Brain Organ that Monitors Danger
Also Seeks Pleasure
An almond-shaped brain organ, the amygdala has long been associated with translating the sensory input of dangerous situations into the emotion of fear, then signaling the body to prepare for an immediate response, either fight, flight or freeze. Now new research, as reported in Science News , reveals its role in the perception of, and response to, far more enjoyable events.
Well connected to regions of the cerebral cortex that process incoming signals from the five senses, the amygdala works in tandem with the prefrontal cortex, the brain’s command center, to determine optimum actions in the face of threats. Modern imaging tools that can zoom in on neuronal activity have allowed researchers to ferret out amygdala cells that identify, assess, and assign value to, potentially beneficial situations.
Feelings provide valuable information about the environment. It makes sense then that the amygdala, as part of the limbic system—a region of the brain that handles emotional experiences and responses to them—would play a crucial role not just in avoiding danger but also in pursuing goals associated with the pleasurable gratification of needs.
One research team, taking advantage of the availability of a small group of patients who had undergone procedures requiring placement of electrodes in their brains, monitored amygdala nerve cells in subjects evaluating the worth of assorted junk foods. Of the 51 amygdala neurons tracked, 16 demonstrated direct correlation with the volunteers’ ratings of individual treats. Results of a study like this also raise questions about the role the amygdala might play in addiction.
In an experiment that looked at the amygdala’s part in decision making, scientists found one set of nerve cells registered good surprises while another registered bad. Having two distinct types of amygdala neurons evaluating surprise according to positive versus negative content insures the most effective responses to unpredictable events.
To determine which of two brain organs directs the show, other scientists explored connections between the prefrontal cortex, also active in assigning value, and the amygdala. Using unfortunate but very smart monkeys, experimenters short-circuited the amygdala in some of them after training all of them to play a computer game where choosing one picture over another garnered a better reward.
The researchers discovered that even without a working amygdala, monkeys still selected pictures leading to the best outcome on most of the trials. Zeroing in on neurons in the orbitofrontal cortex (sensory integration, sense of self, self-reflection) and anterior cingulate cortex (attention, mood regulation, internal states monitoring , decision making), they detected a decrease in neuronal activity in the latter.
More intriguing, monkeys without a functioning amygdala exhibited no emotional response (determined by pupil diameter and heart rate in response to the reward). Retaining the cognitive ability to select wisely, they lacked any emotional engagement with their successful choices.
These findings have implications for childhood trauma survivors whose brains developed to cope with brutal, often random, assaults by people expected, and depended upon, for love and care. Stress and trauma have been found to inhibit neurogenesis (cell growth) in the hippocampus, a part of the brain that, working in concert with the amygdala and prefrontal cortex, consolidates all aspects of experience into narrative memory.
Could changes have occurred in nerve cell growth in the amygdala as well? For example, an incest survivor with childhood-based Post-traumatic Stress Disorder who has a hair-trigger response to any form of unexpected touch might have an impaired amygdala rendering her/him unable to distinguish, on the fly, between good touch and bad. A malfunctioning amygdala might also predispose someone to remain in a dysfunctional or abusive relationship, spurning positive ones, if the ability to discern rewards has been affected by early abuse or neglect.
The curious lack of emotional engagement that monkeys minus amygdalas displayed as they selected available rewards seems like anhedonia, the inability to feel pleasure, a symptom of depression. PTSD sufferers often complain about not being able to feel good or even okay. The monkeys’ disconnection between what their cortices know and what their bodies feel also suggests depersonalization, a type of dissociation associated with trauma, where the victim looks at her/himself from a distance. Later in life, survivors complain that their bodies feel alien to them and that they don’t feel anything.
As neuroimaging expands knowledge about the workings of the brain on the cellular level, the potential exists for greater understanding of the way early abuse and neglect affects the developing brain’s ability to react effectively to life’s ups and downs, especially in relationships with others. More effective treatment could then be devised to ensure recovery for all who live with childhood-based PTSD.