Brain scans set sights on mind's eye.

The mind's eye, which creates mental images of objects and scenes from the outside world, has winked elusively at scientists who have tried to trace its location in the brain-until now. A new study finds that people who visualize various objects experience blood flow surges, signaling enhanced cell activity, in brain areas that handle the earliest stages of visual processing.

The extent and exact location of these cerebral surges depend on the size of the imagined object and resemble activity changes that accompany the actual viewing of objects, assert Stephen M. Kosslyn, a psychologist at Harvard University, and his colleagues. Mental imagery may rely on the brain's ability to generate internal pictures from signals supplied by regions responsible for vision, the scientists contend in the Nov. 30 Nature.

In contrast, some researchers have argued that it is verbal interpretations of what we see that seem visual in retrospect.

"The fact that stored visual information can affect processing in the earliest visual areas suggests that knowledge can fundamentally [influence] what one sees," Kosslyn holds. "We may see different things, depending on what we expect to see, although our expectations are often pretty accurate."

For instance, memories of our physical surroundings may feed into mental images that guide our movements through space with great precision, even when vision is blocked (SN: 8/12/95, p.104).

Kosslyn's group used positron emission tomography (PET) scans to examine blood flow in the brains of 12 men as they performed three types of tasks. For a resting baseline, each volunteer closed his eyes, relaxed, and imagined complete darkness in front of his mind's eye.

In a listening baseline, participants heard a series of item names, such as "anchor," each followed by a direction to make a spatial judgment about the item, such as deciding whether the anchor's rightmost or leftmost point was higher. They had only 1 second to make each spatial judgment and were told not to visualize anything during these trials.

In an imagery task, volunteers heard the names of items and memorized pictures of them. Participants then visualized items as small, medium, or large and made spatial judgments.

Imagery-specific activation of tissue in the primary visual cortex became apparent when imagery data for specific items were compared to the listening baseline for the same objects. The primary visual cortex responded in different patterns to small, medium, and large images.

"This is a nice demonstration that the visual cortex is activated by mental images in the same ways it would be activated by visual perceptions," asserts Larry R. Squire, a neuroscientist at the Veterans Affairs Medical Center in San Diego.

For reasons still unclear, Kosslyn notes, the resting baseline task also activated the visual cortex enough to muddy comparisons between it and the imagery task.