Motivation and emotion/Book/2023/Dopamine and extraversion-introversion

Overview


Ben often finds it difficult to keep up with his wife Trisha. She is a social butterfly and is always assertive and enthusiastic in conversations. She often wants to have others around the house for dinner, and constantly desires outgoing social activities. Ben, on the other hand, appreciates his quiet time, opting to read or engage in activities by himself. He often feels socially drained, and recharges by having a quiet space to think through his thoughts.

Overall, Ben exhibits introverted personality traits and Trisha exhibits extraverted personality traits. How might their individual differences in dopamine system functioning and anatomy influence their levels of trait extraversion?

Focus questions:


 * What is extraversion?
 * What is dopamine?
 * What is the dopamine hypothesis of extraversion?
 * How does dopamine affect extraversion?
 * How do introverts respond to dopamine system activation relative to extraverts?
 * What key theories guide research on dopamine's affect on extraversion?
 * What are the current limitations of this line of research?

History


Extraversion is considered to be a stable factor of personality. Its modern interpretation is predicated on ideas of individual differences of personality, extending back to Hippocrates. However, contemporary discussions of extraversion as a component of personality begin with Carl Jung, a famous Swiss psychiatrist and founder of analytical psychology. In Jung's Psychological Types (1921), he proposed a theory in which human beings are comprised of distinct psychological characteristics, highlighting introverted verses extroverted tendencies existing amongst individuals (see Figure 2).

Contemporary measurement
Throughout the 20th century, the scientific study of personality developed, narrowing and defining its conceptual frame. Between 1990 and 2010, five and six-factor models of personality emerged. Such models continue to prevail in contemporary psychology. Within them, introversion-extraversion, now commonly referred to simply as extraversion, remains a prominent factor of personality (Costa & McCrae, 1992; Lee & Ashton, 2004; DeYoung et al., 2007).

Behaviour
It is genuinely the case that people are different. Indeed, factors of personality such as extraversion influence individual's, thoughts, perceptions, values, and, therefore, behaviour (DeYoung et al., 2007). Whilst different facets or aspects of extraversion exist depending on the model from which it is measured, the overarching consensus amongst psychologists is that the extraversion factor comprises distinctly reliable characteristics. To emphasise, those scoring high in extraversion (low in introversion) are socially assertive and enthusiastic, gregarious, experience high levels of positive emotion, and are overall oriented towards stimulation and gratification of rewards; in contrast, those scoring low in extraversion (high in introversion) are more reserved, have less social motivation, prefer lower levels of stimulation, and are overall oriented towards internal forms of gratification such as thoughts and feelings (Costa & McCrae, 1992; Lee & Ashton, 2004; DeYoung et al., 2007).

What is dopamine?


Dopamine (3,4-Dihydroxyphenethylamine) is an endogenous neurotransmitter. Neurotransmitters are chemical molecules which affect electrical activity of neurons. Neurons are stimulable cells which transmit signals throughout the brain and body (Fries, 2005). Fundamentally, neurons are connected and communicate with each other via large cellular networks. This electrical communication is necessary for motor processes, internal communication, behaviour, thoughts, and more.

Dopamine exists within the simplest biological organisms such as nematodes, right through to complex organisms such as humans (Barron et al., 2010). In humans, dopamine has a wide range of specific and non-specific functions and effects within motivation, including seeking, reward, movement, and learning.

Mechanisms
Dopamine is manufactured in the ventral tegmental area (VTA), hypothalamus, and substantia nigra areas of the brain (Juárez et al., 2016). Broadly, dopamine acts upon a set of five dopamine receptors (D1, D2, D3, D4, and D5) modulating the communication of various brain areas; in turn, these brain areas signal to other parts of the brain and body, influencing facets of motivation and facilitating movement, approach, excitement and 'feel good' emotions, and sustained focus and attention (Mishra et al., 2018).

Movement
Presently, dopamine has been isolated as responsible not merely for the motivation to move, but indeed in movement itself. Movement deficits, such as those present in Parkinson's disease, share a large connection to dysfunction within the dopamine producing systems (Mishra et al., 2018). For example, in a healthy person, neurons in the substantia nigra and basal ganglia communicate via dopaminergic neurotransmission, resulting in smooth and controlled fine tuning of ones movement. Individuals with Parkinson's disease experience progressive degeneration of neurons in the substantia nigra. As a result of such degeneration these individuals suffer a reduction of the availability of dopamine, causing hesitation, stiffness, and an overall lack of movement control (Gepstein et al., 2014).

Positive emotion
Joy, contentment, happiness: These words are frequently associated with the conception of positive emotion, or a state of positivity (Carl et al., 2013). Positive emotion is often experienced as one attains or progresses towards a goal, or is faced with a positive opportunity; in turn, dopamine directly affects physiological mobilisation and motivation within individuals (Carl et al., 2013; Ashby et al., 1999). Positive emotionality, trait extraversion, and extraverted motivations, although conceptually different, have been associated in numerous studies (Costa & McCrae, 1992; Lee & Ashton, 2004; DeYoung et al., 2007), indicating that there is a link between positive emotion and extraversion.

Early studies
Because dopamine plays such a key role in enthusiasm, positive emotion, and seeking 'fun' activities, and because extraversion is typically utilised as an indicator of these things, it was proposed that dopamine and extraversion were intrinsically linked. But how does dopamine affect motivation towards extraverted and introverted behaviour? In the mid-1990's, hypotheses and studies surrounding this question began to emerge.

Depue et al. (1994) hypothesised that activation of specific dopamine receptors would produce extraverted effects such as positive emotionality. In line with their hypothesis, they found that this activation was associated with such increases, implying a relationship between dopamine circuitry and extraversion. Following this, Stelmack and Pivik (1996) theorised that there was an association between individual differences in neuronal wiring and extraversion in certain brain regions. The authors posited that dopamine terminal quantity varied individually, and that this would be associated with differences in extraversion; in short, those having reduced availability of dopamine terminals would be more introverted and vice-versa. This link was empirically supported, shortly thereafter by Ebstein et al. (1996) who discovered that variation on the dopamine D4 receptor gene, which affected D4 receptor quantity, was associated with extraversion, particularly aspects of novelty seeking, indicating that perhaps specific receptors were involved in extraversion. Korr and Kumari (1997) then conducted the first study showing that drug-induced dopamine system arousal was associated with trait extraversion. Just prior to the year 2000, Depue and Collins (1999), now influential in this area of research, proposed a theory that extraverts have higher levels of dopamine receptor activation in the mesocorticolimbic circuit, leading to a lower behavioural activation threshold, meaning that such individuals are more salient towards motivational stimuli.

Between 2000 and 2010


Between 2000 and 2010, the area of research expanded the dopamine hypothesis of extraversion to exploring more specific neurophysiological systems. Early on, Lieberman and Rosenthal (2001) highlighted contradictory findings within this field of research. They concluded that study of particular neurocognitive systems was required to understand individual differences in favour of generalised comprehensive biological theories of extraversion. As such, many studies continued to narrow focus of the dopamine influence, particularly focusing on areas of mesencephalic cell groups (Rammsayer, 2004). This may sound complicated, but it is relatively straight forward to understand. Mesencephalic (or 'midbrain') cell groups are strongly associated with motivation, movement, and reward, and are highly dopaminergically innervated, meaning they contain a high concentration of dopamine nerves (neurons). Generally, the midbrain dopamine system can be divided into two subsystems: The mesostriatal system, involved in movement and coordination and the mesocorticolimbic (see Figure 4) circuit, involved in reward processing and motivation (Rosenbury et al., 2015).

Throughout this time period, studies highlighting the association between the dopamine system and extraversion continued to emerge. Research continued to identify presence of specific dopamine receptor genes as producing significant differences in extraversion. For example, Golimbert et al. (2007) found that particular variations on the dopamine D4 receptor was correlated with such differences in members of the Russian population. However, their results were somewhat unexpected, as they found this correlation amongst women only. Nevertheless, their results appeared consistent with other cross-cultural studies (Bookman et al., 2002; Tsai et al., 2004). Overall, it was generally found that such genetic variation on the dopaminergic D4 receptors was related to extraverted characteristics personality in women, but aggression tendencies in men. These results implied both that the dopaminergic system and its relation to extraversion is complex and dynamic, and that further investigation was required.

2010 to present
Despite expanded research on the dopaminergic systems affect on extraversion in the early 2000's, the relationship was still unclear. For the past decade, investigations into proximal and causal relationships have continued to expand.

The behavioural activation system (BAS) is a 'conceptual nervous system' introduced by Jeffrey Gray (1981) which plays a key role in reward sensitivity, approach, and driving behaviour (DeYoung et al., 2010). In short, it describes traits included within extraversion such as talkativeness and assertiveness, which seem to reflect approach behaviour. In 2013, Wacker et al. conducted the first experiment manipulating dopamine D2 receptors with 200mg of the drug sulpiride, and found that this dopamine D2 receptor blocker inhibited BAS activity, evidenced by lower activation of BAS associated brain areas, and reduced extraverted motivations. Overall, the study implied that the D2 receptor plays a role in extraverted traits. Importantly however, their study was limited insofar as it only included male participants. Nevertheless, this implied further associations of dopamine receptors as being involved with extraverted traits. Those high in trait extroversion have also been shown to have more sensitive reward circuitry associated with behavioural conditioning. Depue and Fu (2013) showed that participants high in trait extraversion retained conditioned behaviour more than other individuals when administered a drug which heightened dopaminergic activity. This suggests that extraverts have increased reward sensitivity and are thus more motivated to pursue reward compared to introverts. Indeed, this specific line of research continues.

Following from this, in a review of the literature, Smillie (2013) highlighted an increasing trend of research focusing on the reward aspects of extraversion, outlining a new yet somewhat undefined reward-processing theory of extraversion to have emerged. By contrast, in a unifying theory of the role of dopamine in personality, Colin DeYoung (2013) proposed that the role of dopamine plays a key role in the determination of personality traits. Broadly, his theory suggests that dopamine is linked to a fundamental dimension of human personality, referred to as "exploration." This dimension encompasses motivation towards novelty seeking, curiosity, creativity, and openness to experience. Despite the increasing trend in research, he argues that dopamine's influence is not limited to just reward-seeking behaviours, as commonly believed, but extends to these broader array of traits and functions.

Responding to subsequent trends towards theoretical approaches, Wacker and Smillie (2015) conducted a review of the literature. In short, they argued that individual differences in extraversion relating to dopamine system variation was inconclusive. They assert that although evidence indicates extraversion is associated with increased dopamine sensitivity, current research puts too large an emphasis on "theoretically consistent associations", concluding that future research must focus on particular causal evidence.

Since then, research has only somewhat expanded. At this time, although the dopamine hypothesis of extraversion has significantly developed, more distinct approaches are being utilised in measurement, and different theory is being applied, influential research in this area has yet to be identified. Nevertheless, some studies are worth mentioning. For example, Goclowska et al. (2019) found that traits associated with exploration were associated with characteristics presented by DeYoung's (2013) unifying theory, further giving credence to his ideas. However, their study was limited somewhat as they did not specifically influence dopamine pathways. Similarly to Wacker (2013), Ohmann et al. (2020) also administered 200mg of sulpiride. Contrary to their expectations, yet along previous lines, they found less effort exhibited (lower motivation) when presented with a reward-based task amongst all participants, regardless of trait extraversion.


 * Case study - Erica

Erica is a university student in her early twenties. She likes to study with her group of friends in the library, and sometimes finds it difficult to clarify her thoughts when she doesn't have her group of friends around. This past weekend, Erica enjoyed interacting with her university peers at a couple of parties, one of which she hosted (see Figure 5). She invited everyone she could and was loud and boisterous while frequently asserting herself in the various group contexts.

{Erica most likely has an: - Introverted personality + Extroverted personality
 * type=""}

{Those scoring very high in introversion generally have: + A tendency to enjoy one-on-one interactions - Dopamine system irregularities
 * type=""}

{The dopamine hypothesis of extraversion suggests that Erica: + Has increased dopaminergic sensitivity - Has decreased dopaminergic sensitivity
 * type=""}

Conclusion
This chapter delved into the intricate connection between dopamine and extraversion, seeking to answer the fundamental question: How does dopamine affect extraversion-introversion? By now, you should have a basic understanding of the differences between extraversion and introversion, and understand it as a continuum, comprehend dopamine and its mechanisms, and grasp dopamine's affects on extraversion.

While dopamine is indeed linked to extraverted motivation and behaviours, it is not a one-size-fits-all connection. The role of dopamine in shaping personality is complex and dynamic, with individual differences and gender-specific variations. Understanding the interactions between dopamine and extraversion will have implications for personal growth and development and researchers should emphasize the need for particular causal evidence on how dopamine affects extraversion.

Revisiting focus questions:


 * What is extraversion? Extraversion is a personality factor characterised by traits of assertiveness and enthusiasm, gratification of rewards, and novelty seeking. Introverts, however, are less assertive and enthusiastic, have less social motivation, and prefer internal forms of gratification.
 * What is dopamine? It is an endogenous neurotransmitter responsible for movement, seeking, reward, motivation and learning.
 * What is the dopamine hypothesis of extraversion? The dopamine hypothesis of extraversion suggests that dopamine is closely linked to extraverted motivation and behaviours based upon physiological differences and dopamines effects.
 * How does dopamine affect extraversion? Dopamine's influence on extraversion lies in its role in motivating individuals towards outgoing and social behaviors. It is associated with the experience of positive emotions and the pursuit of reward.
 * How do introverts respond to dopamine system activation relative to extraverts? Individuals with reduced availability of dopamine terminals may exhibit more introverted tendencies, while those with higher levels of dopamine receptor activation may lean towards extraversion.
 * What key theories guide research on dopamine's affect on extraversion? Theories include the activation of specific dopamine receptors leading to extraverted effects, differences in dopamine terminal quantity being associated with introversion or extraversion, and the role of dopamine receptor genes in personality traits.

Future research in the field of dopamine and extraversion should continue to explore this complex relationship and address limitations, with the following areas being expanded:


 * Causal evidence: Researchers should focus on obtaining more causal evidence to understand how dopamine directly influences extraverted and introverted behaviours and motivation. This may involve experiments that manipulate dopamine levels in controlled settings to observe specific effects on extraversion.
 * Neuroimaging: Neuroimaging technology can provide a more detailed understanding of how specific brain regions and pathways related to dopamine contribute to extraversion. This could help pinpoint the neural mechanisms of dopamine and link them specifically to extraversion.
 * Clinical implications: Understanding the relationship between dopamine and extraversion could have clinical implications. For example, it might help in the development of treatments for conditions where motivation and reward processing are affected, such as depression.
 * Theoretical inconsistency: Addressing theoretical inconsistency in research on dopamine's affect on extraversion is crucial for developing a more accurate and holistic understanding of this complex relationship.

In conclusion, future research on dopamine and extraversion should aim to provide a more comprehensive, nuanced understanding of this relationship, addressing the complexity and individual variability involved. This knowledge can have practical applications for personal development, mental health, and a deeper understanding of human behaviour.