This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.
2004 First Web Paper
Adolescents between the ages of 13 and 19 tend to act impulsively and irrationally. Testing limits, experimenting, and acting without considering future consequences are all part of adolescent behavior according to Dr. Laurence Steinberg of Temple University (1). He states that teenage self-regulation of impulsive behavior does not appear to mature until later in adolescence (1). The perceived rebellious actions of teenagers that were once dismissed as changes in hormones corresponding with the beginning of puberty may actually be due to functional differences in teenage brains. The behavioral differences between adults and teenagers become recognizable due to the increased freedom and decision making that adolescents acquire. Studying the variations in the brains of adolescents and adults provides evidence for the argument that the actions of the nervous system are responsible for observed behaviors.
Two studies have identified differences between adolescent and adult brains. One study conducted by Dr. Arthur Toga of the Laboratory of Neuro Imaging located at UCLA demonstrates that children and adolescents from ages 12 to 16 have less myelination in the frontal lobes of the brain (2). The frontal lobes, located at the front of the cranium, have been identified as the area of the brain that dictates rational behavior and reasoned weighing of consequences (4). Myelin is composed of neural cells that form insolating lipid layers around nerve processes. Myelinated processes can more effectively conduct electrical signals from one neuron to another. The presence of more myelin in adult frontal lobes implies that more neural processes are connecting neurons together. If connections between neurons in adolescent frontal lobes are not as abundant, adolescents may not be as capable of using their frontal lobes Decreased myelination may mean that neurons in the frontal lobes of children and teenagers are not as interconnected and not as capable of communicating via passing signals as the neurons of adult frontal lobes, resulting in decreased ability to make reasoned decisions. Dr. Jay Giedd of the National Institutes of Mental Health also studied the adolescent brain using magnetic resonance imaging. Dr. Giedd has identified a growth period of the neuron bodies or gray matter in the prefrontal cortex, a specific section of the frontal lobes, at ages 11 in girls and 12 in boys (3). Though adolescents contain more gray matter than adults, neurons are connected throughout the teenage years so development and usage of the frontal lobes occurs gradually. Throughout adolescence, the brain decreases the amount of synapses and increases the amount of myelination of certain processes in order to strengthen them (3). He concludes that the adolescent brain has not made adequate neural connections and can be shaped by activities throughout the maturation process (3).
If the connections in the frontal lobes of children and teens are not as developed as the brains of adults, another portion of the adolescent brain may be used in tasks where adults normally process inputs with their frontal lobes. In a study conducted by Dr. Deborah Yurelun-Todd of Harvard University, brain activity was scanned using functional magnetic resonance imaging (5). Both adults and adolescents from ages 11 to 17, who had no diagnosed psychological disorders or brain injuries, were asked to identify the emotion on pictures of faces on a computer screen (5). The expression of the picture shown to the participants was one of fear. The teens typically activated the amygdala while the adults activated the frontal lobes to perform the same task of identifying the expression (5). Because teens and adults are activating different portions of their brains to perform the same task, studying the function of the amygdala may provide an explanation for observed behavioral differences in adolescents and adults.
The amygdala is part of the limbic system and is responsible for emotional reactions. Dr. Jean-Marc Fellous states that the amygdala is responsible for emotional processing and reactionary decision making because lesions of this region interfere with emotional reactions (6). By using the area of the brain that identifies situations with emotions, adolescents react in an impulsive manner more than a reasoned one. The increased activity of the amygdala in teens may be because the frontal lobes have not yet developed a regulatory role in the nervous system. Dr. Richard Davidson of the University of Wisconsin-Madison found that in 500 individuals who had decreased activity in their frontal lobes, they also had decreased ability to regulate emotion (7). Davidson concludes that there may be some interaction between the amygdala and the frontal lobes (7). Like the individuals Davidson studied, adolescents may not have the ability to sufficiently regulate emotional processes because their frontal lobes have not matured. The impulsive behavior of adolescents is due to the increased reliance on the instinctual part of the brain while the area for rational thought, the frontal lobes, develops.
Further evidence to support the nervous system producing all behavior would be to observe different behaviors corresponding to varying neural connections within the frontal lobes. Signals to different neurons would be expected to produce different types of behavior if interactions of the nervous system are responsible for producing behavior. Since the frontal lobes are still forming myelinated connections between neurons during adolescence, environmental factors can influence development of varying connections. Dr. Giedd identifies the time between 13 and 18 when connections are made as the "use it or lose it principle (3)." He says that the activities that teens participate in will influence the connections made in the brain (3). If the neuron process connections are not properly made through sufficient stimulus, reduced function of the frontals lobes can result. Different environmental inputs can influence the development of teenage frontal lobes (3). If Dr. Giedd is correct that connections can be influenced by different stimuli, monitoring a child's behavior, setting rules and seeing that they are obeyed should promote the development of regulatory connections in the frontal lobes. An individual who grows up in an environment where regulation of emotions is encouraged would be expected to have different myelinated processes than an individual where such activity is not promoted. Environmental inputs may have an important role in forming connections between neurons that leads to increased reasoning ability and self-regulation of emotional behavior. More studies are needed to support Dr. Giedd's theory. One potential study would be to map white matter, myelinated processes, in children who grew up in various household environments.
The structure of the adolescent brain provides an explanation for the perceived teenage behavior of irrationality and impulsiveness. This behavior can be attributed to activation of the amygdala the region of the brain responsible for emotional behavior. Mature frontal lobes may regulate the actions of the amygdala and allows individuals to reason through situations instead of acting on instinct. Poor connections between neurons formed during adolescence may lead to less emotional regulation as an adult. Since differences in the adult and adolescent brains can be correlated to different types of behaviors, the variations in the brains of adolescents and adults provides evidence for behaviors being produced by the activity of the nervous system. Future studies to further correlate adolescent behavior to functional brain differences would include functional magnetic resonance imaging study of the use of the amygdala over the frontal lobes and further evidence that the frontal lobes do regulate the activity of the amygdala.
1)The Study of Abnormal Psychopathology in Adolescence, This is the web version of Dr. Steinberg's paper that outlines some normal and abnormal adolescent behaviors.
2)Teenage Brain: A Work in Progress, This site from the National Institute of Mental Health presents several studies on the development of the teenage brain, mainly through MRI imaging.
3)Adolescent Brains are Works in Progress, This site from Frontline presents data obtained from Dr. Jay Giedd's studies of the development of the adolescent brain. Dr. Giedd focuses on prefrontal cortex development study, but also addresses Corpus Callosum and Cerebellum development.
4)Frontal Lobes, This site gives some background on frontal lobe structure and function. Some research on possible frontal lobe abnormalities and consequences are also presented.
5)Deciphering the Adolescent Brain, This is a web version of an article published in the Harvard University Gazette that presents the research performed by Dr. Deborah Yurelun-Todd. She studies the use of the amygdala as opposed to the frontal lobes in children and adolescents.
6)Emotional Circuits and Computational Neuroscience, This site is the online version of a paper by Dr. Jean-Marc Fellous and colleagues Jorge L. Armony and Joseph E. LeDoux. They determine that many emotional responses originate from the amygdala.
7)Brain's Inability to Regulate Emotion Linked to Impulsive Violence, Research conducted by Dr. Davidson on the regulatory role that the frontal lobes play is presented in this article.
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