This study will test a new electrical technique called direct current (DC) polarization that is able to change brain activity in subtle ways for a short time. A recent study showed that, depending on its direction, the current could make people perform a little better or perhaps slightly worse on a test of the function of the brain's frontal lobe. This study will use positron emission tomography (PET) scanning to examine how DC polarization affects brain activity.

Healthy volunteers between 20 and 70 years of age who are right handed and who are not taking any medications that affect the brain may be eligible for this study. Candidates are screened with a brief medical history and neurological evaluation.

Participants have a PET scan on three different days at least 3 days apart. Each scanning session takes 2-1/2 to 3 hours. For the scan, radioactive water is injected into the body through a vein. Subjects lie on a bed that slides in and out of the doughnut-shaped scanner, with their head held from the back by a padded holder and in front by a custom-molded plastic mask with holes for the eyes, nose, and mouth. DC electrodes made of wet sponges are placed on the right side of the head and over the left eye and are held in place with elastic bandages.

Three kinds of DC polarization are tested. In two tests the current is the same, but in opposite directions. The third is a sham (placebo) condition with no current delivered. Each of the three scans is separated by at least 3 days. On each day, a series of scans is done in a single session. Before each injection of tracer, the DC current is turned on. This may cause a tingling or slight burning on the skin under the electrodes, which disappears when the current is turned off. In each session, the subject receives 16 injections of tracer about 8 minutes apart, with DC polarization turned on for 4 out of the 8 minutes.

During most of the 8-minute periods, subjects are shown a pattern of dots about every 2 seconds. Sometimes the subject just looks at the patterns, and sometimes subjects are asked to push a button corresponding to the pattern they saw just before the current one. Sometimes they will be asked to push a button corresponding to the pattern that came before that one and so on, up to three patterns before the current one. The task lasts for about 2 minutes each time, with time to relax in between.