Psychological scales have been designed to measure these hypothesized systems and study individual differences in personality.
[9] People with less sensitive systems are not easily aroused and seek additional stimulation, resulting in an extraverted personality.
[9] People with more sensitive systems are overstimulated and try to avoid additional stimulation, resulting in an introverted personality.
[9] The development of the biopsychological theory of personality occurred during Gray's time at Oxford where he was a fellow and lecturer.
[10] Gray's main critique of Eysenck’s theory was that introverts are not more sensitive to conditioning, but are more responsive to non reward and punishment.
The evidence Gray collected for his hypothesis on the biological basis of personality comes from blink tests done on humans and studies done on animals injected with sodium amobarbital.
Specifically, Gray's theory concentrated on understanding how reward or punishment related to anxiety and impulsivity measures.
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.
[6] Using a voxel-based morphometry analysis, the volume of the regions mentioned was assessed to view individual differences.
[15] The behavioral activation system (BAS), in contrast to the BIS, is based on a model of appetitive motivation - in this case, an individual's natural disposition to pursue and achieve goals.
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.
[21] 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.
Researchers besides Gray have shown interest in this theory and have created questionnaires that measure BIS and BAS sensitivity.
Carver and White created a scale that has been shown to validly measure levels of individual scores of BIS and BAS.
For example, people who were rated as having stronger reactions to stimuli should be lower in Extraversion as the strong sensation is uncomfortable to them, according to Eysenck’s theory.
[26] If a person with bipolar disorder self-reports high sensitivity to BAS, it means that a manic episode could occur faster.
The low BIS group was composed of convicted prostitutes who had been found to take part in illegal behavior.
It is a specifically designed questionnaire linking to Gray's theory referencing the SR to the BAS and the SP to the BIS.