Biology 202
1999 First Web Reports
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Personality: a Neurobiological Model of Extraversion

David Mintzer

Underlying the question of whether brain equals behavior is the possibility that one's personality may be understood on a neurobiological level. Personality affects how a person will behave in certain situations. Peoples' attitudes towards their environments, their dispositions, personal preferences and dislikes all help determine their everyday actions. If behavior is controlled by the nervous system, these factors which make up a person's personality must also fall under its direction. This does not refer to whether one's personality is a result of environment or genomic make-up. It has already been proposed that personality is 50%-70% hereditary and that home environment has little impact on child development (1,2). Here, nature vs. nurture is largely irrelevant; regardless of where one's personality comes from, it reflects chemical and electrical processes occurring within the nervous system.

The higher ordered personality trait which has been most studied for its neurobiological link is extraversion. On a hierarchical level extraversion often describes a person who is sociable, active, assertive and impulsive (3,4). It seems likely that these traits fall under neuronal influence. The first theory which attempted to explain personality biologically linked extraversion with general arousal level of the nervous system. Hans Eysenck proposed that the ascending reticulocortical activating system regulated arousal levels by opening and closing channels for incoming stimulation. J. Gray expounded on the physiology of extraversion through an animal model. The septohippocampal system regulates anxiety while septal-lateral hypothalamic and medial forebrain bundle influenced impulsivity. Studies have not fully supported these theories. EEG readings do not show a strong link between level of brain arousal and extraversion (5).

Recently Richard Depue (4) has developed the model further, proposing that extraversion is not a change in overall arousal level. Rather it is a motivated behavior; extraverts demonstrate an increased sensitivity to reward signals (incentive motivated behavior.) They seek social dominance and achievement, and react accordingly in social situations. According to Depue there are three criterion necessary to establish the neurobiological basis of such a personality: a) define the network of neural structures associated with the trait, b) explain how individual differences occur within the functioning of that network and c) identify the sources of individual differences. Using an animal model Depue found that the ventral tegmental area dopamine projection system facilitates incentive motivated behavior in rats. Dopamine agonists and antagonists in the ventral tegmental area facilitate and impair respectively exploratory, aggressive, social and sexual behavior. Similar phenomena is observed in humans-- dopamine activating drugs tend to stimulate an enhanced interaction with the environment. Individual differences in dopamine levels, as expected with extraversion, can result both genetically and environmentally. Different strains of inbred mice display differing dopamine and extraversion levels. While this supports the genetic explanation for individual personality differences, experience also plays a role. Ventral tegmental area dopamine release is modified throughout development through dendritic branching and growth and formation and destruction of new synapses. Increased stimulation of the ventral tegmental area can bring about such changes, evidence that experience can alter brain chemistry.

Further supporting Depue's theory, a direct relationship has been found between dopamine response and extraversion levels. Extraverted subjects were found (indirectly) to have significantly higher levels of dopamine than non-extraverted subjects. The enzyme monoamine oxidase, a dopamine inhibitor, is inversely related to extraversion.

Other research has described this dopamine model of personality as a behavioral on and off switch (6). The BFS (behavioral activation or facilitation system) regulates exploratory and goal-directed behavior through the release of dopamine. The off switch (behavioral inhibition system) is believed to be controlled by serotonin levels. Low levels cause impulsiveness and oversenesitivity (characteristics of extraversion) while high levels can cause anxiety (characteristic of intraversion). Extreme levels of either dopamine or serotonin play a role in personality disorders.

Of course a complex personality trait such as extraversion is not controlled solely by two or three neurotransmitters. It is most likely an elaborate interaction between many different chemicals and neurons in the brain. However, dopamine is a good place to begin investigation. Dopamine has far reaching influences in the nervous system. A single dopamine releasing neuron can form 500,000- 1,000,000 synapses with neighboring neurons. The amine neurotransmitters (dopamine, serotonin, norepinephrine) have been shown to be active in brain structures associated with emotion, motivation and cognition, all processes involved in personality formation (4).

The observed correlations between neurotransmitters and personality leads to a number of questions regarding self-determination, free will and choice. Do we have inherent predispositions towards certain personalities and behaviors? Do we have control over these predispositions? Even deeper, what is this sense of "I"? Does this biological theory of personality, where our attitudes, behaviors and dispositions are mediated by chemical levels, allow for any sense of self outside of these chemical reactions? Such questions lead us to doubt one's own responsibility for one's actions. Can somebody be held accountable for actions controlled by chemicals?

On an empirical level, neurobiology investigates these questions with a reductionist approach, examining chemical release at synapses and correlating such activity with behavior. Steven Rose, in his article "Lifelines: Biology, Freedom, Determinism" disputes this approach, asserting that reductionism has great difficulty establishing a causal relation between observed chemical phenomena and behavior (7). Just because certain chemicals are released during certain behavior, does not mean the chemical caused the behavior. As the title implies, Rose believes in the freedom and self-determination of organisms. Autopoiesis, a process whereby the organism develops itself, interacting with and affecting its environment, allows for these concepts.

Personality research has a long history in the field of psychology. Only in the last forty years, however, have scientists begun to look for biological explanations. Is it possible that eventually neurobiology will be able to explain personality on the same level as motor responses or our sense of hearing? It is unlikely in the near future that biology will confirm or refute Freud's theories on personality development. Still, the chemical basis of personality give us great insight into how and why we behave as we do. It can help explain why different people react differently to similiar situations. Perhaps most importantly, it offers the potential to alter such behavior, from treating mild cases of anxiety to severe personality disorders.

 

WWW Sources

1) Neurobiology's role in personality and emotion (membership needed to access article)

2) Do parents really matter? (membership needed to access article)

3) Biology of Personality

4) Neurobiologyo f the structure of personality: Dopamine, facilitation of incentive motivation, and extraversion. Depue, Richard A. & Collins, Paul F.

5) Psychobiology of Personality, Zuckerman, Marvin. Cambridge ; New York : Cambridge University Press, 1991

6) The Chemistry of Personality

7) Lifelines: Biology, Freedom, Determinism. Rose, Steven.




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