"Below the cortex, the human brain bears a striking resemblance to those of much older species," writes Thomas B. Czerner in What Makes You Tick? The Brain in Plain English (2001). "Just as they do in most animals, neurons in the medulla oblongata and pons located at the base of your brain, where it begins to taper into your spinal cord, steadily and reliably regulate your vegetative functions, automatic bodily activity such as your heartbeat and respiration."
In Descartes' Error: Emotion, Reason, and the Human Brain (1994), Antonio R. Damasio explains that "the first hint that amygdale and emotion might be related can be found in the work of Heinrich Klüver and Paul Bucy, who showed that surgical resection of the part of the temporal lobe containing the amygdala created affective indifference, among a variety of other symptoms."
Brain stem structures and the reticular formation:
In Evolving Brains (2000), John Allman cites research that shows when the amygdalae are damaged, one loses the ability to discern emotions, particularly fear and anger expressed in another's face or intoned in another's voice. Allman writes: "The role of the amygdale in the perception of facial expressions was beautifully shown by Ralph Adolphs and his colleagues, who studied a remarkable patient who had suffered a bilateral amygdalar damage without significant injury to other parts of the brain. Although this patient had normal vision and could perceive faces, she was unable to discriminate the emotional content in the negative facial expressions of fear and anger. Thus all faces appeared to be smiling or neutral to her, even those which were actually frightened or angry." Damage to the amygdalae also impair one's ability to descern emotion in another's speech. Allman writes: "Sophie Scott and her colleagues found that amygdalar lesions also disrupted the ability to perceive the emotional content of speech intonation even though their patient had normal hearing. As with facial expressions in Adolphs's patient, the auditory expressions of fear and anger were the most impaired in this patient."
The illustration below (image links to source) is borrowed from the Society for Neuroscience website. This image appears in an article on post-traumatic stress disorder (PTSD). In addition to depicting the amygdala, the illustration depicts the location of the prefrontal cortex. The prefrontal cortex is often referred to simply as the frontal lobe or frontal cortex. This area of the neocortex lies in front of cortical motor areas. We will discuss the frontal cortex and cortical motor areas later in this narrative. For now, suffice to say that the frontal cortex (or prefrontal cortex) is involved in executive functions and the expression of personality. The Wikipedia entry for prefrontal cortex explains that this area of the brain orchestrates "thoughts and actions in accordance with internal goals." The point to be made here, regarding the subcortical amygdalae, is that during normal functioning, the enlarged prefrontal cortex in humans can modulate emotional impulses generated in the amygdalae.
Continued stressful circumstances, however, can potentiate amygdalae functioning, allowing it to become more powerful—some might even say willful—over time, sometimes exerting subcortical control over our human cortical reasoning. Such potentiated activity can exacerbate symptoms of mental illness, including obsessions and compulsions.
In Why Zebras Don't Get Ulcers: The Acclaimed Guide to Stress, Stress-Related Diseases, and Coping (2004), Robert M. Sapolsky Robert M. Sapolsky emphasizes that while the glucocorticoids released during stressful episodes may disrupt hippocampal function and the memory-forming processes, those same glucocorticoids make amygdalae synapses more excitable, allowing neurons to grow more of the cables that connect the cells to each other.
"One way to understand OCD is that the normal cortical inhibition of the amygdala is malfunctioning and that the anxiety responses induced by the amygdala therefore become more intrusive and chronic in patients with OCD. …," write Denys, Zohar, and Westenberg in "The Role of Dopamine in Obsessive-Compulsive Disorder: Preclinical and Clinical Evidence." Dopamine comes into play in response to amygdalae-generated anxiety in that dopamine drives seeking activity. Seeking activity includes not only the search for food, drink and sex but–in times of anxiety and fear–access to safety. Denys, Zohar, and Westenberg write: "When dopamine is increased, the ability of the prefrontal cortex to suppress the affective responses generated in the amygdala is attenuated." We will discuss neurocircuitry related to obsessions, compulsions, and other symptoms more fully in Parts 2 and 3 of CorticalBrain.com.
It seems that evolution has—over eons of time—conserved and promoted replication of neuron firing patterns that produce behaviors which contribute to reproduction and survival, thereby automating certain movements. These movements—called fixed-action patterns—may have something to do with obsessions and compulsions.
Especially regarding PTSD, past experience is a key. Neuroscientists have found that experience shapes amygdala processing over time. You could say that the amygdalae learn, over time, the level of danger that should be associated with any particular stimulus. In defining incentive salience, authors Vilayanur S. Ramachandran and Lindsay M. Oberman expertly describe the process by which amygdalae can predict danger. In "Broken Mirrors: A Theory of Autism," Scientific American, November 2006, the authors write:
In The Emotional Brain: The Mysterious Underpinnings of Emotional Life (1996), Joseph LeDoux provides an excerpt from Heinrich Klüver's and Paul Bucy's report of a monkey's behavior after removal of the temporal lobes, which include the amygdalae. Klüver and Bucy report that the monkey does not exhibit anger and fear and seems unable to recognize objects.
LeDoux explains that Klüver and Bucy "referred to this collection of symptoms as 'psychic blindness,' by which they meant that the animals had perfectly good visual acuity but were blind to the psychological significance of stimuli." LeDoux also notes that, after removal of the temporal lobes, the monkeys became "hypersexual, attempting to copulate with other monkeys of the same sex or with members of other species (sexual activities seldom if ever practiced by 'normal' monkeys)."
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