To dream, perchance to scan.

Brain scans obtained from sleeping men have helped illuminate the neural system that makes dreaming possible, reports a team of investigators.

During rapid eye movement (REM) sleep, the brain's visual system retains its power to generate images but cannot process external sensations, the group notes in the Jan. 2 Science. Several inner-brain structures involved in memory and emotion exhibit heightened activity during REM sleep. In contrast, frontal lobe areas that integrate visual information with other sensations and make possible temporary recall of related items display sharp drops in neural effort, contends neuroscientist and report coauthor Allen R. Braun of the National Institutes of Health in Bethesda, Md.

These physiological features of REM sleep lay the groundwork for characteristic elements of dreams, such as intense emotions, bizarre happenings that seem unquestionably real, fractured time sequences, and a surreal lack of reflection about all the strange goings-on, the researchers theorize.

Braun and his colleagues obtained positron emission tomography (PET) scans of blood flow in the brains of 10 men during REM sleep and compared the images to scans taken during another stage of sleep--slow wave sleep--and to scans taken while the men were awake. Dreams occur most often and most vividly during REM sleep, the investigators assert.

REM sleep was characterized by low blood flow in the frontal lobes and in the tissue at the back of the brain that receives initial visual information from the eyes. A drop in flow reflects lessened neural activity. Blood flow surged in areas that orchestrate visual scenes and that contribute to emotion and long-term memory.

In an earlier PET scan study, researchers had found that the amygdala--a brain structure involved in processing fear and other emotions--becomes particularly active during REM sleep (SN: 9/21/96, p. 184).

Long-time sleep researcher David Foulkes, formerly of the Georgia Mental Health Institute and now working independently in Atlanta, doubts that these new brain studies shed light on the anatomy of dreaming--precisely because they focus solely on REM sleep.

Much evidence suggests that dreams of the same kind and of comparable intensity occur in nonREM sleep, Foulkes argues. Thus, he says, no physiological property unique to REM sleep can explain dreaming.

In the absence of environmental input or voluntary selfcontrol, consciousness--a cognitive system for producing awareness of sensations and memories--creates dreams, Foulkes theorizes. Under some circumstances, ranging from relaxed wakefulness to REM sleep, this mechanism results in dreaming.

People develop the capacity to dream during childhood, says Foulkes, who notes that dreams do not assume a storylike form until age 7 or 8. Maturation of the frontal lobes may be required for dreams with a narrative structure, in his view.

The new PET findings offer valuable insights into the anatomy of dreaming, counters psychiatrist J. Allan Hobson of Harvard Medical School in Boston. Optimal brain conditions for dreaming exist only during REM sleep, he holds.

Braun's data support the notion that dreams reflect the activation of an individual's "emotional memory bank," with little direction or interpretation from the frontal lobe, he proposes.

"The PET results are consistent with Freud's idea that dreams have meaning," Hobson says, because they draw on neural repositories of memories. "But they challenge his theory that an unconscious dream censor [which would be located in the frontal lobes] screens out and masks particularly disturbing wishes."

Whatever the anatomy of dreaming, notes Braun, the core functions of REM sleep remain poorly understood.