientists have taken the first step toward delineating the location and characteristics of a cluster of interrelated patches of brain tissue that orchestrate vision. Until now, knowledge about these regions came almost entirely from monkey experiments that relied on invasive methods such as electrical stimulation of the living brain.

"We're at the threshold of understanding much more about the human visual cortex and its possible role in higher mental functions, such as language comprehension," asserts Martin I. Sereno, a neuroscientist at the University of California, San Diego.

Sereno and his colleagues used functional magnetic resonance imaging (MRI) to study the brains of seven volunteers, they report in the May 12 Science. Functional MRI creates images of brain anatomy and determines localized activity from measurable responses of the brain to a strong external magnetic field.

MRI images taken while volunteers viewed a slowly rotating semicircle revealed activity in the cortex, the brain's outer layer, that changed according to the angle of gaze registered by the retina. Images taken while viewing a thick ring that slowly expanded or contracted yielded information on cortical areas sensitive to the point in the visual field at which a participant's gaze was directed.

A computer program transformed MRI scans of each participant's brain from a bunched cortical surface to a flat expanse of tissue. Data from the two visual trials were then fitted into these flattened images, which allowed researchers to isolate all activated parts of the cortex.

The researchers mapped out five adjacent segments of the visual cortex by noting magnetic signal changes from one region to the next. The location of only one of those areas, the primary visual cortex, or V1, had already been traced in humans, Sereno says.

Humans and monkeys display a number of similarities in the shape and organization of these visual areas, he notes. However, mapped areas of the human visual cortex place far more emphasis on processing information from the center of gaze than do corresponding areas of the monkey cortex. For some reason, the human visual cortex evolved a preference for extracting information from the center of the visual field when assembling elements of a scene, Sereno proposes.

Further MRI work may uncover links between the visual cortex and some types of linguistic ability, he adds. Part of the visual cortex not mapped in the current study lies just below Wernicke's area, a structure involved in speech and language comprehension, according to Sereno.