Chemical Basis of Life
of the brain that are quiet in the brains of unaf-
fected individuals. Knowing the site of altered
brain activity can help researchers develop more
directed drug therapy.
In addition to highlighting biochemical
activities behind illness, PET scans allow biolo-
gists to track normal brain physiology. Figure 2E
shows that different patterns of brain activity
are associated with learning and with reviewing
something already learned.
These PET images demonstrate brain changes that accompany learning. The top and bottom views show di±
erent parts of the same brain. The
“naive” brain on the left has been given a list of nouns and asked to visualize each word. In the middle column, the person has practiced the task, so he can picture
the nouns with less brain activity. In the third column, the person receives a new list of nouns. Learning centers in the brain show increased activity.