The new research was presented at the 34th Society for Neuroscience annual meeting in San Diego.
New research--in both humans and animals--shows differences in the structure and functioning of adolescent brains compared with preadolescents or adults that correspond to such teenage behaviors as immature decision making, increased risk taking, and impulsive behaviors. As a result of this research, scientists now urge that puberty be studied as a separate stage of development--one distinctly different from the life stages of children or adults.
"Adolescents' brains seem to bias their decision-making capabilities in the direction of favoring short-term benefits, even when these benefits are weighed against potential long-term detriments," says Jonathan Cohen, MD, PhD, of the department of psychology at Princeton University.
In one study, Gregory Berns, MD, PhD, and his colleagues at Emory University School of Medicine found that hyperactivity in the reward circuits of adolescent brains compared with adult brains may underlie adolescents' immature decision making.
The researchers intermittently administered primary rewards of squirts of juice and water to adolescents aged 13 to 17 and to adults aged 30 to 50, while simultaneously viewing their brain activity with functional magnetic resonance imaging. The adolescents' brains showed significant activation in the medial prefrontal cortex and in the brainstem, near the ventral tegmentum and substantia nigra, compared with adults.
In previous studies, the researchers found that this reward model showed good specificity for targeting brain regions heavily innervated by the midbrain dopamine system. Other recent research has suggested that dopamine may play a role in the learning of behaviors associated with reward and pleasure. Consistent with this view, the model demonstrated that when the rewards were administered in an unpredictable manner, a key reward structure of the brain--the striatum--was more active, suggesting that "reward" to the brain may have more to do with the predictability of an event than with how pleasurable it is.
"These studies suggest that adolescent decisions may, in part, be due to a greater biological sensitivity to either rewards themselves or, more likely, the novelty of rewards," says Berns. "It is possible that a reward or novelty associated with the reward may trigger hyperactivity in still developing brain reward structures and circuits."
Another study found differences in risk-taking behaviors in pubescent rats. Georgia Hodes, a graduate student at Rutgers University working with Tracey J. Shors, PhD, used an elevated plus maze to study exploratory behavior of male and female rats before, during, and after puberty. The maze contained "safe" (closed and darker) areas and more "anxiety-provoking" (open and exposed) areas. The researchers analyzed videotapes of rats as they maneuvered freely through the maze during 10-minute sessions. The elevated plus maze is a task used to measure anxiety, which is created by the conflict between the animal's desire to explore and its fear of open spaces. An animal's entering the exposed areas of the maze indicates lower anxiety, while extension of the rat's head into the open area without actually entering it (stretched attenuated posture) indicates assessment of risk.
The researchers found that both male and female pubescent rats were more likely to enter and spent more time in the open and exposed areas than did the adult male and female rats, suggesting that the pubescent rats were more willing to explore and less anxious than the adults. The prepubescent rats showed more active behavior, not necessarily more anxious behavior, compared with pubescent and adult rats.
Pubescent rats were more likely to extend their bodies into the anxiety-provoking areas than prepubescent rats. Adult rats demonstrated similar extension behaviors, but did not enter the anxiety-provoking areas, while the pubescent animals abruptly entered the areas after assessing risk, suggesting that the pubescent rats were more impulsive than the adults. No sex differences were found at any age.
"Puberty appears to be a specialized stage of development in which males and females are more likely to explore novel environments," Hodes says. "This is likely an adaptive strategy that emerges with foraging behavior. It would also have reproductive advantages by enhancing contact with new members of the opposite sex."
In other work, scientists are using the extreme example of risk-taking among psychopaths to elucidate normal behavior and brain structure and function. Prominent characteristics of psychopaths are their poor decision-making ability and heightened risk taking.
Diana Fishbein, PhD, and her colleagues at the Research Triangle Institute International in Baltimore scanned the brains of 13 psychopaths and 15 nonpsychopaths using positron emission tomography (PET) while the individuals completed two sets of trials from a computerized neurocognitive task--the Rogers Decision-Making Task (RDMT). Before being scanned, the participants scored in either the "primary psychopathy" or nonpsychopathy ranges on a questionnaire that measures psychopathic characteristics.
The computerized RDMT measures risk-taking tendencies and decision-making ability by presenting participants with various choices that involve "gambling" a certain number of points based on the likelihood that their choice will be correct. Participants can make two choices: choices that are less likely to be correct, but result in a greater number of points earned if they are correct or a greater number of points lost if they are incorrect; or choices that are more likely to be correct, but result in a fewer number of points earned if they are correct or a fewer number of points lost if they are incorrect. Risk taking on this task is characterized by choosing the least likely choice to be correct in pursuit of a greater reward (i.e., earning more points), even though the penalty for being incorrect may be greater (i.e., losing more points) than making a choice that is more likely to be correct.
Differences in behavior and corresponding brain activity were found between psychopaths and nonpsychopaths, after adjusting for group differences in severity of prior illicit drug use. Individuals within the primary psychopathy range made more risky choices--choices that were more unfavorable (making the least likely choice in pursuit of a greater reward that results in a more likely penalty) during both trials. They also accrued fewer points and had longer reaction times before making choices than did the nonpsychopaths. Although both groups decreased their risky choices during the second trial, the nonpsychopaths reduced their risky choices significantly more than the psychopaths.
These behaviors appeared to correspond with the PET observations of brain activity, again after controlling for severity of prior illicit drug use, Fishbein says. Although several brain areas were activated in both psychopaths and nonpsychopaths during the RMDT trials, the investigators also found several differences in brain activity. Nonpsychopaths showed significantly higher activation in the bilateral orbital frontal regions, the left lenticular nucleus, and the left parietal cortex than psychopaths. Psychopaths showed more activation than nonpsychopaths in the left anterior cingulate, the right hippocampus, the right insula, and the bilateral caudate. Both groups showed activation in the right orbital frontal cortex, the bilateral thalamus, the cerebellum, and the parietal lobes, and deactivation in the left superior frontal gyrus, the bilateral mid-cingulate cortex, the anterior cingulate, the lenticular nucleus, and the insula.
"The psychopath group's greater activation of the anterior cingulate cortex and insula is likely related to their greater focus on attaining high rewards and lack of consideration of the potential penalties that may occur," says Fishbein. "Activation of the hippocampus in the psychopath group is likely related to the lack of appropriate emotional response to choices that incur penalties.
"Psychopaths' performance deficits correspond to less activation in the prefrontal cortex and more activation in the brain structures sensitive to error monitoring and reward," says Fishbein. "These activations suggest a lack of frontal inhibitory control over risky decisions."