PET was developed in 1972, and now six American labs use it in research. To do a PET scan, scientists inject radioactive (but safe) water into the volunteer’s bloodstream. The “hot” blood finds its way to the brain. Since active regions use more blood than inactive ones, they light up with radioactivity that is captured on special detectors surrounding the person’s head. In one experiment reported last week by Squire and Marcus Raichle of Washington University, 18 volunteers studied a list of 15 common words, four to eight letters long, on a computer screen. Next they looked at 20 three-letter word fragments, one at a time. After each one they said the first word that came to mind. PET found a marked drop in activity in part of the cortex that processes sights (diagram). “This tells us that when the brain processes a visual stimulus [such as a written word], some trace of the stimulus is left,” says Squire. “So when the same stimulus is presented again, less neural activity is required to recognize it.” It’s as if the sight of a word has etched a channel in the cortex, so that the next time the eyes see the word the brain need do very little work to process it.
In fact, less may be better. In the next round the volunteers tried to remember which word from the original list matched a fragment. The PET showed hot spots in the hippocampus, a little seahorse-shaped organ deep in the brain that forms long-term memories. No surprise there: the subjects were trying to remember. The odd thing was that a spot on the frontal lobe, involved in thinking, also lit up when subjects tried to remember the words, “but was most active in people who remembered fewer of the words,” says Raichle. The less thinking the brain did, it seems, the better memory worked. And vice versa.
A separate PET study revealed the brain to be pretty nimble when it comes to switching circuits. Raichle showed common nouns to 11 adults and asked them to respond with an appropriate verb. “Hammer” might elicit “hit,” for instance. Four areas of the brain lit up, including the cerebellum and part of the cortex. After 15 minutes of practice, the subjects were tested again on the same nouns. Not a single one of the original areas lit up. Only the brain’s motor system, which controls muscles, showed activity. So much for the admonishment against putting mouth in motion before engaging brain: it seems that if the brain knows the answer cold, it doesn’t have to think much. Yet when the volunteers saw a new list of nouns, the original thinking areas turned on again. The abrupt change, says Raichle, suggests that when a task “is novel and requires conscious thought, the brain utilizes resources quite different from those required for the simple repetition of a word. You can essentially rearrange the brain in 15 minutes. It blew my socks off.”
In other papers at the meeting, researchers presented the first evidence that some intellectual abilities of men vary with hormone levels. (Other studies have shown such a correlation with women’s monthly hormonal fluctuations: when estrogen levels are low, women do better on spatial tests.) J. S. Janowsky and colleagues at the University of Oregon reported that when elderly men wore skin patches that raised their testosterone levels–in some, to those of a 20-year-old–their spatial skills improved significantly. Is lots of testosterone good for spatial skills? It may not be so simple. In a related finding, Doreen Kimura of the University of Western Ontario reported that men scored higher on spatial tests–finding simple figures hidden in a complex picture, or mentally rotating objects in space–in spring than in autumn. Springtime is when testosterone levels are lowest. Earlier experiments had shown that men with below-average testosterone levels do better than their fellows on spatial tests. So do women with above-average testosterone levels. Apparently, when it comes to spatial skills there is an optimum hormone level, somewhere below the average male’s and above the average female’s. If the findings hold, maybe male engineers should stick to designing bridges in the springtime, and not autumn.
DIAGRAM: A New Window on the Mind (RAICHLE–WASHINGTON UNIVERSITY)
controls motor function
gathers scent perceptions, a center of emotions
forms long-term memories
controls balance
receives and processes sensory information, site of higher thinking
The PET scan shows the area of the brain that became active when a test taker tried to remember a list of words. Both the right side of the hippocampus, seat of visual memory, and the cortex, site of higher thinking, lit up.
