15.4: Specific Emotions

Fear / Anger

Nearly everyone has experienced the prototypical fear response: Imagine reading a book when you see a spider skittering along the wall. Suddenly, you’ll feel your heart racing, your breathing increase, dryness of your mouth, and your palms sweating. You probably won’t notice the dilation of your pupils or the change in liver activity and digestion. This sympathetic nervous system activity is downstream of being presented with a fearful stimulus.

But upon closer inspection, it was no spider at all - just an errant piece of brown fuzz picked up by a draft. Within minutes, your body’s physiology returns back to normal.

This anecdote points out a few important features about the fear response. First, the onset of the fear response is quick, and so is the dissipation of the fear. Second, it is triggered by exposure to a perceived threat, regardless of whether the stimulus is a genuine threat or not (the overwhelming majority of spiders are clinically harmless to humans!). Third, the fear response is greatly modified by knowledge and experience - an entomologist would recognize that the spider is a harmless house spider and would, instead of fear, display curiosity, interest, boredom, or other emotions. On the other hand, someone who has been bitten by a dangerous spider and sent to the hospital when younger would have a much stronger physiological response.

Fear is likely the most evolutionarily ancient emotion, and is highly protective. When encountering a hungry mountain lion, faces displaying the traits of fear (enlarged eyes, flared nostrils, and a slightly open mouth accompanying a gasp) would signal to others nearby that a threat is nearby, which helps initiate heightened alertness and the appropriate fight-or-flight response.

Figure 15.14 The fear response is likely an evolutionarily protective behavior, since it provides social cues about dangers.

Patient SM

Patient SM is a notable case study of a person who does not experience the fear response. Born with an extremely rare genetic condition called Urbach-Wiethe disease, Patient SM progressively developed calcification in her amygdala bilaterally, causing destruction and cell death. Like other people with Urbach-Wiethe disease, she had no severe significant cognitive deficits, except for the inability to experience fear.

In one test, she was shown a variety of emotionally charged videos, then asked to rate the intensity of each clip with respect to different emotions. SM found clips from America’s Funniest Home Videos to be just as funny as the control patients, and she found the clips of disgusting toilets to be just as repulsive. But, when presented with clips depicting ghost hauntings or suspenseful serial killers on the loose, SM rated these stimuli as being non fearful.

Other studies challenged Patient SM with more concrete threats. Researchers brought SM to a pet store, where she asked to handle the snakes. She was stopped by an employee before she put her hand into a tarantula cage out of curiosity. The researchers also took her to the Waverly Hills Sanatorium haunted house, where she bravely led a group of strangers through the house populated by actors dressed as monsters and ghosts. Although she did not display any fearful behaviors, such as hesitation to walk through the darkened corridors, patient SM reported the sensation of exhilaration and enthusiasm, akin to riding a roller coaster.

Figure 15.15 Patient SM exhibit no physiological fear response following presentation of typically scary stimuli, such as snakes or a haunted house.

Clinical connection: Plasmatoxmosis

Plasmatoxmosis is a disease caused by infection of the parasite Toxoplasma gondii. Plasmatoxmosis may cause symptoms characteristic of a generic immune response, such as fatigue, body aches, and fever. Some people may be completely asymptomatic; however, others experience severe side effects like seizures or birth defects when an infection happens during pregnancy.

T. gondii is only capable of reproducing within cats, but the parasite has developed a very unusual evolutionary adaptation that permits propagation of the species, called the manipulation hypothesis. Cats excrete T. gondii via feces, which is eaten by rats and mice, where the parasites take up residence in their temporary host. T. gondii causes modifications in amygdala neurons, changing the rodent fear response, in turn making them attracted to cat-related odors rather than fearful of them. The fearless rodents are therefore less likely to avoid a hungry cat, increasing their risk of being eaten, which begins the cycle again.

Figure 15.16 A toxoplasma infection changes the rodent fear response, causing them to be less afraid of predators. At either point, plasmatoxmosis can infect humans with potentially severe outcomes during pregnancy.

Some research suggests that a T. gondii infection subtly affects human behavior in cat owners, hinting that T. gondii infection is associated in suicides and the onset of schizophrenia.

She also was asked to recall some of her past real-life, fear-provoking experiences, such as when she was attacked in a domestic violence incident or was held up by a stranger at knife point in a public park. In none of these cases did she ever report feeling fear, although she was upset and angered at the situation. The destruction of her amygdala seemed to make her resilient against PTSD: the day after being threatened with a knife to her throat, she walked past the very same park bench.

Fear and anger are two very closely related emotions. As with many other emotions, anger manifests in complex ways. The anger spectrum runs from low (irritation) to high (rage), and from quick (lashing out) to persistent (vengeful). Strong anger can provoke anti-social behavior such as violence.

Since anger may precede violence, angry faces function as a warning during evolutionary survival situations or during interpersonal conflict. Seeing angry faces may prompt awareness, initiating the sympathetic nervous system response in case “fight or flight” becomes necessary. As with fear, the emotional response to angry faces or body postures is partially guided by the amygdala, resulting in communication with the rest of the body through cortisol release downstream of the HPA axis.

Environmental factors such as childhood maltreatment or expectations are partially responsible for how a person responds to a particular anger-provoking stimulus. Internal homeostatic conditions also influence the anger response, just as how low blood sugar increases aggression and drives negative or hateful feelings in the face of a challenge (the portmanteau “hangry” was added to the Oxford Dictionary in 2018).

Neurobiological factors also contribute to the anger response. In addition to amygdala circuits, regions in the frontal cortex decrease activity during acts of aggression, suggesting that frontal circuits actively inhibit the limbic system, which drive our more “primitive” responses. Altered frontal cortical action may therefore account for one reason why two different people would react to the same anger-provoking stimulus in different ways.

Clinical connection: Borderline Personality Disorder (BPD)

Borderline Personality Disorder (BPD) is an emotional dysregulation disorder characterized by disproportionately intense emotional reactivity to environmental triggers. For instance, they may experience anger or hostility in response to minor inconveniences. These emotional changes may persist for unusually long durations, up to hours or days, which often puts strain on interpersonal relationships. They also experience a fear of abandonment, an unstable sense of self, suicidal ideation or attempts, and dissociation (an emotional detachment from events).

It is estimated that 1.4% of people have BPD. It is often comorbid with other psychiatric disorders, such as anxiety, depression, or substance use disorders. The main treatment approach is psychotherapy; medications are not always effective at reversing the symptoms.

Celebrities such as Saturday Night Live actor Pete Davidson, musician Amy Winehouse, and Princess Diana likely had BPD. Arguably, the fictional character Anakin Skywalker from the Star Wars prequel films also displays many diagnostic traits of BPD.

Figure 15.17 The HPA axis regulates both sympathetic nervous system activity and the stress response.

Stress

Stress is a natural and healthy bodily response that is similar to the prototypical “fightor-flight” response: elevated heart rate and blood pressure, increased respiration rate (shortness of breath), energy usage, and perspiration, among others. Physiologically, stress also causes a weakening of the immune response, a slowing of growth, and decreases in kidney filtration.

Stress can either result from purely physical stimuli (exposure to constant cold; chronic dehydration), a purely psychological threat (resentment; religious guilt), or a combination of both (living paycheck to paycheck).

The human stress hormone cortisol triggers the physiological response to stress. Cortisol is a signaling molecule synthesized from cholesterol which readily passes through cell membranes where it can activate intracellular glucocorticoid receptors (GRs). Once the ligand-receptor complex is formed, it translocates into the nucleus, where it functions as a transcriptional regulator. Nearly every cell contains GRs, meaning that an elevation of systemic cortisol can influence several organs to change their activity.

A small amount of stress, sometimes called eustress, is beneficial and generally performance-enhancing, such as in the moments leading up to the beginning of an athletic competition. The opposite, distress, is characterized by the inability to cope with rising demands, leading to increased anxiety and other maladaptive responses. Distress is the kind of stress that people experience when they are facing multiple rapidly-approaching deadlines, difficulties in paying rent, or relationship issues.

Biochemically and physiologically, eustress is the same as distress. The major difference is the psychological interpretation or perception of the stressor. Eustress is seen as a challenge which someone can work to overcome and is often short lived or temporary. Chronic long-term distress causes all variety of negative health outcomes, such as cardiovascular disease, diabetes, and obesity, all risk factors for premature death.

Figure 15.18 A molecule of cortisol (red triangle) diffuses through the cell membrane and binds to intracellular glucocorticoid receptors (GR, green).

The early medical descriptions of stress were put forth by Hans Selye in the 1930’s. As a medical student, he observed that chronicallyill humans, regardless of their specific condition, often experienced a similar set of late-stage negative health outcomes, such as gastric ulcers, high blood pressure, and heart attacks. Collectively, he called this response the general adaptation syndrome.

Selye observed that extracts from different internal organs led to these health changes in rats, eventually leading to premature death. As a good scientist, Selye also injected a second group of rats with a harmless saline solution as a control. Surprisingly, Selye observed that these control animals also exhibited the general adaptation syndrome.

Clinical connection: Ulcers

Ulcers are injuries to the mucous membranes of the stomach or intestines resulting in abdominal pain, nausea, and changes in appetite (although some patients report no symptoms.) In severe cases, ulcers can cause bleeding or perforation of the stomach, which could be fatal. An estimated 4% of people have ulcers, although this is likely an underestimation.

For several years, ulcers were believed to be caused solely by stress. However, later research suggested that colonies of the gut bacterium Helicobacter pylori was found to contribute to the formation of 80% of ulcers, a discovery that earned Drs. Barry Marshall and Robin Warren a Nobel Prize in 2005. Today, the evidence suggests that neither factor exclusively leads to ulcer formation: Nearly half of all people have the H. pylori colonies in their gut, but not all these people develop ulcers. Also, about 30% of people with ulcers do not harbor H. pylori, and people taking oral antibiotics that have eliminated their H. pylori colonies can still develop ulcers.

Figure 15.19 H. pylori interacts with stress and may result in stomach ulcers.

It turns out that even minor stressors, such as the movement of the rat cage, grabbing of the animal, and administering an injection led to an eightfold increase in plasma levels of epinephrine, a hormone upstream of the stress response. Selye concluded that the general adaptation syndrome was not a result of the injections of organ extract, but rather because he induced a tremendous stressor to all the rats.

Like Selye, we use nonhuman models to study how the nervous system changes in response to stress. Rodents can be exposed to chronic-restraint stress, where they are put into a narrow cylinder for part of their day. Food restriction stress is a rodent model of starvation or dietary malnutrition. The forced-swim test is best known as an assessment of depressive behavior, but it can also be used to induce stress. Food restriction and the forced swim test increases plasma levels of the neurohormone corticosterone (the rodent analog of cortisol).

Psychological stressors also induce the stress response. Common stress paradigms include exposure to socially-dominant bully rats or predator-associated cues like fox urine, which contains a group of chemicals called kairomones. Kairomones cause behavioral changes in other species, whereas pheromones refer to interspecies chemical communication. Social isolation is also a stressor for rodents, and for this reason, rats and mice are normally group housed.

Love

Love can be thought of as an intensely strong attachment towards a person (romantic love, lust), a thing (passion project), or concept (patriotism as a love for country, or altruism as a love for fellow humans). However, it is very difficult to put a strict biological definition on these varied concepts of love.

This section focuses on interpersonal love, of which there are several unique forms, each resulting in different behavioral outcomes. For example, romantic love drives physical attraction, lust, and sexual activity. Parental love, on the other hand, encourages self-sacrifice and hyper-attentiveness towards a newborn. Some behaviors are shared between the two forms of love, such as respect.

Romantic love

Romantic love drives much of human behavior and has been documented thoroughly in the arts all the way from Homer’s Iliad through Shakespeare’s Romeo and Juliet up to modern Taylor Swift.

The majority of human societies have embraced social monogamy, the romantic relationship characterized by a pair of people who share resources, parenting duties, and exhibit preferential mating. Outside of humans however, only 9% of mammalian species form socially monogamous pairs, while at least 75% of bird species maintain socially monogamous relationships (which may explain the origin of the phrase “love birds”).

Figure 15.20 Unlike mammals, most bird species form socially monogamous bonds after mating.

Dr. Helen Fisher, an anthropologist and a leader in the field of love research, suggests that love can be divided into three closely interconnected components. These three are guided in part by different signaling pathways, and lead to somewhat different behavioral outcomes.

1. Lust

Lust (or libido) refers to a very strong desire for sexual gratification. These behaviors are largely driven by the actions of the sex hormones testosterone, estradiol, and progesterone, released downstream of activation of the HPG axis. As hormones, they are synthesized from cholesterol and circulate through the bloodstream to influence the body in many ways.

Both testosterone and estradiol contribute to sex-seeking behaviors in men and women, where increasing testosterone levels drive up sexual desire.

In the brain, the sex hormones strongly influence the medial preoptic area (mPOA) of the anterior hypothalamus. The mPOA contains a sexually dimorphic area, the part of the brain that exhibits the biggest morphological difference between males and females: in humans, it is about twice as large in males with double the number of neurons throughout childhood and early adulthood. In the rat, it is up to 8 times larger in males than females, and if this area is lesioned, rats exhibit decreased motivation to engage in sex.

The amygdala also plays a significant role in mediating lust, and lesions may either result in hypersexuality (Kluver-Bucy syndrome) or a decrease in responding to socially-derived sex cues.

In addition to changing libido, the sex hormones contribute to sexual differentiation and maturation (mascularization with testosterone, such as muscle mass increases and hair growth), differences in immune system activity (decreases with higher testosterone), and pain tolerance (higher tolerance with higher testosterone) for example. Progesterone is less of a behavioral driver and functions more as a signal that prepares female bodies for changes in preparation for childbirth, such as in the uterus and breasts.

2. Attraction

Attraction is characterized by high energy investment and preoccupation towards a small number of people. From an evolutionary perspective, attraction may have developed to discriminate between multiple reproductive partners, allowing the focusing of limited resources towards fewer partners.

In Fisher’s theory, attraction is strongly related to the action of dopamine and norepinephrine. In humans, the reward circuitry (Section 11.2) is involved in feelings of love. Using fMRI, Fisher presented pictures of a patient’s romantic partner to them and identified increases in the blood flow to dopaminergic midbrain areas such as the ventral tegmental area and the striatum. This finding compares with imaging studies that observed increases in blood flow to insula, premotor, and hypothalamus as well as striatum in response to highly erotic, sexual imagery. These studies suggest that romantic love and lust have different driving neural structures underlying these behaviors.

Norepinephrine functions to increase attention, alertness, and energy, which accounts for the exhilarated feeling you may feel when spending time with a potential partner.

3. Attachment

Attachment is the long-term accompanied by feelings of comfort and emotional stability. Attachment also contributes to behaviors that maximize offspring survivability, such as sharing parenthood responsibilities and protectiveness towards offspring. The major neurochemical drivers of this form of love are oxytocin and vasopressin.

We have learned about mammalian pair bonding through experiments with prairie voles (Microtus ochrogaster), rodents that are indigenous to central North America. Atypical of mammals, prairie voles exhibit social monogamy after mating, where the two voles will cohabitate and share parental duties, such as mutual resource collection, nest building, and caring for their young.

Figure 15.21 Prairie voles, rodents found in the wild across central north America, form monogamous relationships after mating.

An experimental test called the partner preference paradigm can be used to assess vole monogamy. Here, a three-chamber testing apparatus is used where a test vole is placed in a central chamber. In the other two chambers are voles who have been harnessed into their rooms, unable to leave. The test vole is then free to move between any of the three chambers. When the test vole has mated with one of the harnessed voles, they will choose to spend more time with their “partner” vole compared to the novel “stranger” vole.

Figure 15.22 In a partner preference test, a mated prairie vole will choose to spend more time with their partner compared to the stranger, both of which are tethered to their chamber.

Not only do mated prairie voles prefer the company of their partner, they also demonstrate behaviors similar to some human romantic relationships. For example, mated prairie voles living together display selective aggression against a stranger, “intruder” voles, a behavior called “mate-guarding”. They also spend significant time mutually caring for their young, their pair bonding can be modified by psychoactive substance use, and they show increased anxiety when they are separated from their partner vole.

Other nonhuman studies have compared the prairie voles with physiologically comparable animal models, such as the montane vole or meadow voles. While these other rodents are similar in size, they have slightly different geographic distributions. Notably, they have radically different mating habits, as they exhibit social promiscuity rather than monogamy.

Comparing between monogamous and promiscuous species has allowed researchers to identify several neurotransmitter signals that are implicated in pair bonding. Oxytocin and vasopressin seem to play important roles in vole pair bonding, as inhibition of either of these signals decrease partner preference. Additionally, differences in dopamine receptor levels and corticotropin releasing factors are observed between vole species.

Parental love

Parental love refers to instinctive affection towards one’s offspring. Parental love behaviors include nurturing (collecting and sharing resources), protecting (promoting aggression against “intruders”), and preparing one’s young for their adult life (risk assessment training).

In evolutionary theory, parental love serves to improve the odds of passing of one’s genes through the following generation. Most large mammals (humans included) are K-selected species, which benefit most from a small number of high-quality offspring that require substantial parental investment, as opposed to large numbers of offspring with little investment.

An extension of parental love is familial love or kinship, the protection and preferential support of one’s extended genetic relations. Because blood relatives share common genes, increasing the survivability of these family members has benefits for passing genes to the next generation. In the words of the geneticist Jack Haldane, “I’d gladly give my life for two brothers or eight cousins.”

Many behaviors related to mammalian motherhood are accompanied by changes in neural activity. Nursing, for instance, is feeding behavior that is regulated through a positive feedback cycle. Offspring suckling activates the mother’s somatosensory afferents. Through a series of oxytocin-dependent circuits across the hypothalamus, suckling ultimately increases lactation through the milk letdown reflex. Auditory sensory inputs such as the sounds of a crying baby can also trigger this reflex. Sometimes, just thinking about the baby can induce letdown.

Figure 15.23 Suckling (bottom left) triggers somatosensory inputs, which send afferents into hypothalamus (top), increasing blood levels of oxytocin. Oxytocin increases accumulation of milk in the mammary glands (bottom right), which encourages increased suckling.

Some changes are dependent on neural plasticity. For example, after childbirth, the auditory areas of rodents rewire to become more sensitive to high frequency sounds. This adaptation allows the mothers to better detect the ultrasonic vocalizations that are emitted by offspring when they are distressed or hungry. Olfactory areas also change in order to become more sensitive to the particular odorants given off by their young, allowing them to better identify their offspring. In humans, these olfactory changes result in decreased aversion towards traditionally aversive stimuli (urine or fecal matter) when they originate from their children.

Disgust

Disgust is an evolutionarily protective emotion that helps protect an organism from being exposed to toxic or dangerous substances. It also leads to disease avoidance, which is why we dislike the smell of rotten foods or fecal matter, and why we may be repulsed by blood. As in the previous examples, disgust often happens in response to specific sensory stimuli (chapter 9).

The most common physiological response to disgusting stimuli is nausea, which can promote vomiting. Additionally, nausea decreases appetite, which prevents further intake of potentially toxic compounds.

Disgust is not inherently ingrained and has a strong learned cultural component that can be modified by experience. Americans may be turned off by potent moldy cheeses, which are delicacies in many European countries, or fermented fish products, commonly used as a flavor enhancer in South East Asian cuisine.

Figure 15.24 Typical face produced in response to a disgusting stimulus.

Disgust extends beyond physical or chemical stimuli, and also manifests itself in thought. For instance, disgust is associated with sexual contact leading to low reproductive success, such as with animals or incestuous relationships. Antisocial behaviors can also provoke the disgust behavior, such as feeling physically nauseated when reading about serial killers.

The canonical “disgust face” is observed across several cultures. The curling of the upper lip and wrinkling of the nose decrease air flow through the nasal cavity, and squinting minimizes eye surface exposure. An opening of the mouth is a precursor for spitting out the disgusting flavors.

The insula is involved in mediating perception of disgust and displays changes in fMRI activity in response to disgust-related cues.

Clinical connection: Obsessive-compulsive disorder (OCD)

OCD is a common psychiatric condition affecting an estimated 3% of the population, characterized by persistent intrusive thoughts or the need to perform some ritualistic series of actions or motions. OCD exists on a spectrum, and in severe cases, can severely impair the quality of a patient’s life. OCD is often treated with psychiatric intervention or SSRIs like fluoxetine (more information in chapter 16).

Some studies implicate abnormal disgust sensitivity as contributing to OCD. Patients were shown a variety of disgustprovoking stimuli, such as unsanitary toilets or parasitic infections. If the threshold of disgust sensation is particularly low a patient may engage in their perseverative behavior more frequently.

Figure 15.25 Morphological differences are observed in the volume of the insular cortex between people with OCD and neurotypical patients.