Copyright © 2007-2017 Russ Dewey
Neurologist Paul D. MacLean invented the term limbic system. MacLean's theory of the triune brain (three-part brain) identified three areas of differing phylogenetic or evolutionary age. Although the model is simplistic, there is some truth to it.
The oldest part of the brain, sometimes called the reptilian brain, is present in virtually all animals. It controls behavior dominated by olfaction–the sense of smell.
The olfactory lobe of the brain, directly over the sinus cavities in humans, is a primitive structure found in all animals. It is called the rhinencephalon, literally the "smell brain." This area controls many animal activities that typically involve the sense of smell: food gathering, courtship, mating, and warning of predators.
What is MacLean's "triune brain" concept?
The second part of the brain, which MacLean claimed was unique to mammals, is the limbic system. As it turns out, animals other than mammals do have rudiments of a limbic system, but it is larger in mammals. MacLean (1949) originally called this the visceral brain because of its close ties to the autonomic nervous system, which influences the gut (viscera).
The third brain area is the neocortex (literally "new cortex"), which corresponds to the cerebral hemispheres in humans. As we saw in Chapter 2, humans have far more neocortex than any other species. In MacLean's simple system, the neocortex produces the intellect, which overlies and controls the impulses of the limbic system.
In 1952 MacLean coined the term limbic system (to replace visceral brain) and the label stuck. It is now widely used to describe the deep forebrain structures, below the cerebral cortex but above the brain stem.
Is the limbic system exclusively devoted to emotion?
The limbic system is basically an area, not a structure. It consists of a dozen different specialized structures. The limbic system is not exclusively devoted to emotion. It contains the hippocampus, which is vital to the cognitive process of assembling event memories, as well as the hypothalamus, which regulates the autonomic nervous system.
The limbic system also contains several structures crucial to motivation and emotion. We already discussed the pleasure centers, which include areas in the ventral tegmental area (VTA), nucleus accumbens, and the septum, all in the limbic system.
The limbic system is also home to the amygdala, part of a system for detecting threats to the organism. You might recall from Chapter 2 that a rat with its amygdala removed "will walk up to a sleeping cat and even nibble on its ear."
What is the apparent function of the amygdala?
The function of the amygdala in humans has been illuminated by patients with damaged or surgically removed amygdalas. One patient, known as S.M., had damage to the amygdala on both sides due to a rare genetic disorder.
S.M. was studied for years by researchers at the University of Iowa. They described S.M. as generally in a good mood, despite adversity in her life such as living in a dangerous area with poverty, crime, and drugs (Tranel, Gullickson, Koch and Adolphs, 2006).
In questionnaires, S.M. reported no fear. She reported other emotions such as joy, happiness, and sadness. She remembered some incidents of fear from childhood, before her disease destroyed her amygdalas.
Feinstein, Adolphs, Damasio, and Tranel (2010) reported how S.M. visited a pet store with snakes and spiders. She showed no fear, repeatedly asking to touch the snakes and a tarantula.
How did a woman with damaged amygdalas behave?
S.M. had been held at gunpoint and knifepoint during a domestic incident, but she felt no fear. She was aware that her fearlessness was abnormal.
S.M. did not react to external threats (including horror movies). However, she did respond with great anxiety to inhaling carbon dioxide, which produces feelings of suffocating.
This was a valuable tip to researchers, who verified the phenomenon with two other patients with bilateral amygdala damage. "They and S.M. said these fearful feelings were entirely novel to them."
The fear occurred again if the carbon dioxide was administered again. These and other studies that the amygdala is specifically tuned to external threats (Feinstein et al., 2013).
S.M. had trouble recognizing negative social cues such as untrustworthiness in others. She had no sense of "personal space" and stood extremely close to strangers, even nose-to-nose with direct eye contact, without discomfort.
Apart from her indifference to external threats, S.M. was normal emotionally. She had a high degree of empathy for others and had three normal children.
S.M.'s high degree of empathy is intriguing in view of another case involving surgical removal of the amygdala. Richard-Mornas, Mazzietti, Koenig, Borg, Convers, and Thomas-Anterion (2014) described a woman who developed "hyper-empathy" after removal of her right amygdala and hippocampus.
She described a "new ability to decode others mental states," confirmed by her family. This was a permanent change; it lasted 13 years before the researchers wrote about it.
The woman reported feelings of great upset if she saw suffering on news programs. She scored above average on a test involving recognizing emotions in 36 photographs. Her reactions also occurred when meeting people in person or reading about characters in novels.
The reports of fearlessness and empathy are compatible. Empathy is ruled out by a feeling of being threatened. In war, for example, people demonize an enemy and do not empathize with them. When the amygdala is removed, a threat-detection system is disabled. Empathy then predominates.
Feinstein, J. S., Adolphs, R., Damasio, A., & Tranel, D. (2011) The Human Amygdala and the Induction and Experience of Fear. Current Biology, 21, 34-38. doi:http://dx.doi.org/
Feinstein, J. S., Buzza, C., Hurlemann, R., Follmer, R. L., Dahdaleh, N. S., Coryell, W. H., Welsh, M. J., Tranel, D., & Wemmie, J. A. (2013). Fear and panic in humans with bilateral amygdala damage. Nature Neuroscience. Retrieved from: https://digest.bps.org.uk/
Richard-Mornas, A., Mazzietti, A., Koenig, O., Borg, C., Convers, P., & Thomas-Anterion, C. (2014) Emergence of hyper empathy after right amygdalohippocampectomy. Neurocase, 20, 666-670. doi:10.1080/13554794.2013.826695
Tranel D, Gullickson G, Koch M, & Adolphs R (2006). Altered experience of emotion following bilateral amygdala damage. Cognitive Neuropsychiatry, 11, 219-232. doi:10.1080/13546800444000281
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Copyright © 2007-2017 Russ Dewey