The Neurobiology of Emotion and the Amygdala

Submission Procedures: Students were reminded regarding the "Nuku disaster" and encouraged to submit assignments via the Nuku site rather than just Qualtrics to facilitate grading and feedback. No late penalties apply for those who used the alternative portal on time.
Revision Quiz Topics:
- Vasopressin Receptors: High expression of vasopressin receptors, particularly in male animals, is a significant determinant of mating strategies.
- Parental Investment and Mating Strategies: Monogamy is more common in birds than in mammals. This is attributed to the high thermal requirements for egg incubation, necessitating a partner to provide food or take over incubation duties.
- Mammalian vs. Avian Reproduction: Theoretically, female mammals can raise offspring alone via lactation. In contrast, avian offspring survival increases significantly with biparental arrangements, especially in harsh or rapidly changing environments where resources are scarce.

Rationality vs. Emotion: Historically, philosophers have viewed reason as a superior faculty and emotion as something to be suppressed. This is exemplified by characters like Spock from Star Trek, known for his lack of emotion and reliance on rational thinking.
The Purpose of Emotion: Emotions are not merely "feelings" but evolved functional systems designed to solve evolutionary problems:
- Social Communication: Facilitates bonding and group living as social animals.
- Rapid Processing: Allows for quick responses to environmental stimuli, which is essential for survival.
- Resource and Status Management: Feelings like jealousy, anxiety, and anger arise when resources are shared unfairly or when ensuring status for resource access.
Negativity Bias: Humans possess a strong "Bad is Stronger than Good" (BBB) bias. We are not primarily built for happiness but for threat detection. The cost of a "false alarm" (a racing heart) is low compared to the high cost of ignoring a real threat (death), leading to a "safety-first" evolutionary gearing.

Emotion is the result of a network across the brain rather than a single center. The three primary "characters" in this system are:

The Amygdala: Often called the fear or emotional center. It is almond-shaped and specifically detects the salience and relevance of stimuli.
The Prefrontal Cortex (PFC): Serves a regulatory role. It generally inhibits the overactivity of the amygdala. Damage to certain regions of the PFC leads to heightened emotional reactivity.
The Hippocampus: Known as the memory center. it provides context for emotions. It allows for refined responses based on past experiences and is crucial for "fear extinction" (learning that a previously dangerous stimulus is no longer a threat).

System Integration: The amygdala is part of the limbic system, which includes the olfactory bulb (explaining why smells trigger strong immediate emotions), the thalamus, the PFC, and the hippocampus.
Internal Nuclei: While small, the amygdala contains separate sub-regions that receive signals from the cortex and send signals to the forebrain and hypothalamus.
Dual Processing Pathways:
- The Low Road: Information travels from the Thalamus directly to the Amygdala. This pathway is fast but unrefined, allowing for immediate reactions to potential threats (e.g., jumping away from a rope that looks like a snake).
- The High Road: Information travels from the Thalamus to the Cortex, then to the Amygdala. This is a slower, more careful evaluation that allows for error correction (e.g., realizing the "snake" is actually a rope).

Biological Cascade: The amygdala triggers the Hypothalamus to release Corticotropin-Releasing Hormone ( $CRH$ ), which eventually elevates cortisol levels.
Short-term Benefits: Elevated cortisol prepares the body for action by increasing heart rate and making glucose more available in the blood for a "fight or flight" response.
Chronic Stress and Neurotoxicity: Prolonged exposure to cortisol has neurotoxic effects, particularly in the hippocampus:
- Dendritic branching thins out, resembling a "winter tree" with fewer connections.
- The hippocampus and PFC shrink (atrophy), while the amygdala can become enlarged.
- This creates a vicious cycle where the regulatory regions (PFC/Hippocampus) are too weak to inhibit the hyperactive amygdala.
Immune Suppression: High cortisol levels compromise the immune system and throw inflammatory markers into disarray, as survival takes priority over long-term health maintenance.

Patient SM: A woman with complete bilateral amygdala damage due to a rare genetic disorder.
- She experiences happiness, sadness, and disgust, but is incapable of feeling fear.
- She lacks the "highlighter" function of the brain (salience); she does not remember emotionally charged stories better than neutral ones.
- She shows no fear of snakes, spiders, or haunted houses.
Klüver-Bucy Syndrome (Animal Models): Monkeys with amygdala lesions become extremely docile and lose innate fears (e.g., of snakes). However, they exhibit social boundary issues, such as standing too close to other monkeys, leading to social rejection or attack.
Social Attention and the Eye: Patient SM fails to look at the eye region of faces, which is critical for detecting fear.
- Cooperative Eye Hypothesis: Human eyes evolved with large white parts (sclera) to provide high contrast, helping social animals communicate emotions and coordinate activities like hunting.

Meditation and Reappraisal: Mindfulness and cognitive reappraisal activate the Ventromedial Prefrontal Cortex ( $vmPFC$ ), which strengthens the inhibitory connection to the amygdala. This reduces amygdala reactivity over time.
Suppression: Merely trying to inhibit a behavioral response (suppression) engages the lateral PFC but leaves amygdala activity ongoing; the emotion is felt but not expressed.
Genetic Factors: Serotonin Transporter Polymorphism ( $5-HTTLPR$ ) affects reactivity:
- Short Allele ( $s$ ): Associated with higher amygdala engagement and hyper-reactivity to stress. Linked to increased risk for anxiety and PTSD.
- Long Allele ( $l$ ): Associated with lower amygdala reactivity.
Startle Response: Measuring startle amplitude (the physical jump) is a way to test amygdala sensitivity. Research shows a dose-dependent effect where higher levels of $CRF$ correlate with higher startle responses.

Question: Does the enlarged amygdala in chronically stressed individuals imply the sympathetic nervous system is more engaged?
Answer: Yes, it can mean the sympathetic system is over-engaged, or the parasympathetic system is under-engaged, signifying a loss of autonomic balance.