Triune brain myth

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Chapter 4 endnote 61, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

This illusory arrangement of [brain] layers, which is sometimes called the “triune brain,” remains one of the most successful misconceptions in human biology.

MacLean’s “triune brain” idea[1] proposes a reptilian core for appetites, such as hunger and sex, cloaked in a mammalian limbic system for passions/emotion, which itself is controlled by a cerebral cortex for rationality. This idea presupposes (wrongly) that evolution is unrelentingly progressive, moving towards ever more sophisticated animal forms, with humans at the pinnacle. MacLean imposed a phylogenetic gloss on brain anatomy, arguing that the brain was structured in a hierarchical fashion because it had evolved that way.

The so-called triune brain is a myth. Experts in brain evolution no longer take it seriously.[2] Nevertheless, it arises again and again in the history of science. A succession of notable thinkers have proposed an animal brain swaddled in a blanket of humanity, as we will learn in chapter 8, including Charles Darwin’s “stamp of our lowly origin,”[3] and Paul Broca’s “grand limbic lobe.”[4] Broca’s work helped inspire the 20th-century neuroanatomist James Papez to propose an “emotion system” in the brain, which MacLean extended to become the so-called “limbic system.”[5] To MacLean, this “system” was an evolutionarily old network that was virtually unchanged in the brains of humans and non-human mammals, while the rest of the cortical mantle evolved atop to hold cognition. And voilà, the “triune brain” concept was born.

Some of the problems with the triune brain hypothesis are nicely summarized by Georg Striedter.[2] See also criticisms of the limbic system concept.

Ancient origins of the triune brain concept

Both Plato and Aristotle believed in a theory of human nature that inspired the triune brain concept.

For Plato (around 400 BCE), the psyche (mind and soul) consists of three parts: rational thoughts, passions (which today we would call emotions), and appetites like hunger and sex drive. Rational thought was in charge, controlling the passions and appetites, an arrangement that Plato described as a charioteer wrangling two winged horses. Plato placed emotions in the heart, appetites in the liver and possibly other parts of the gut, and rationality in the head. MacLean tattooed these divisions onto the human brain, with emotions and appetites residing in the subcortical and limbic circuity that controls the body, and rationality in the cerebral cortex (which was mistakenly thought to have nothing to do with body-budgeting or emotion).

This “fish to human” progression in the "triune brain concept" dates back to Aristotle, who ordered all species linearly from simple to complex (scala naturae).

Notes on the Notes

  1. MacLean, Paul D. 1952. "Some psychiatric implications of physiological studies on frontotemporal portion of limbic system (visceral brain)." Electroencephalography and Clinical Neurophysiology 4 (4): 407-418.
  2. 2.0 2.1 Striedter, Georg F. 2006. “Précis of Principles of Brain Evolution.” Behavioral and Brain Sciences 29 (1): 1–36.
  3. Darwin, Charles. (1871) 2004. The Descent of Man, and Selection in Relation to Sex. London: Penguin, p. 689.
  4. Broca, Paul. 1877. "Sur la circonvolution limbique et la scissure limbique." Bulletins de la Société d'anthropologie de Paris 12 (1):  646-657.
  5. Papez, James W. 1937. "A proposed mechanism of emotion." Archives of Neurology & Psychiatry 38 (4): 725-743.