Triune brain

Last updated January 07, 2024

The triune brain is a model of the evolution of the vertebrate forebrain and behavior, proposed by the American physician and neuroscientist Paul D. MacLean in the 1960s. The triune brain consists of the reptilian complex (basal ganglia), the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed each as independently conscious, and as structures sequentially added to the forebrain in the course of evolution. According to the model, the basal ganglia are in charge of our primal instincts, the limbic system is in charge of our emotions and the neocortex is responsible for objective or rational thoughts.

Since the 1970s, the concept of the triune brain has been subject to criticism in evolutionary and developmental neuroscience^[1] and is regarded as a myth.^[2] Although it overlaps in some respects with contemporary understanding of the brain,^[3] the triune brain hypothesis is no longer espoused by comparative neuroscientists in the post-2000 era ^[4] due to harsh criticism against it.^[5]

MacLean originally formulated his model in the 1960s and propounded it at length in his 1990 book The Triune Brain in Evolution. The triune brain hypothesis became familiar to a broad popular audience through Carl Sagan's Pulitzer prize winning 1977 book The Dragons of Eden .

Aspects

Reptilian complex

"Reptilian complex" (also known as the "R-complex", "reptilian brain" or "lizard brain") was the name MacLean gave to the basal ganglia, structures derived from the floor of the forebrain during development. The term derives from the idea that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean proposed that the reptilian complex was responsible for species-typical instinctual behaviours involved in aggression, dominance, territoriality, and ritual displays.^[6]

Paleomammalian complex

This consists of the septum, amygdalae, hypothalamus, hippocampal complex, and cingulate cortex. MacLean first introduced the term "limbic system" to refer to this set of interconnected brain structures in a paper in 1952. MacLean's recognition of the limbic system as a major functional system in the brain was widely accepted among neuroscientists, and is generally regarded as his most important contribution to the field. MacLean maintained that the structures of the limbic system arose early in mammalian evolution (hence "paleomammalian", with paleo- meaning old) and were responsible for the motivation and emotion involved in feeding, reproductive behaviour, and parental behaviour. ^{[ citation needed ]}

Neomammalian complex

This consists of the cerebral neocortex, a structure found uniquely in higher mammals, and especially humans. MacLean regarded its addition as the most recent step in the evolution of the mammalian brain, conferring the ability for language, abstraction, planning, and perception. ^{[ citation needed ]}

Interactions between structures

The triune brain model argues that these structures are relatively independent from one another, but that they are still connected to each other in some form or another.^[7]

The model views different cognitive behaviors as caused by three different entities instead of one. The reptilian complex is said to control all of the instinctual and impulsive actions, while the neomammalian complex is responsible for keeping the primitive instincts constrained. An example is controlling the impulse of eating. It seems that if one is hungry, then that means the reptilian complex is commanding the body to eat. However, an individual has the rational choice not to eat when hungry, and this rational thought is said to be controlled by the neomammalian complex. The model thus suggest that these two (and three depending on the situation) structures are in a perpetual battle to control the body.

These interactions between the neocortex and the reptilian brain often seem competitive as the conscious thought generated by the neocortex can suppress the primitive thoughts generated by the reptilian complex. Thus, the model suggests that the interactions between structures are not constructive, but that they are conflicting due to the anatomical separation of the brain. ^[8]

This separation of structures proposed an underlying difference between consciousness and unconscious behaviour and argued that the reason why humans are such intelligent and conscious species is due to the not-so-common neocortex that they possess, unlike most other animals.^[8] This detachment contributes to the idea that the three complexes interact with each other separately rather than a single construct interacting with itself.

Status of the model

MacLean originally formulated the triune brain hypothesis in the 1960s, drawing on comparative neuroanatomical work done by Ludwig Edinger, Elizabeth C. Crosby and Charles Judson Herrick early in the twentieth century.^[9]^[10] The 1980s saw a rebirth of interest in comparative neuroanatomy, motivated in part by the availability of a variety of new neuroanatomical techniques for charting the circuitry of animal brains. Subsequent findings according to human brain evolution expert Terrence Deacon, have refined the traditional neuroanatomical ideas upon which MacLean based his hypothesis. Deacon mentioned that 'the evolutionary addition of different parts of the brain is simply not realistic. However, all the parts of the brain were already existing, they were just further developed upon as the homosapien species evolved and gained life experiences.'^[11]

For example, the basal ganglia (structures derived from the floor of the forebrain and making up MacLean's reptilian complex) were shown to take up a much smaller portion of the forebrains of reptiles and birds (together called sauropsids) than previously supposed, and to exist in amphibians and fish as well as mammals and sauropsids. Because the basal ganglia are found in the forebrains of all modern vertebrates, they most likely date to the common evolutionary ancestor of the vertebrates, more than 500 million years ago, rather than to the origin of reptiles.^{[ citation needed ]}

Recent behavioral studies do not support the traditional view of sauropsid behavior as stereotyped and ritualistic (as in MacLean's reptilian complex). Birds have been shown to possess highly sophisticated cognitive abilities, such as the toolmaking of the New Caledonian crow and the language-like categorization abilities of the grey parrot.^[12] Structures of the limbic system, which MacLean proposed arose in early mammals, have now been shown to exist across a range of modern vertebrates. The "paleomammalian" trait of parental care of offspring is widespread in birds and occurs in some fishes as well. Thus, like the basal ganglia, the evolution of these systems presumably dates to a common vertebrate ancestor.^[10]^[13]

Finally, recent studies based on paleontological data or comparative anatomical evidence strongly suggest that the neocortex was already present in the earliest emerging mammals.^[10] In addition, although non-mammals do not have a neocortex in the true sense (that is, a structure comprising part of the forebrain roof, or pallium, consisting of six characteristic layers of neurons), they possess pallial regions, and some parts of the pallium are considered homologous to the mammalian neocortex. While these areas lack the characteristic six neocortical layers, birds and reptiles generally possess three layers in the dorsal pallium (the homolog of the mammalian neocortex).^[10]^[13] The telencephalon of birds and mammals makes neuroanatomical connections with other telecencephalic structures^[10] like those made by neocortex. It mediates similar functions such as perception, learning and memory, decision making, motor control, conceptual thinking.

Lay interest

The triune model of the mammalian brain is seen as an oversimplified organizing theme in the field of comparative neuroscience.^[15] It continues to hold public interest because of its simplicity. While inaccurate in many respects as an explanation for brain activity, structure and evolution, it remains a commonly used concept as the "neocortex" represents that cluster of brain structures involved in advanced cognition, including planning, modeling and simulation; the "limbic brain" refers to those brain structures, wherever located, associated with social and nurturing behaviors, mutual reciprocity, and other behaviors and affects that arose during the age of the mammals; and the "reptilian brain" refers to those brain structures related to territoriality, ritual behavior and other "reptile" behaviors.^{[ citation needed ]}

Howard Bloom, in his book The Lucifer Principle , references the concept of the triune brain in his explanations of certain aspects of human behavior. Arthur Koestler made MacLean's concept of the triune brain the centerpiece of much of his later work, notably The Ghost in the Machine . English novelist Julian Barnes quotes MacLean on the triune brain in the foreword to his 1982 novel Before She Met Me. Peter A. Levine uses the triune brain concept in his book Waking the Tiger to explain his somatic experiencing approach to healing trauma.^{[ citation needed ]}

Glynda-Lee Hoffmann, in her book The Secret Dowry of Eve, Women's Role in the Development of Consciousness, references the triune theory explored by MacLean and goes one step further. Her theory about human behavior, and the problems we create with that behavior, distinguishes the prefrontal cortex as uniquely different from the rest of the neocortex. The prefrontal cortex, with its agenda of integration, is the part of the brain that can get the other parts to work together for the good of the individual. Hoffmann claims that in many humans the reptilian cortex (agenda: territory and reproduction; in humans that translates to power and sex) is out of control, and the amygdala stokes the fear that leads to more bad behavior.^[16]

Related Research Articles

The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. In vertebrates, a small part of the brain called the hypothalamus is the neural control center for all endocrine systems. The brain is the largest cluster of neurons in the body and is typically located in the head, usually near organs for special senses such as vision, hearing and olfaction. It is the most energy-consuming organ of the body, and the most specialized, responsible for endocrine regulation, sensory perception, motor control, and the development of intelligence.

The central nervous system (CNS) is the part of the nervous system consisting of the brain and spinal cord, the retina and optic nerve, and the olfactory nerve and epithelia. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric and triploblastic animals—that is, all multicellular animals except sponges and diploblasts. It is a structure composed of nervous tissue positioned along the rostral to caudal axis of the body and may have an enlarged section at the rostral end which is a brain. Only arthropods, cephalopods and vertebrates have a true brain, though precursor structures exist in onychophorans, gastropods and lancelets.

The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of the allocortex. It is separated into two cortices, by the longitudinal fissure that divides the cerebrum into the left and right cerebral hemispheres. The two hemispheres are joined beneath the cortex by the corpus callosum. The cerebral cortex is the largest site of neural integration in the central nervous system. It plays a key role in attention, perception, awareness, thought, memory, language, and consciousness. The cerebral cortex is part of the brain responsible for cognition.

The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.

Synapsids are one of the two major clades of vertebrate animals in the group Amniota, the other being the sauropsids, which include reptiles and birds. The synapsids were the dominant land animals in the late Paleozoic and early Mesozoic, but the only extant group that survived into the Cenozoic are the mammals. Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name. The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

Sauropsida is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to include both extinct stem-group relatives of modern reptiles, as well as birds. The most popular definition states that Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile. Sauropsids, on the other hand, include all amniotes more closely related to modern reptiles than to mammals. This includes Aves (birds), which are now recognized as a subgroup of archosaurian reptiles despite originally being named as a separate class in Linnaean taxonomy.

The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex, as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In the human brain, the cerebrum is the uppermost region of the central nervous system. The cerebrum develops prenatally from the forebrain (prosencephalon). In mammals, the dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into approximately symmetric left and right cerebral hemispheres.

<span class="mw-page-title-main">Neocortex</span> Mammalian structure involved in higher-order brain functions

The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning and language. The neocortex is further subdivided into the true isocortex and the proisocortex.

Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed and predicted brain mass for an animal of a given size, based on nonlinear regression on a range of reference species. It has been used as a proxy for intelligence and thus as a possible way of comparing the intelligence levels of different species. For this purpose, it is a more refined measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects. Expressed as a formula, the relationship has been developed for mammals and may not yield relevant results when applied outside this group.

Paul Donald MacLean was an American physician and neuroscientist who made significant contributions in the fields of physiology, psychiatry, and brain research through his work at Yale Medical School and the National Institute of Mental Health. MacLean's evolutionary triune brain theory proposed that the human brain was in reality three brains in one: the reptilian complex, the limbic system, and the neocortex.

Evolutionary neuroscience is the scientific study of the evolution of nervous systems. Evolutionary neuroscientists investigate the evolution and natural history of nervous system structure, functions and emergent properties. The field draws on concepts and findings from both neuroscience and evolutionary biology. Historically, most empirical work has been in the area of comparative neuroanatomy, and modern studies often make use of phylogenetic comparative methods. Selective breeding and experimental evolution approaches are also being used more frequently.

<span class="mw-page-title-main">Habenula</span>

The habenula is a small bilateral neuronal structure in the brain of vertebrates, that has also been called a microstructure since it is no bigger than a pea. The naming as little rein describes its elongated shape in the epithalamus, where it borders the third ventricle, and lies in front of the pineal gland.

The evolution of mammalian auditory ossicles was an evolutionary process that resulted in the formation of the bones of the mammalian middle ear. These bones, or ossicles, are a defining characteristic of all mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.

Polyvagal theory (PVT) is a collection of proposed evolutionary, neuroscientific, and psychological constructs pertaining to the role of the vagus nerve in emotion regulation, social connection and fear response. The theory was introduced in 1994 by Stephen Porges. There is consensus among experts that the assumptions of the polyvagal theory are untenable. PVT is popular among some clinical practitioners and patients, but it is not endorsed by current social neuroscience.

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References

↑ Cesario, Joseph; Johnson, David J.; Eisthen, Heather L. (8 May 2020). "Your Brain Is Not an Onion With a Tiny Reptile Inside". Current Directions in Psychological Science. 29 (3): 255–260. doi: 10.1177/0963721420917687 . S2CID 218960531.
↑ Georg F. Striedter (2005). Principles of Brain Evolution. Sunderland, MA: Sinauer Associates. ISBN 978-0-87893-820-9.^{[ page needed ]}
↑ Panksepp, J. (2003). Foreword to Cory, G. and Gardner, R. (2002) The Evolutionary Neuroethology of Paul MacLean: Convergences and Frontiers. ISBN 978-0-275-97219-6 ^{[ page needed ]}
↑ Kiverstein, Julian; Miller, Mark (6 May 2015). "The embodied brain: towards a radical embodied cognitive neuroscience". Frontiers in Human Neuroscience. 9: 237. doi: 10.3389/fnhum.2015.00237 . PMC 4422034 . PMID 25999836. S2CID 17811190.
↑ Reiner, A. (1990). The triune brain in evolution: Role in paleocerebral functions. Science, 250(4978), 303-306.
↑ Sax, Boria (15 October 2017). Lizard. Reaktion Books. ISBN 978-1-78023-872-2 . Retrieved 9 June 2020.
↑ Steffen, Patrick R.; Hedgesss, Dawson; Matheson, Rebekka (2022). "The Brain Is Adaptive Not Triune: How the Brain Responds to Threat, Challenge, and Change". Frontiers in Psychiatry. 13. 802606. doi: 10.3389/fpsyt.2022.802606 . ISSN 1664-0640. PMC 9010774 . PMID 35432041.
1 2 "Triune Brain - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 9 December 2022.
↑ Reiner, A. (12 October 1990). "The Triune Brain in Evolution. Role in Paleocerebral Functions. Paul D. MacLean. Plenum, New York, 1990. xxiv, 672 pp., illus. $75". Science. 250 (4978): 303–305. doi:10.1126/science.250.4978.303-a. PMID 17797318. S2CID 11754163.
1 2 3 4 5 Striedter, G. F. (2005) Principles of Brain Evolution. Sinauer Associates.^{[ page needed ]}
↑ "A theory abandoned but still compelling". medicine.yale.edu. Retrieved 28 September 2022.
↑ Patton, Paul (December 2008). "One World, Many Minds: Intelligence in the Animal Kingdom". Scientific American. Retrieved 29 December 2008.
1 2 Butler, A. B. and Hodos, W. Comparative Vertebrate Neuroanatomy: Evolution and Adaptation, Wiley^{[ page needed ]}
↑ Boraud, Thomas; Leblois, Arthur; Rougier, Nicolas P. (December 2018). "A natural history of skills" (PDF). Progress in Neurobiology. 171: 114–124. doi:10.1016/j.pneurobio.2018.08.003. PMID 30171867. S2CID 52143060.
↑ Smith, C.U.M. (15 January 2010). "The Triune Brain in Antiquity: Plato, Aristotle, Erasistratus". Journal of the History of the Neurosciences. 19 (1): 1–14. doi:10.1080/09647040802601605. PMID 20391097. S2CID 24578071.
↑ Hoffmann, Glynda-Lee (2003). The Secret Dowry of Eve: Woman's Role in the Development of Consciousness. Simon and Schuster. ISBN 978-1-59477-561-1.^{[ page needed ]}^{[ non-primary source needed ]}