Motivation and emotion/Book/2017/Biological factors in emotional reactivity

Overview
Have you ever heard someone described as "an emotional person"? Although happiness is an emotion, you probably do not picture an excitable, happy person when you hear this phrase. More likely, you picture a person who is easily upset, cannot control their emotions, or is sensitive to emotional events. In other words, you picture someone with negative affective style, poor emotion regulation, or who is highly emotionally reactive (Table 1).

Table 1.

Definitions of affective style, emotion regulation, and emotional reactivity.



Imagine you failed a university course. The immediacy, intensity, and duration of your response represent emotional reactivity. If you are immediately overwhelmed by immense disappointment, and struggle to recover, you would score highly on a measure of emotional reactivity. Emotional reactivity can be studied using self-report, observational and physiological measures. The Emotional Reactivity scale (ERS; Nock et al., 2008), and the Perth Emotional Reactivity scale (PERS; Becerra, Preece, Campitelli, & Scott-Pillow, 2017) are self-report measures. Although positive and negative reactivity are often considered dichotomous, a dip in one does not necessarily correspond to a rise in the other; they are distinct dimensions (Becerra et al., 2017). The PERS differentiates between these dimensions, while the ERS does not (Becerra et al., 2017). Heart rate, skin conductance, and patterns of brain activation are measured to infer arousal (degree of activation), because emotional reactions are assumed to coordinated expressive, physiological, and experiential responses (Mauss, Levenson, McCarter, Wilhelm, & Gross, 2005).

This chapter explores the biological causes and correlates of emotional reactivity, and their implications. Biological theories of emotion propose a finite number of basic emotions based on neural substrates, innate universal expressions, and unique feeling-motivational states (Izard, 1992). Neural substrates are the primary focus of this chapter, followed by applied perspectives relating reactivity to genetic predisposition and psychological well-being. As most literature on emotional reactivity uses positive and negative valence as umbrella terms for variants of joy and interest, and variants of fear, disgust, sadness and anger respectively, this chapter will follow suit. Interestingly, Arnold's foundational cognitive appraisal theory of emotion (1960) also supports bivalent categorisation, suggesting emotions result from subcortical appraisals of whether stimuli are beneficial or harmful. The underlying physiology of individual differences in emotional reactivity has important implications for understanding behaviour and improving psychological well-being.

Emotional reactivity and the brain


The brain is complex, and separating the neurological correlates of emotional reactivity and emotion regulation is challenging (Domes et al., 2010). Reaction and regulation seem sequential, however top-down and bottom-up emotional processes are bidirectional and difficult to differentiate. Keep in mind emotional reactions both affect and are affected by regulation.

The forebrain, comprising the cerebral cortex and the limbic system, is where most differences in emotional reactivity arise. The cerebral cortex is the outermost layer of the brain, and has distinct areas involved in movement, sensation, and executive functions. The limbic system is a subcortical collection of structures important for emotion and motivation, and receives information from both cortical and mid/hindbrain structures. See Figure 2 for a depiction of terms used to describe anatomical locations.

Prefrontal cortex
The prefrontal cortex (PFC) is the area of the cerebral cortex corresponding approximately to the forehead (Figure 3). The PFC exhibits cognitive control when information is conflicting or ambiguous, guiding behaviour based on prior learning and current goals (Miller & Cohen, 2001). Miller and Cohen's model of cognitive control (2001), found that contextual cues from the PFC bias other regions towards enhanced or reduced reactivity to emotional stimuli, supporting its role in reactivity, beyond top-down regulation.

Consistent with the idea of stable PFC activity influencing emotional reactivity, the valence hypothesis suggests cortical asymmetry represents a diathesis, or predisposed valent-specific sensitivity to emotional stimuli (Wheeler, Davidson, & Tomarken, 1993). Lateralised dominance is proposed to bias reactivity of other brain regions, and reflect trait characteristics (Table 2). Left hemispheric dominance is associated with higher behavioural approach system (BAS) scores, and right with higher behavioural inhibition system (BIS) scores (Sutton & Davidson, 1997). BAS and BIS represent trait sensitivity to incentives or threats, and are associated with extroversion and neuroticism respectively (Carver & White, 1994). The valence hypothesis suggests these traits predict reactivity, supported by findings that left hemispheric dominance was associated with increased reactivity to a positive film, whereas right hemispheric dominance was associated with increased reactivity to a negative film (Wheeler et al., 1993). However, findings that right PFC inhibition enhances, rather than reduces attention to aversive stimuli (Berger, Domes, Balschat, Thome, & Höppner, 2017) are inconsistent with the valence hypothesis. Although emotional reactivity and BIS/BAS tendencies represented by cortical asymmetry are clearly related, it could be argued that sensitivity to potential incentives or threats are more accurately conceptualised as motivational than emotional tendencies.

Table 2.

Factors associated with left or right hemispheric dominance.



There is a weak, negative correlation between dimensions of positive and negative reactivity, with most people responding more strongly to one than the other (Becerra et al., 2017). What patterns of brain activation might be associated with individuals who score high on both BIS and BAS, or neuroticism and extraversion measures? Would they be more or less emotionally reactive than someone with low scores on both measures?

Dorsolateral PFC
The dorsolateral PFC (dlPFC) has connections with areas important for motor control and voluntarily shifting gaze (Miller & Cohen, 2001), consistent with its role in executing planned behaviours. This also reflects its involvement in approach and avoidance behaviours and representations of goal states. Imaging techniques have shown this region to be active in both up and down-regulation of emotional responses, likely suppressing distraction and maintaining focus on regulatory goals (Eippert et al., 2006). Stimulation increasing left dlPFC activity resulted in less time spent looking at threatening stimuli, thereby reducing reactivity (Chen, Basanovic, Notebaert, MacLeod, & Clarke, 2017), demonstrating how frontal regions proactively influence reactivity.

Ventral PFC
The ventromedial PFC (vmPFC) and orbitofrontal cortex(OFC) lie in the lower prefrontal region and contribute to top-down regulation of limbic regions (Hare et al., 2008; Eippert et al., 2006). During adolescence, the ventral PFC shows increased response to both happy and sad expressions, indicating its function is not valent-specific (Hare et al., 2008). The vmPFC may represent both positive and negative states in the absence of immediate incentives (Davidson & Irwin, 1999). Berridge and Kringelbach (2013) differentiate between representation and causation of emotion, concluding that prefrontal regions, particularly the OFC, are representative, rather than causal. While prefrontal regions may be necessary to consciously experience emotion, emotion does not originate from these regions, but from structures in the limbic system.

Limbic system
The limbic system forms a border around the brainstem and includes the olfactory bulb, hypothalamus, hippocampus, amygdala and cingulate gyrus of the cerebral cortex. This system is linked with the thalamus and basal ganglia and represents emotional behaviours, motivation, long-term memory and olfaction (Kalat, 2019).

Anterior cingulate cortex
The anterior cingulate cortex (ACC) lies ventral to other cortical regions, and is highly connected to the limbic system. It is sometimes considered a prefrontal area (eg. Eippert et al., 2006), and is thought to connect the limbic system with the PFC, acting as "an interface between emotion and cognition" (Allman, Hakeem, Erwin, Nimchinksy, & Hof, 2001). This ACC regulates amygdala activity either directly, or by recruiting prefrontal regions to exhibit top-down control (Eippert et al., 2006).

Amygdala
The amygdala has been highlighted by cognitive (Arnold, 1960) and biological theorists, as essential in activating emotion. It is seen as an origin of emotional experience, and communicates with cortical structures, and lower brain regions involved in preparing the body to respond to threats (Arnsten, 2009). Although often associated with fear, the amygdala plays a broader role in determining whether a situation is harmful or beneficial (Yaniv, Desmendt, Jaffard, & Richter-Levin, 2004). Women, who are stereotypically more emotional than men, show greater amygdala reactivity in response to neutral stimuli (Domes et al., 2010); it is the appraisal of a stimulus as relevant to one's well-being, not the stimulus itself, that leads to emotion. Just as in cortical regions, amygdala lateralisation contributes to valent-specific affective sensitivity. Right amygdala dominance is associated with higher dispositional negative affect (Davidson & Irwin, 1999), while stronger left amygdala responses to happy faces are predicted by higher extroversion scores (Canli, Sivers, Whitfield, Gotlib, & Gabrieli, 2002).

Basal ganglia
The basal ganglia are highly connected subcortical structures relevant to emotional reactivity, including the caudate nucleus and the nucleus accumbens (NAcc). The caudate nucleus, like the amygdala, is more reactive to positive stimuli in extraverts (Canli et al., 2001). This region is often associated with reward, or positive emotion, however this has been challenged by a study finding stronger caudate responses to negative than positive or neutral pictures (Carretie et al., 2009). The NAcc is now being investigated as an origin-point of emotion. A review by Berridge and Kringelbach (2013) supported the causal role of the NAcc in pleasure responses, describing a "keyboard mechanism" for valence generation of hedonic like-dislike emotions. Specific locations or "hedonic hotspot circuits" along the medial shell of the NAcc were found to trigger different mixtures of desire or fear, based on location, just as the location of keys on a keyboard determine the sounds resulting from playing (Berridge, & Kringelbach, 2013; Reynolds & Berridge, 2008).

Bringing it together


Different regions of the brain do not work in isolation, but use chemicals called neurotransmitters to form pathways and feedback loops. A stimulated neuron in one area releases a neurotransmitter, which may activate or inhibit neighbouring neurons. Those neurons then send excitatory or inhibitory messages to their neighbours, and so on. Perhaps you have heard simplified explanations of brain chemistry, such as "dopamine causes pleasure" or, "oxytocin causes love". In reality, their roles are complex. For exampleː
 * Oxytocin administration reduced fear-related amygdala activity among individuals with social anxiety (Labuschagne et al., 2010), and borderline personality disorder (Simeon et al., 2011), but did not cause them to fall in loveǃ
 * Dopamine is involved in stress as well as pleasure, which may be better attributed to endorphins (Berridge & Kringelbach, 2013).
 * Serotonin can be both anxiogenic and anxiolytic, depending on the type of receptor activated (Charney, 2004).

The bidirectional connections between the ventral PFC and the amygdala enable suppression of amygdala responses, and predict habituation to emotional stimuli (Hare et al., 2008) (Figure 3). During down-regulation of stress responses, prefrontal regions such as the OFC engage, reducing amygdala activity. During acute stress, the amygdala activates hypothalamic pathways which releases high levels of norepinephrine and dopamine, inhibiting PFC function and reducing capacity for down-regulation (Arnsten, 2009). Artificially reducing dlPFC activity also increased reactivity to aversive and neutral stimuli, through impairing regulatory capacity and indirectly disinhibiting the amygdala (Berger et al., 2017). This results in automatic responses based on salient features, rather than cognitive appraisal or goal-states (Arnsten, 2009). 

{Dominance in which hemisphere is associated with reactivity to negative stimuli? - Left hemisphere + Right hemisphere - Ventral hemisphere - Neither hemisphere
 * type=""}

{Which pattern of neural connectivity is likely to be found in an adolescent with strong responses to negative emotional stimuli? - Low connectivity between dlPFC and amygdala - High connectivity between dlPFC and amygdala + Low connectivity between vmPFC and amygdala - High connectivity between vmPFC and amygdala
 * type=""}

Epigenetics
Epigenetic changes are changes in gene expression. Explained simply, experiences can switch genes on or off (Figure 4), enabling environmental factors to effect long-lasting physiological changes. Some people are more sensitive to environmental influence than others. Boyce and Ellis (2005) termed this "biological sensitivity to context", and argued that high phenotypic reactivity may lead to negative outcomes in stressful settings, but also increase susceptibility to positive influences. Epigenetic processes adapt response systems to the environment they develop in; stress reactivity may reflect the placticity of stress response systems (Boyce & Ellis, 2005). The diathesis-stress model proposes that underlying vulnerability predisposes an individual to be adversely impacted by stressors. The differential susceptibility hypothesis is an alternative perspective which suggests individuals differ in plasticity, with highly reactive individuals disproportionately affected by enriching, and negative experiences (Belsky & Pluess, 2009), consistent with Boyce and Ellis' findings (2005).

Emotional reactivity and psychopathology
Increasingly, emotional reactivity is implicated in psychopathology. Exceptionally high and low reactivity is associated with poor psychological outcomes (eg. M’Bailara et al., 2009; Reichenberger, Wiggert, Agroskin, Wilhelm, & Blechert, 2017). Altered emotional reactivity is a feature of depression (Reichenberger et al., 2017), borderline personality disorder (Glaser, Os, Mengelers, & Myin-Germeys, 2008), and bipolar disorder, even during asymptomatic periods (M’Bailara et al., 2009). Further, emotional reactivity significantly explains the relationship between psychopathology and self-injurious thoughts and behaviours (Nock et al., 2008).

One version (polymorphism) of a gene involved in transporting serotonin (5-HTTLPR) is being investigated for its role in reactivity in mood disorders. The short polymorphism of this gene reduces serotonin availability, resulting in increased amygdala activity and decreased coupling between the amygdala and regulatory prefrontal areas (Hariri & Holmes, 2006). This polymorphism is thought to increase stress sensitivity, magnifying the risk of developing disorders including depression, anxiety and post-traumatic stress disorder (Caspi, Hariri, Holmes, Uher, & Moffitt, 2010). 
 * If stress reactivity reflects environmental sensitivity (Boyce & Ellis, 2005), might the short 5-HTTLPR polymorphism be found disproportionately among individuals with exceptional as well as poor mental health?



{Complete this sentence. High ̠ ̠ ̠ ̠ ̠ ̠ ̠ ̠ ̠ ̠ ̠ ̠ ̠ reactivity can lead to poor psychological outcomes. - Positive - Negative + Positive or negative - Polymorphism
 * type=""}

Real world applications
By now you might be asking, "this is interesting, but how does it help me? Will knowing this stop me crying in cartoons or freaking out before exams?" The answer is probably not, but there are things you do everyday that impact your reactive tendencies for better or worse.

Why do things seem better in the morning?


You've probably heard that there’s nothing a good night's sleep won’t fix. This may be an overstatement, but there is a biological explanation for why a seemingly insurmountable stressor may seem more approachable in the morning. During rapid eye movement (REM) sleep, amygdala responses to affective stimuli from the previous day become depotentiated (van der Helm et al., 2011). Amygdala activity is reduced during recall, prefrontal connectivity is re-established, and subjective emotional reactivity is reduced (van der Helm et al., 2011). This may be attributable to reduced adrenergic activity during REM sleep, or the benefit of REM sleep on emotional memory consolidation, which reduces stimulus novelty. Unfortunately, while REM sleep reduces emotional reactivity, reactivity can also reduce REM sleep quality. Deprivation of REM sleep increases emotional reactivity and prevents habituation to emotional stimuli (Rosales-Lagarde et al., 2012).

Other day-to-day influences
Self-control, mindfulness and environmental factors also alter emotional reactivity. According to the strength model of self-control, self-control is a limited resource and can be depleted through resisting short-term benefits in favour of long-term goals (Baumeister, Vohs, & Tice, 2007). Depletion of self-control also resulted in increased amygdala activity when repeatedly exposed to negative emotional scenes instead of habituation. (Wagner, & Heatherton, 2013). Depleted participants showed less vmPFC-amygdala connectivity than controls and increased amygdala activity in response to neutral scenes (Wagner, & Heatherton, 2013). This may be why after a stressful day in which you resisted yelling at your boss (short-term gain), to remain well-placed for promotion (long-term goal), you are more likely to go home and snap at your family. Luckily, if you do offend a loved one, they will likely be less upset the following day, after some REM sleep!

Appreciating little things is another way to boost well-being. "Flourishers" (individuals with optimal mental health) are more reactive to pleasant events than others, and positive reactivity was predicted by mindfulness (Catalino, & Fredrickson, 2011). Mindfulness is beneficial to emotional reactivity for both beginners and experts, although skill level is associated with different patterns of neural activity (Taylor et al., 2011). For beginners but not experts, viewing emotional stimuli in a mindful state was associated with reduced left amygdala activity. Compared to beginners, experts showed relative deactivation of cortical areas. This may be because experts have learned to observe, rather than suppress experiences. Mindfulness attenuates chronic reactivity by replacing evaluation, or appraisal of affect, with sensory-based representations of emotion (Farb et al., 2010).

An alternative strategy for increasing positive and reducing negative reactivity, is to "tune" the NAcc "keyboard" by situating yourself in a preferred environment. Stressful environments expand fear-generating zones, whereas pleasant environments expand appetitive-generating zones (Reynolds & Berridge, 2008). In effect, being somewhere you like simultaneously decreases your potential for negative reactions and increases your potential for positive reactions. 

{Which behaviour will increase negative emotional reactivity? + Exercising self-control - Experiencing REM sleep - Practicing mindfulness as a beginner - Practicing mindfulness as an expert
 * type=""}

Conclusion
Emotional reactivity is a small part of emotional experience with far reaching implications ranging from neuroplasticity to psychological well-being. The primary brain regions involved in emotional reactivity are the PFC and the limbic system. While cortical areas bias other regions towards particular response patterns, the amygdala and the NAcc contribute causally to emotional activation (Berridge & Kringelbach, 2013). High reactivity may reflect heightened sensitivity to positive and negative environmental factors, increasing susceptibility to epigenetic changes (Boyce & Ellis, 2005). While reactivity is implicated in psychopathology, it can be altered to improve psychological well-being through sleep management, mindfulness, and simply visiting your favourite place. Biological predispositions are not set in stone. Hopefully, understanding the biological processes underlying emotional reactivity will give you insight into your own tendencies, and empower you to take advantage of your brain's ability to change.