Unit 15: Emotion II: Aggression

LO: Articulate the difference between anger and aggression

Anger is a basic emotion, a feeling that can be triggered by things like frustration, hurt feelings, or stress.

• Aggression is not an emotion, but is one possible behavioral result of anger. For example, an angry person might punch someone. In studies of humans, anger and aggression can be readily distinguished because people can state that they are angry even if they do not act on that feeling. In animals, researchers can only infer anger by the aggressive behaviors an animal exhibits, such as making a scary sound or a threatening posture.

LO: Distinguish predatory and affective aggression

Aggressive behaviors can be categorized based on their purpose and display:

• Predatory Aggression (Quiet Biting Attack):

  • Purpose: Used for obtaining food (prey).

  • Display: Involves few vocalizations and low sympathetic arousal (no feeling of anger). The attack is typically targeted at the head and neck of the prey.

  • Motivation: Hunger or obtaining resources.

• Affective Aggression (Threatening/Defensive):

  • Purpose: For show; usually intended to intimidate and defend rather than kill.

  • Display: Involves high vocalizations (growling, hissing) and an intense sympathetic arousal (fight-or-flight response). The attack is typically indiscriminate, targeting anywhere on the body.

  • Motivation: Anger, fear, or defense of territory/young.

LO: Describe the effects of amygdala stimulation and lesions of the amygdala on aggression

The amygdala plays a critical role in mediating fear and aggression:³

Manipulation	Effects on Aggression
Bilateral Amygdala Lesions	Reduces fear and aggression in all animals tested. Animals become fearless and placid, exhibiting the characteristics seen in Klüver-Bucy syndrome.
Amgdala Stimulation	Leads to a state of increased vigilance and can elicit violent aggression in some animals (like cats) or reported anxiety and fear in humans.

LO: Discuss how changes in testosterone impact aggressive behavior

Testosterone, the primary male sex hormone, has been shown to impact aggressive behavior, although the link is complex:

• Positive Correlation: There is a correlation between levels of male androgen (testosterone) and aggressiveness.

• Abolishing Aggression: Castration, which eliminates circulating testosterone, typically abolishes aggressive behavior.

• Restoration: Testosterone replacement therapy can restore aggressive behavior.

• Seasonal Effects: In some species, aggressive behavior is linked to seasonal spikes in testosterone.

LO: Summarize the role of the hypothalamus in aggression by citing key experimental breakthroughs

The hypothalamus is a central component in the brain's aggression circuitry, as demonstrated by key experiments:

• Hess's Breakthrough: In the 1920s, Swiss physiologist Walter Hess showed that electrical stimulation of the hypothalamus could elicit affective aggression in cats (i.e., a threatening display).

• Flynn's Breakthrough: In the 1960s, John Flynn refined this work by demonstrating that stimulation of the medial hypothalamus produced affective aggression (threat), while stimulation of the lateral hypothalamus produced predatory aggression (quiet biting attack). These findings established the hypothalamus's role in controlling the expression of both types of aggression.

LO: Describe sham rage and identify lesions that elicit sham rage

Sham rage is a behavioral state in which animals exhibit all the physical characteristics of rage (hissing, arching back, struggling, increased heart rate and blood pressure) but do so spontaneously or in response to minimal provocation. Critically, this rage is not directed at specific objects or people; it is a sham because it lacks context and direction.

• Sham rage is elicited by lesions of the cerebral hemispheres (cortex), but only if the posterior hypothalamus remains intact. If the lesion includes the posterior hypothalamus, sham rage disappears. This indicates that the posterior hypothalamus is critical for the expression of rage, while the cortex normally acts to inhibit this expression.

LO: Compare the effects of stimulating the medial and lateral hypothalamus with traditional electrodes

Traditional electrical stimulation studies revealed distinct roles for two hypothalamic regions in aggression:

• Stimulating the Medial Hypothalamus: Elicits affective aggression (threatening, defensive posture, high vocalization). This circuit is likely involved in the display of defensive and emotional aggression.

• Stimulating the Lateral Hypothalamus: Elicits predatory aggression (quiet biting attack, directed at the head/neck). This circuit is likely involved in the goal-directed behavior of hunting

LO: Discuss how optogenetic approaches were used to study the hypothalamus in mice

Optogenetics, which uses light to activate genetically targeted neurons, was used to study the hypothalamic circuits underlying aggression with high specificity:

• Targeting: Researchers targeted two distinct populations of neurons in the ventromedial hypothalamus (VMH).

• Findings: They found that genetically-defined populations of VMH neurons drove two different behaviors:

  1. Fighting/Aggression: One population of neurons was dedicated to fighting behavior.

  2. Mating: A separate population of neurons was dedicated to mating behavior.

• Significance: This approach allowed the researchers to show that these complex, motivated behaviors are controlled by discrete, genetically defined neural pathways within the hypothalamus.

LO: Describe the serotonergic regulation of anger and aggression

Serotonin (5-HT) plays a significant role in modulating anger and aggression, generally acting as an inhibitor of aggressive impulses.

• Regulation: Serotonin appears to regulate aggressive behavior, rather than generating it directly.

• Hypothesis: The serotonin deficiency hypothesis suggests that aggression is inversely related to the amount of serotonin turnover (release and synthesis) in the brain.

• Low Serotonin: Low serotonin turnover is associated with increased aggression and risky behavior in animal models. The less active the serotonergic neurons are, the more aggressive the animals are.

• Role in Humans: Low levels of serotonin metabolite (5-HIAA) have been found in the cerebrospinal fluid of people who exhibit highly aggressive behavior or who attempt suicide.

LO: Diagram the neural circuit for anger and aggression

Anger and aggression are mediated by a distributed network of interconnected structures:

1. Input/Evaluation: Sensory input about a threat is processed by the Amygdala (Amy) and Dorsal Striatum (DS).

2. Command Center: Output from the amygdala and DS is integrated by the Hypothalamus (HYP), which organizes the appropriate aggression response.

3. Expression: The hypothalamus sends projections to the Periaqueductal Gray (PAG) in the brain stem, which orchestrates the specific behavioral acts of aggression (e.g., freezing, attack).

4. Regulation: The entire circuit is heavily regulated by the Prefrontal Cortex (PFC), particularly the ventromedial PFC (vmPFC), which can evaluate and regulate the behavior by inhibiting the amygdala/DS activity to suppress aggressive impulses. Serotonin (5-HT) is a key neuromodulator across this entire circuit.

LO: Evaluate how cellular diversity in the hypothalamus underpins its functional heterogeneity

The hypothalamus regulates a wide and heterogeneous array of vital functions (temperature, hunger, thirst, sex, sleep, aggression). This functional complexity is supported by its cellular diversity:

• The hypothalamus contains a high degree of cellular diversity, including different neuronal subtypes.

• This diversity allows for the segregation of functions into discrete, specialized microcircuits.

• As seen in the optogenetic studies, adjacent populations of genetically-defined neurons can be dedicated to controlling fundamentally different, complex motivated behaviors (e.g., fighting vs. mating).

• This heterogeneity means the hypothalamus is not a single command center, but a collection of highly specialized microcircuits working in concert.