Brain Structures and Hemispheric Specialization in Emotion

Researchers have a very clear and concrete understanding of the specific brain areas that control movement.
While some questions remain, the mapping of movement to the brain is well-established.
In contrast, the relationship between brain areas and emotion is much less certain.
There are very few brain areas that can be tied concretely to emotion, particularly when moving beyond basic emotions like fear and aggression.

The limbic system is defined as a set of structures in the brain that essentially surround the thalamus and involve parts of the thalamus itself.
Historically, this system has been believed to be the primary seat of emotion in the brain. However, the concept of the limbic system goes "in and out of fashion" in the science of emotion.
There is limited evidence to suggest that the limbic system acts as a unified whole to control emotion.
It is difficult to point to specific structures within the system and assign them definitive, singular emotional roles.

Olfactory Bulb: Located in the limbic system, it is responsible for the sense of smell (olfaction). Smell is a powerful trigger for emotional memories.
- Example: Smelling a perfume or cologne that has not been encountered for years can suddenly trigger vivid memories and associated emotions.
- Example: Many people report that the "smell of the sea" brings back specific emotional memories.
Hippocampus: This structure is very strongly involved in memory.
Fornix: A white matter structure that connects the hippocampus to various subcortical structures; it also plays a significant role in memory.
Cingulate Gyrus: Specifically, certain areas around this structure are thought to potentially be involved in the experience of anger.
Hypothalamus: Often included in diagrams of the limbic system (sometimes with a question mark), this structure is critical for motivation. Motivation and emotion are deeply interconnected, justifying its occasional inclusion in emotional models.
Amygdala: A critical structure for emotion, specifically regarding attacking and escape behaviors. It will be discussed in further detail regarding fear.

Researchers utilize functional Magnetic Resonance Imaging (fMRI) and Positron Emission Tomography (PET) to identify which brain parts are active during emotional experiences.
The theoretical expectation is that if a specific part of the brain controls an emotion (e.g., happiness), that area should show significant activity during a scan when the subject feels that emotion.
Collated Study Results: Summarized data across numerous studies indicate that there are no clear, specific brain areas associated with individual emotions.
- Happiness: Studies show activity distributed "all over the brain" with no localized center.
- Sadness: Activity is also distributed, though perhaps slightly less so than happiness.
- Disgust and Anger: There is more localized evidence for these, but it is not definitive across all neuroimaging scans.
- Fear: While associated with the amygdala, this is often difficult to demonstrate solely through these types of imaging studies.

The artificial nature of the MRI environment may pollute the results of emotional studies:
- Subjects are confined to a very small, cramped space.
- Subjects must remain perfectly still to allow the scanner to capture clear images.
- The machine produces loud, distracting noises described as "grinding, banging, and thumping" as magnets switch on and off.
Authenticity Problem: It is difficult for a subject to actually feel an authentic emotion like happiness or sadness on command while in such a restrictive and noisy environment, rather than just "pretending" to feel it.

The strongest cortical relationship identified is between the insula (or insular lobe) and the emotion of disgust.
Anatomy: The insula is a hidden lobe. To reveal it, one must pull the temporal lobe away and pull the cortex on the other side of the Sylvian fissure upwards (the gyri of the insula).
Evidence from Injury: People who have suffered a stroke or other brain injury to the insula often lose the ability to experience disgust.
- They typically fail to react to triggers like foul smells or the sight of rotting meat, which would elicit strong disgust in most people.

There is documented evidence of increased activity in the medial prefrontal cortex when people feel angry.
Inhibition Hypothesis: This area is also active during non-emotional tasks, specifically when a person must inhibit a behavior or choose a difficult correct option over an easy one.
It is speculated that the medial prefrontal cortex is not "causing" anger, but is instead acting to suppress or inhibit the physical urges and aggressive responses associated with anger.

The amygdala is located at the end of the hippocampus.
Etymology: The name "amygdala" comes from the Latin word for "almond," because its shape resembles an almond still in its shell.
The amygdala is composed of various different nuclei (parts) that contribute to its function.
In humans, it is primarily linked to anxiety and fear. In animals, there is also a very clear link between the amygdala and aggression.

A broad way of categorizing emotion is through the lens of hemispheric dominance, specifically focusing on "Approach versus Withdrawal."
Left Hemisphere - Behavioral Activation System (BAS):
- Associated with approach behaviors.
- Linked to low-to-moderate levels of arousal.
- A person with an active BAS is more likely to engage with or approach others.
- This is not strictly "positive"; one can approach others in either a friendly or an aggressive manner.
Right Hemisphere - Behavioral Inhibition System (BIS):
- Linked to withdrawal and the inhibition of behavior.
- Associated with higher levels of arousal.
- A person with an active BIS is more likely to stay away from others or withdraw from social interaction.

Depression: There is evidence that some people with depressive symptoms show higher activity in the right hemisphere compared to the left.
This suggests their Behavioral Inhibition System is more active than their Behavioral Activation System, which is consistent with the social withdrawal seen in depression.
Treatment: Some depression treatments involve brain stimulation aimed at increasing activity in the left hemisphere to encourage approach behaviors.