Gray's biopsychological theory of personality
One of the most widely accepted theories in terms of biological models in psychology is the biopsychological theory of personality proposed by Jeffrey Alan Gray in 1970. Gray hypothesized two systems controlling behavioral activity, the Behavioural Inhibition System (BIS) and the Behavioural Activation System (BAS). The BIS is thought to be related to sensitivity to punishment as well as avoidance motivation, while the BAS is thought to be related to sensitivity to reward as well as approach motivation.
Using psychological test scales designed to correlate with the attributes of these hypothesized systems, neuroticism has been found to be positively correlated with the BIS scale, and negatively correlated with the BAS scale. 
The Biopsychological Theory of Personality is similar to another one of Gray's theories, reinforcement sensitivity theory. The Biopsychological Theory of Personality was created after Gray disagreed with one of his colleague's theories. This colleague was Hans Eysenck, and his arousal theory dealt with biological personality traits.  Eysenck looked at the ascending reticular activating system (ARAS) for answering questions about personality. The ARAS is part of the brain structure and has been proposed to deal with cortical arousal, hence the term arousal theory. Eysenck compared levels of arousal to a scale of introversion versus extraversion. The comparison of these two scales was then used to describe individual personalities and their corresponding behavioral patterns.  Gray disagreed with Eysenck's theory because Gray believed that things such as personality traits could not be explained by just classical conditioning.  Instead, Gray developed his theory which is based more heavily on physiological responses than Eysenck's theory.
Gray had a lot of support for his theories and experimented with animals to test his hypotheses.  Using animal subjects allows researchers to test whether different areas of the brain are responsible for different learning mechanisms. Specifically, Gray's theory concentrated on understanding how reward or punishment related to anxiety and impulsivity measures. His research and further studies have found that reward and punishment are under the control of separate systems and as a result people can have different sensitivities to such rewarding or punishing stimuli. 
Behavioral Inhibition System
The Behavioral Inhibition System (BIS), as proposed by Gray, is a neuropsychological system that predicts an individual's response to anxiety-relevant cues in a given environment. This system is activated in times of punishment, boring things, or negative events.  By responding to cues such as negative stimuli or events that involve punishment or frustration, this system ultimately results in avoidance of such negative and unpleasant events.  According to Gray's Theory, the BIS is related to sensitivity to punishment as well as avoidance motivation. It has also been proposed that the BIS is the causal basis of anxiety.  High activity of the BIS means a heightened sensitivity to nonreward, punishment, and novel experience. This higher level of sensitivity to these cues results in a natural avoidance of such environments in order to prevent negative experiences such as fear, anxiety, frustration, and sadness. People with a highly active BIS have been shown to learn more effectively through use of punishment than by reward. The physiological mechanism behind the BIS is believed to be the septohippocampal system and its monoaminergic afferents from the brainstem.
Behavioral Activation System
The Behavioral Activation System (BAS), in contrast to the BIS, is based on a model of appetitive motivation - in this case, an individual's disposition to pursue and achieve goals. The BAS is aroused when it receives cues corresponding to rewards and controls actions that are not related to punishment, rather actions regulating approachment type behaviors. This system has an association with hope.  According to Gray's theory, the BAS is sensitive to conditioned appealing stimuli, and helps curb impulsivity.  It is also thought to be related to sensitivity to reward as well as approach motivation. The BAS is sensitive to nonpunishment and reward. Individuals with a highly active BAS show higher levels of positive emotions such as elation, happiness, and hope in response to environmental cues consistent with nonpunishment and reward, along with goal-achievement. In terms of personality, these individuals are also more likely to engage in goal-directed efforts and experience these positive emotions when exposed to impending reward. The physiological mechanism for BAS is not known as well as BIS, but is believed to be related to catecholaminergic and dopaminergic pathways in the brain. Dopamine is a neurotransmitter commonly linked with positive emotions, which could explain the susceptibility to elation and happiness upon achieving goals which has been observed. People with a highly active BAS have been shown to learn better by reward than by punishment, inverse to BIS as mentioned above. 
Compare and Contrast
Together, the two systems work in an inverse relationship. In other words, when a specific situation occurs, an organism can approach the situation with one of the two systems. The systems will not be stimulated at the same time and which system is dominant depends on the situation in terms of punishment versus reward.  This phenomenon of the differentiation between the two systems is thought to occur because of the distinct areas in the brain that becomes activated in response to different stimuli. This difference was noted years ago through electrical stimulation of the brain. 
The behavioral activation system and behavioral inhibition system differ in their physiological pathways in the brain. The inhibition system has been shown to be linked to the septo-hippocampal system which appears to have a close correlation to a serotonergic pathway, with similarities in their innervations and stress responses. On the other hand, the activation, or reward system, is thought to be associated more with a mesolimbic dopaminergic system as opposed to the serotonerigic system. 
The two systems proposed by Gray differ in their motivations and physiological responses. Gray also proposed that individuals can vary widely in their responsiveness of the behavioral inhibition system and the behavioral activation system. It has been found that someone who is sensitive to their BIS will be more receptive to the negative cues as compared to someone who is sensitive to their BAS and therefore responds more to cues in the environment that relate to that system, specifically positive or rewarding cues. Researchers besides Gray have shown interest in this theory and have created questionnaires that measure BIS and BAS sensitivity. Carver and White have been the primary researchers responsible for the questionnaire. Carver and White created a scale that has been shown to validly measure levels of individual scores of BIS and BAS. This measure focuses on the differences in incentive motivations and aversive motivations. As previously mentioned these motivations correlate to impulsivity and anxiety respectively. 
Applications of the Biopsychological Model
Since the development of the BAS and BIS, tests have been created to see how individuals rate in each area. The questionnaire is called the Behavioral Inhibition System and Behavioral Activation System Questionnaire. 
People can be tested based on their activation of either systems by using an EEG. These tests will conclude whether a person has a more active BIS or BAS. The two systems are independent of each other. 
These tests can determine different things about a person's personality. They can determine if a person has more positive or negative moods.  Using psychological test scales designed to correlate with the attributes of these hypothesized systems, neuroticism has been found to be positively correlated with the BIS scale, and negatively correlated with the BAS scale.
Now doctors and other professionals can determine if a person with bipolar disorder is on the brink of a manic or depressive episode based on how they rate on a scale of BAS and BIS sensitivity. If a person with bipolar disorder self-reports high sensitivity to BAS, it means that a manic episode could occur faster. Also, if a person with bipolar disorder reports high sensitivity to BIS it could indicate a depressive phase. 
The BAS/BIS Questionnaire can also be used in the cases of criminal profiling.  Previous research as reported by researchers MacAndrew and Steele in 1991 compared two groups on opposite spectrum levels of fear and the response of a variety of questions. The two groups in the study varied on levels of BIS, either high or low, and were selected by the researchers. One group was composed of women who had experienced anxiety attacks and together made up the high BIS group. The low BIS group was composed of convicted prostitutes who had been found to take part in illegal behavior. Main findings showed that the responses to the questionnaires were distinctly different between the high BIS group and the low BIS group, with the convicted women scoring lower. Results from this study demonstrate that questionnaires can be used as a valid measurement to show differences in the behavioral inhibition systems of different types of people. 
Future research or implications
As mentioned previously, psychological disorders have been analyzed in terms of the behavioral inhibition and activation systems. Understanding the differences between the systems may relate to an understanding of different types of disorders that involve anxiety and impulsivity. To date, there are many types of anxiety disorders that deal with avoidance theories and future research could show that the behavioral activation system plays a large role in such disorders and may have future implications for treatment of patients.
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