The discovery that the brain undergoes a second growth spurt during adolescence has potential implications not only for teen driving but also for other behavioral problems that have their roots in this period of development.

Just as bad experiences or the absence of proper stimulation can stunt a young child's brain during the first surge in brain growth, too much of the wrong kind of stimulation may adversely affect the vulnerable adolescent brain, researchers say.

For example, brain-imaging studies show that if normal, healthy adolescents are exposed to violent videos for a long time, they show similar neural network patterns in the areas governing emotions and self-control as youngsters who are chronically aggressive, according to two Indiana University School of Medicine researchers.

Viewing violent videos does not turn all adolescents into aggressors, of course. But scientists now want to know what might make some youngsters more vulnerable to aggression after watching violent scenes.

"The brain is reacting biologically to the experiences that the teenagers are having," said clinical psychologist William Kronenberger, who worked with radiologist Dr. Vincent Mathews on the study.

Heavy marijuana smoking also can restructure the adolescent brain in ways that are disturbingly similar to the neural pathways found in schizophrenics, said Dr. Sanjiv Kumra and Dr. Manzar Ashtari of the Albert Einstein College of Medicine in New York.

Like the Indiana team, the researchers used magnetic resonance imaging, or MRI, to peer into the brain. In this case they compared the brains of adolescent schizophrenics and healthy teens who had smoked marijuana heavily for a year.

Repeated exposure to marijuana, they found, might interfere with development of the fiber bundles connecting the brain area that processes speech to the frontal cortex where executive decisions are made.

"The same pattern of abnormalities that you could see in schizophrenia you could also see in adolescents who don't have schizophrenia but who just smoke marijuana," Kumra said. Several studies of adolescent marijuana smokers suggest that they are two to four times more likely to develop schizophrenia than non-smokers, she said.