Researchers have long suspected that the chemistry of the brain largely influences personality and emotions. Now, a Cornell clinical psychologist has shown for the first time how the neurotransmitter dopamine affects one type of happiness, a personality trait and short-term, working memory.
"One personality trait in humans is how sensitive and responsive we are to incentives and rewards," said Richard Depue, professor of human development and family studies and director of the Laboratory of Neurobiology of Personality and Emotion at Cornell. Depue is an expert in the neurobiology of personality, emotion and temperament with particular expertise in the neurotransmitters dopamine, serotonin and norepinephrine. "Some of us are motivated by signals of incentive-reward and pursue goals, and others are not."
A major reason for the difference, he argues, is related to different levels of or responsiveness to dopamine, one of the chemical substances that transmits nerve impulses through the brain.
From a series of experiments with humans and based on what was already known from animal studies, Depue has concluded that dopamine is strongly related to the trait some researchers call extraversion, but Depue and his colleagues prefer to refer to it as "positive emotionality."
"This is the first time it has been shown in humans that a central nervous system neurotransmitter is associated strongly with an emotional trait in humans," Depue said.
The higher the level of dopamine, or the more responsive the brain is to dopamine, the more likely a person is to be sensitive to incentives and rewards. "When our dopamine system is activated, we are more positive, excited and eager to go after goals or rewards, such as food, sex, money, education or professional achievements," Depue said.
To examine this relationship, Depue first measured this trait in volunteers using personality tests. He then used Ritalin, an amphetamine widely prescribed for attention deficit disorder, to activate the dopamine system. How much the dopamine system is activated can be assessed by levels of a hormone (prolactin) in the blood and by changes in the rate of spontaneous eye blinks, which previous studies have shown to be significant.
Depue found that how reactive someone is to dopamine highly correlates with high scores on positive emotionality. People who responded easily to the drug and showed an increase in spontaneous eye blinks had a more active dopamine system in general and, Depue suspects, feel happier than others in response to incentives.
"We have strong evidence that the feelings of being elated and excited because you are moving toward achieving an important goal are biochemically based, though they can be modified by experience," Depue said.
He published his findings on dopamine's relationship to personality in the Journal of Personality and Social Psychology (1994, Vol. 67), on neurobiological factors in personality and depression in the European Journal of Personality(1995, Vol. 9) and on the neurochemistry of the incentive reward behaviors and how these behaviors are related to a universal personality trait in a forthcoming issue of Behavioral and Brain Science. In addition, he published the neurobiological implications for personality, emotion and personality disorder in the new book, Major Theories of Personality Disorder, edited by Jon Clarkin and Cornell Professor Mark Lenzenweger (Guilford Press, 1996).
By better understanding the role of dopamine in humans and how temperament types and personality traits can be driven biochemically, we can glean insight into personality and psychological disorders, Depue suggests. "There is now overwhelming evidence that 50 to 70 percent of individual variation in personality trait scores, for example, is related to genetic influence," he said.
He also points out that some research suggests that low levels of serotonin, which can result in irritability and volatile emotions, also may make people more responsive to dopamine. These people, therefore, may be more susceptible to drugs that activate the dopamine system, such as cocaine, alcohol, amphetamine and, to a lesser extent, opiates and nicotine. One theory is that different dopamine receptors in the brain may be related to different types of abuse and that people who have particularly low dopamine functioning may be more susceptible to depression and Parkinson's disease.
In related research, Depue has shown that dopamine is strongly related to how well the prefrontal cortex holds information. "To hold in short-term memory a spatial map of the environment, for example, you must have the dopamine system activated; without it, you can't do this type of cognitive functioning," Depue concludes from his research in this area.
Depue now is measuring the emotional responses of volunteers to emotional film clips before and after their dopamine systems have been stimulated or with the use of a placebo.
Depue's work is supported, in part, with grants from the National Institute of Mental Health.