The amygdala comprises two almond-sized-and-shaped portions of the mid-brain, that are becoming more and more common targets for study. The numbers of research reports involving the amygdala has been skyrocketing with each passing year. As of the date of this blog, September 9, 2009, PubMed reports 21,633 papers in which the amygdala is mentioned, 1933 of them being major review articles.
And while there are two units (left and right sides of the brain) to the amygdala, it is more often discussed as if it were one organ, so the singular term “amygdala”, and the common usage plural “amygdalas,” not the Latin plural “amygdalae” is used here and throughout most of the medical literature.
The amygdala is connected with the prefrontal cortex, an area involved with our highest intellectual properties, and receives sensory inputs from it all the time. But the amygdale is not a part of the conscious brain. It does not think through images or situations in a sophisticated way. Instead it is part of the limbic system, an evolutionarily highly conserved area that was well developed in animals before man, and is part of an active vigilance-for-survival mechanism.
The amygdale is like a guard dog that never sleeps. It instantly senses danger and calculates magnitude of risk in microseconds. It then passes the message on to the hippocampus, which then routes the message to memory and action centers which help initiate responses and activate coping mechanisms.
Simultaneously , the amygdale causes cause a flood of cortisol, a notable stress hormone, throughout the body. Indeed the “gut sense” that one is in danger is quite literally through the action of the amygdale in prompting “fight-or-flight” hormone surges.
Descartes’ Rational Mind vs. Body Dualism Exploded by Damasio and LeDoux’ s Emotional Brain and Amygdala
Nonetheless, it has become increasingly clear that the amygdale is not just a fear machine.
The study of the function of the amygdale has helped resolve one of the great debates in neuroscience over the last 250 years: Whether the Body (in this case, the amygdala and the brain) governed the Mind (in this case, the sense of consciousness, self-awareness, thought processing, learning and memory), or whether these were two truly distinct operations.
This debate has largely been framed through a challenge to the thinking of Rene Descartes, 1596-1650, a French philosopher and scientist, who posited that man had a rational mind, intellect or soul whose functioning was largely independent of the body and its body parts and which did not have to obey the laws of physics. He gave the mind and thought processes a higher place, saying “I think, therefore, I am.”
In other words, explanations of the operation of the mind, soul or intellect ----including problems of consciousness, thought, learning and memory----cannot, according to Descartes, be reduced to the laws governing biology, chemistry & physic, which clearly have dominion over the body and its parts.
Therefore, according to Descartes, the study of thought processes and learning were properly outside the field of medicine, and suitable only for philosophy courses.
Descartes did allow for the pineal gland to be the possible location of the soul or intellect, but basically saw clear jurisdictional lines separating the body and most of its organs from all things that are the product of the “rational” mind: thoughts, memories, and especially the acquisition of new knowledge.
However, it is now increasingly well-argued in works by two of the last century’s greatest neuroscientists, Antonio Damasio (University of Iowa), author of Descartes’ Error: Emotion, Reason and the Human Brain (1995), and of The Feeling of What Happens: Body and Emotion in the Making of Consciousness (2000)and Joseph LeDoux (NYU), author of The Emotional Brain: The Mysterious Underpinnings of Emotional Life (1996) and of The Synaptic Self: How Our Brains Become Who We Are, that an actively functioning emotional-alarm-circuit amygdala (clearly a body part) allows for new information to be better incorporated into memory (clearly a intellectual process that Descartes saw as “rational” and not Body-Part-Independent) and subsequently into action.
Indeed our sense of reality and what we pay attention to (in a sense our own construction of the conscious world in which we see ourselves living ) are in many ways amygdala-dependent .
Or in the view of Damsio and LeDoux, “ We feel, as well as think, therefore, we are who we are in the world as we perceive it”
The amygdala may very well do this by helping the brain identify salient points of new inputs (whether they have red or green flags indicating either danger or reward) and to prioritize them by the use of the “magnitude dial” of the amygdale (important enough to pay close attention, or not so important or threatening and therefore, something to ignore).
Ultimately, as Damasio and LeDoux would argue, we tend better to learn and remember those things that have particular, added emotional emphases in our lives, and we have to work harder at recall to retrieve information that has no special meaning.
In other words, we remember that on July 3rd, our mother died, but we cannot recall what we had for breakfast on November 15th, because that was not an unusually emotionally charged occurrence.
Incoming information about a death in the family is processed by the amygdale as a fearful new fact, which gives it additional weight in its routing, storage, and availability for retrieval within the brain. Whether you had oatmeal or eggs is likely not.
A well-functioning amygdale also appears to help us interpret the emotional state of others through accurately processing the emotions they display through their facial expressions. This conveys an evolutionarily important advantage, because seeing the signs of either collective panic (danger) or group jubilation (opportunity) in the demeanor of others enables you better to take action sooner.
You need not personally see the coming tidal wave to realize that you must run to higher ground. Likewise, you can determine if your parent or teacher is happy with you, or angry, before they ever say a word.
Shrunken or Damaged Amygdala
It is perhaps no surprise that persons with ADHD who have trouble focusing have been revealed in some studies to have a diminished amygdalas, reducing their ability to see what is important and to pay attention to it.
Likewise, persons with Aspberger’s Syndrome and more severe Autistic Spectrum Disorders, have diminished amygdalas, suggesting that their inability to zero in on people’s faces or activities, keeps them unaware of the emotional states of others. They have a reduced ability to formulate a Theory of Mind that is important to learning throughout the lifespan.
In other words, they cannot pay attention to what is being demonstrated by others and imitate it as part of learning or social integration, because they view all incoming information as having equal threat or reward potential, and therefore being of no particular worth of paying attention to one thing or person over another thing or person.
At the worst, some autistics see all incoming information as being consistently threatening, and will disengage from the outside world, because such a constant threat level is unsustainable owing to the inner turmoil it generates.
A similar pattern of behavior, where suspiciousness and a sense of being attacked, is found in some patients with Alzheimers, when the beta-amyloid plaques that are characteristic of the disease, interfere with the operation of the amygdala.
A legal hot-button issue is that some brain imaging studies have found that criminal sociopaths have diminished amygdalas. (see the work by Yaling Yang, an expert on the brain abnormalities of psychopaths and pathological liars, and her co-workers, at UCLA, below). Shrunken or malfunctioning amygdales are associated with a lesser ability to determine the emotional states of other persons, so that presumably, the criminal had little or no ability to realize the depth of pain and suffering he or she was inflicting. This is argued by some criminal defense attorneys as allowing for a medical or psychiatric disability that eliminates or reduces culpability for their crimes.
However, usage of such defenses also opens up the possibility that if the diminished amygdalas are the cause of the criminal behavior, then perhaps the criminal should be confined to an asylum for the criminally insane until the amygdalas “get back to normal,” an anatomical recovery that may take longer than the usual prison sentence, assuming it ever happens.
Persistent, Dysfunctional Fearful Memories and PTSD
Obviously one of the major clinical concerns where the amygdala may play a role is in patients with persistent anxiety and depression. A wide variety of brain function and imaging studies do in fact, implicate amygdala dysfunction or hyperactivity activity in the linkages of the amygdale to the hippocampus or the prefrontal cortex.
Therapies that have achieved the most success seem to have help to break, at least temporarily, the association of some reasonably neutral stimuli (for example, a loud noise) with a particularly traumatic memory (for example, a bomb going off). In other words, they disrupt, at least temporarily, the negative Pavlovian cycle of stimulus and automatic response.
But a developmental brain study breakthrough in the journal Science by Gogolla et al. cited in detail below, shows why this may actually be easier in children than in adults.
It turns out that at about four weeks of age (which translates into some years within early human childhood) in lab rats that had been exposed to Pavlovian fearful conditioned stimuli, the memory of that fearful stimuli, and the associations that came with it, seemed permanent, and could be brought again and again to the surface with only modest levels of adverse stimuli, even after several months (many years in human aging). They couldn’t forget.
However, before that age , these painful associations and the memory of the fear became extinct.
The difference appears to be a protective protein coat that encapsulates certain areas of the amygdala beginning at four weeks.
The coat appears to eliminate, or at least greatly reduce the loss of fearful memories, generated by training with negative Pavlovian stimulus and responses, perhaps as an adult survival aid. In other words, adults who remembered what was dangerous had an evolutionary advantage over those who kept forgetting.
However, the same team of scientists that discovered this memory-entrapping protein coat for the amygdala, also came up with a line of rats that were genetically deficient in producing that protein and developing that amygdale coat.
These rats lost their fearful memories and anxiety routinely past the four weeks critical point, despite having been exposed to a similar negative Pavlovian stimulus and response training. In other words, no coat means no recall of fearful memories or triggering of “Pavlovian panic” in the rats.
Whether or not this could somehow be adapted to humans with PTSD is uncertain, and the wisdom of effectively erasing a large portion of, if not all, adverse memories and fear responses would surely be a matter of great ongoing medical and psychiatric debate.
Tony Stankus, FSLA, tstankus@uark.edu Professor, Life Sciences Librarian & Science Coordinator
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Really, what we want now, is not laws, against crime, but a law against insanity. (Mark Twain, American writer)
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