Arousal

Arousal is the physiological and psychological state of being awoken or of sense organs stimulated to a point of perception. It involves activation of the ascending reticular activating system in the brain, which mediates wakefulness, the autonomic nervous system, and the endocrine system, leading to increased heart rate and blood pressure and a condition of sensory alertness, desire, mobility, and reactivity.
Arousal is mediated by several neural systems. Wakefulness is regulated by the ARAS, which is composed of projections from five major neurotransmitter systems that originate in the brainstem and form connections extending throughout the cortex; activity within the ARAS is regulated by neurons that release the neurotransmitters norepinephrine, acetylcholine, dopamine, serotonin, and histamine.
Activation of these neurons produces an increase in cortical activity and subsequently alertness.
Arousal is important in regulating consciousness, attention, alertness, and information processing. It is crucial for motivating certain behaviours, such as mobility, the pursuit of nutrition, the fight-or-flight response, and sexual activity. It also holds significance within emotion, having been included in theories such as the James–Lange theory of emotion and being represented as one of the axes in most dimensional models for classifying emotions. According to Hans Eysenck, differences in baseline arousal level lead people to be extraverts or introverts.
The Yerkes–Dodson law states that an optimal level of arousal for performance exists, and too little or too much arousal can adversely affect task performance. One interpretation of the Yerkes–Dodson Law is the "Easterbrook cue-utilisation hypothesis".
Easterbrook's hypothesis suggests that under high-stress conditions, individuals tend to focus on a narrower set of cues and may overlook relevant information, leading to a decrease in decision-making effectiveness.

Neurophysiology

Wakefulness is regulated by the ascending reticular activating system, which is composed of five major neurotransmitter systems – the norepinephrine, acetylcholine, dopamine, serotonin and
histamine, systems – that originate in the brainstem and form connections which extend throughout the cerebral cortex. When stimulated, these systems produce cortical activity and alertness.
The noradrenergic system is a bundle of axons that originate in the locus coeruleus and ascends up into the neocortex, limbic system, and basal forebrain. Most of the neurons are projected to the posterior cortex which is important with sensory information, and alertness. The activation of the locus coeruleus and release of norepinephrine causes wakefulness and increases vigilance. The neurons that project into the basal forebrain impact cholinergic neurons that results in a flood of acetylcholine into the cerebral cortex.
The acetylcholinergic system has its neurons located in the pons and in the basal forebrain. Stimulation of these neurons result in cortical activity, shown from EEG records, and alertness. All of the other four neurotransmitters play a role in activating the acetylcholine neurons.
Another arousal system, the dopaminergic system, releases dopamine produced by the substantia nigra. The neurons arise in the ventral tegmental area in the midbrain, and projects to the nucleus accumbens, the striatum forebrain, limbic system, and prefrontal cortex. The limbic system is important for control of mood, and the nucleus accumbens signal excitement and arousal. The path terminating in the prefrontal cortex is important in regulating motor movements, especially reward oriented movements.
The serotonergic system has almost all of its serotonergic neurons originating in the raphe nuclei. This system projects to the limbic system and the prefrontal cortex. Stimulation of these axons and release of serotonin causes cortical arousal and impacts locomotion and mood.
The neurons of the histaminergic system are in the tuberomammillary nucleus of the hypothalamus. These neurons send pathways to the cerebral cortex, thalamus, and the basal forebrain, where they stimulate the release of acetylcholine into the cerebral cortex.
All of these five systems are linked and show similar redundancy. The pathways described are ascending pathways, but there also arousal pathways that descend. One example is the ventrolateral preoptic area, which release GABA reuptake inhibitors, which interrupt wakefulness and arousal. Neurotransmitters of the arousal system, such as acetylcholine and norepinephrine, work to inhibit the ventrolateral preoptic area.

Importance

Arousal is important in regulating consciousness, attention, and information processing. It is crucial for motivating certain behaviors, such as mobility, the pursuit of nutrition, the fight-or-flight response, and sexual activity. Arousal is also an essential element in many influential theories of emotion, such as the James–Lange theory of emotion or the Circumplex Model, among other models for describing emotions. According to Hans Eysenck, differences in baseline arousal level lead people to be either extraverts or introverts. Later research suggests that extroverts and introverts likely have different arousability. Their baseline arousal level is the same, but the response to stimulation is different.
The Yerkes–Dodson law states that there is a relationship between arousal and task performance, essentially arguing that there is an optimal level of arousal for performance, and too little or too much arousal can adversely affect task performance. One interpretation of the Yerkes–Dodson law is the Easterbrook cue-utilisation theory. It predicted that high levels of arousal will lead to attention narrowing, during which the range of cues from the stimulus and the environment decreases. According to this hypothesis, attention will be focused primarily on the arousing details of the stimulus, so that information central to the source of the emotional arousal will be encoded while peripheral details will not.
In positive psychology, arousal is described as a response to a difficult challenge for which the subject has moderate skills.

Personality

Introversion and extraversion

Eysenck's theory of arousal describes the different natural frequency or arousal states of the brains of people who are introverted versus people who are extroverted. The theory states that the brains of extroverts are naturally less stimulated, so these types have a predisposition to seek out situations and partake in behaviors that will stimulate arousal. Whereas extroverts are naturally under-stimulated and therefore actively engage in arousing situations, introverts are naturally overstimulated and therefore avoid intense arousal. Campbell and Hawley studied the differences in introverts versus extroverts responses to particular work environments in the library. The study found that introverts were more likely to choose quiet areas with minimal to no noise or people. Extroverts were more likely to choose areas with much activity with more noise and people. Daoussiss and McKelvie's research showed that introverts performed worse on memory tasks when they were in the presence of music compared to silence. Extroverts were less affected by the presence of music. Similarly, Belojevic, Slepcevic and Jokovljevic found that introverts had more concentration problems and fatigue in their mental processing when work was coupled with external noise or distracting factors. The level of arousal surrounding the individuals greatly affected their ability to perform tasks and behaviors, with the introverts being more affected than the extroverts, because of each's naturally high and low levels of stimulation, respectively.

Emotional stability vs. introversion-extraversion

or emotional instability and extroversion are two factors of the Big Five Personality Index. These two dimensions of personality describe how a person deals with anxiety-provoking or emotional stimuli as well as how a person behaves and responds to relevant and irrelevant external stimuli in their environment. Neurotics experience tense arousal which is characterized by tension and nervousness. Extroverts experience high energetic arousal which is characterized by vigor and energy. Gray claimed that extroverts have a higher sensitivity to reward signals than to punishment in comparison to introverts. Reward signals aim to raise the energy levels. Therefore, extroverts typically have a higher energetic arousal because of their greater response to rewards.

Four personality types

theorized that there are four personality types: choleric, melancholic, sanguine, and phlegmatic.
Put in terms of the five factor level of personality, choleric people are high in neuroticism and high in extraversion. The choleric react immediately, and the arousal is strong, lasting, and can easily create new excitement about similar situations, ideas, or impressions. Melancholic people are high in neuroticism and low in extraversion. The melancholic are slow to react and it takes time for an impression to be made upon them if any is made at all. However, when aroused by something, melancholics have a deeper and longer lasting reaction, especially when exposed to similar experiences. Sanguine people are low in neuroticism and high in extraversion. The sanguine are quickly aroused and excited, like the cholerics, but unlike the cholerics, their arousal is shallow, superficial, and shortly leaves them as quickly as it developed. Phlegmatic people are low in neuroticism and low in extraversion. The phlegmatic are slower to react and the arousal is fleeting.
The contrasts in the different temperaments come from individuals variations in a person's brain stem, limbic system, and thalamocortical arousal system. These changes are observed by electroencephalogram recordings which monitor brain activity. Limbic system activation is typically linked to neuroticism, with high activation showing high neuroticism. Cortical arousal is associated with introversion–extraversion differences, with high arousal associated with introversion. Both the limbic system and the thalamocortical arousal system are influenced by the brainstem activation. Robinson's study concluded that melancholic types had the greatest natural frequencies, or a "predominance of excitation", meaning that melancholics have a higher internal level of arousal. Sanguine people had the lowest overall levels of internal arousal, or a "predominance of inhibition". Melancholics also had the highest overall thalamocortical excitation, whereas cholerics had the lowest intrinsic thalamocortical excitation.
The differences in the internal system levels is the evidence that Eysenck used to explain the differences between the introverted and the extroverted. Ivan Pavlov, the founder of classical conditioning, also partook in temperament studies with animals. Pavlov's findings with animals are consistent with Eysenck's conclusions. In his studies, melancholics produced an inhibitory response to all external stimuli, which holds true that melancholics shut out outside arousal, because they are deeply internally aroused. Pavlov found that cholerics responded to stimuli with aggression and excitement whereas melancholics became depressed and unresponsive. The high neuroticism which characterizes both melancholics and cholerics manifested itself differently in the two types because of the different levels of internal arousal they had.