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Introversion and extraversion
Hans 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 extrovert's are naturally less stimulated, so these types have a predisposition to seek out situations and partake in behaviors that will stimulate arousal. Therefore introverts are naturally over-stimulated, so they avoid intense arousal whereas extroverts are naturally under-stimulated, so actively engage in arousing situations. Campbell and Hawley (1982) 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 (1986) 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 (2001) 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
Neuroticism or emotional instability and extraversion 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. Extraverts experience high energetic arousal which is characterized by vigor and energy. Gray (1981) claimed that extraverts have a higher sensitivity to reward signals than to punishment in comparison to introverts. Reward signals aim to raise the energy levels. Therefore extraverts typically have a higher energetic arousal because of their greater response to rewards.

The Four Personality Types
Hans Eysenck theorized that there are four personality types: choleric, melancholic, sanguine, and phlegmatic, each embodying either high or low levels of two dimensions: neuroticism and extraversion. 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 (or more introverted). 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 (or more emotionally stable) 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 Electroencephalogram or EEG recordings which monitor brain activity. Limbic system activation is typically linked to neuroticism, which high activation showing high neuroticism, which is known to impair learning. Cortical arousal is associated with introversion-extraversion differences, with high arousal indicating introversion, which is known to facilitate learning. Both the limbic system and the thalamocortical arousal system are influenced by the brain stem activation. Robinson's study (1982) concluded that melancholic types had the greatest natural frequencies, or a "predominance of excitation," meaning that melancholics (who are characterized by introversion) have a higher internal level of arousal. Sanguine people (or those with high extraversion and low neuroticism) had the lowest overall levels of internal arousal, or a "predominance of inhibition ." Melancholics also had the highest overall thalamocortical excitation, whereas cholerics (those with high extraversion and high neuroticism) 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. 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, characterized by both melancholics and cholerics both manifested themselves differently because of the different levels of internal arousal both types had.

Cannon-Bard theory
The Cannon-Bard Theory is a theory of undifferentiated arousal, where the physical and emotional states occur at the same time in response to an event. This theory states that an emotionally provoking event results in both the physiological arousal and the emotion occurring concurrently. For example, a dear family member dies. A potential physiological response would be tears falling down your face and your throat feeling dry. You are "sad." The Cannon-Bard theory states that the tears and the sadness both happen at the same time. The process goes: event (family member dies) --> physiological arousal (tears) AND emotion (sadness) simultaneously.

James-Lange theory
The James-Lange Theory describes how emotion is caused by the bodily changes which come from the perception of the emotionally arousing experience or environment. This theory states that events cause the autonomic nervous system to induce physiological arousal, characterized by muscular tension, heart rate increases, perspiration, dryness of mouth, tears, etc. According to James and Lange, the emotion comes as a result of the physiological arousa l. The bodily feeling as a reaction to the situation IS the emotion. For example, someone just deeply insulted you and your family. Your fists ball up, you begin to perspire, and you are tense all around. You feel that your fists are balled and that you are tense. You then realize that you are angry. The process here is: event (insult) --> physiological arousal (balled fists, sweat, tension) --> interpretation (I have balled fists, and tension) --> emotion (anger: I am angry). This type of theory emphasizes the physiological arousal as the key, in that the cognitive processes alone would not be sufficient evidence of an emotion.

Schachter-Singer Two-Factor theory
The Schachter-Singer Two-Factor Theory or the cognitive labeling theory takes into account both the physiological arousal and the cognitive processes that respond to an emotion provoking situation. Schachter and Singer's theory states that an emotional state is the product of the physiological arousal and the cognition appropriate state of arousal. Meaning, that cognition determines how the physical response is labeled, either as "anger," "joy," "fear," etc. Emotion is a product of the interaction between the state of arousal as well as how one's thought processes appraise the current situation. The physiological arousal, however, does not label the emotion, but the cognitive label does. For example, let's say you are being pursued by a serial killer. You will be sweating and your heart will be racing, which is your physiological state. Your cognitive label will come from accessing your quickly beating heart and sweat as "fear." Then you will feel the emotion of "fear," but only after it has been established through cognition. The process is: the event (serial killer chasing you) --> physiological arousal (sweat, heart racing) --> cognitive label (reasoning; this is fear) --> emotion (fear).

Arousal and memory
Arousal is involved in the detection, retention, and retrieval of information in the memory process. Emotionally arousing information can lead to better memory encoding, therefore influencing better retention and retrieval of information. Arousal is related to selective attention during the encoding process by showing that people are more subject to encode arousing information than neutral information. The selectivity of encoding arousing stimuli produces better long-term memory results than the encoding of neutral stimuli. In other words, the retention and accumulation of information is strengthened when exposed to arousing events or information. Arousing information is also retrieved or remembered more vividly and accurately.

Although arousal improves memory under most circumstances, there are some considerations. Arousal at learning is associated more with long-term recall and retrieval of information than short-term recall of information. For example, one study found that people could remember arousing words better after one week of learning them than merely two minutes after learning them. Another study found that arousal affects the memory of people in different ways. Eysenck found an association between memory and the arousal of introverts versus extroverts. Higher levels of arousal increased the amount of words retrieved by extroverts and decreased the amount of words retrieved by introverts.

Arousal and preference
A person’s level of arousal when introduced to stimuli can be indicative of his or her preferences. One study found that familiar stimuli are often preferred to unfamiliar stimuli. The findings suggested that the exposure to unfamiliar stimuli was correlated to avoidance behaviors. The unfamiliar stimuli may lead to increased arousal and increased avoidance behaviors.

On the contrary, increased arousal can increase approach behaviors as well. People are said to make decisions based on their emotional states. They choose specific options that lead to more favorable emotional states. When a person is aroused, he or she may find a wider range of events appealing  and view decisions as more salient, specifically influencing approach-avoidance conflict. The state of arousal might lead a person to view a decision more positively than he or she would have in a less aroused state.

The reversal theory accounts for the preference of either high or low arousal in different situations. Both forms of arousal can be pleasant or unpleasant, depending on a person’s moods and goals at a specific time. Wundt’s hedonic curve and Berlyne’s hedonic curve differ slightly from this theory. Both theorists explain a person’s arousal potential in terms of his or her hedonic tone. These individual differences in arousal demonstrate Eysenck’s theory that extroverts prefer increased stimulation and arousal, whereas introverts prefer lower stimulation and arousal.

Problems associated with arousal
Arousal is associated with both anxiety and depression. Depression can influence a person’s level of arousal by interfering with the right hemisphere’s functioning. A study found arousal in women to be slowed in the left visual field due to depression, indicating the influence of the right hemisphere.

Arousal and anxiety have a different relationship than arousal and depression. People who suffer from anxiety disorders tend to have abnormal and amplified perceptions of arousal. The distorted perceptions of arousal then create fear and distorted perceptions of the self. For example, a person may believe that he or she will get sick from being so nervous about taking an exam. The fear of the arousal of nervousness and how people will perceive this arousal will then contribute to levels of anxiety.