The Triune Brain Theory, developed by neuroscientist Paul D. MacLean in the 1960s, offers a simplified perspective on the structure and functionality of the human brain. This theory proposes that the human brain is composed of three distinct layers that evolved sequentially:
- Reptilian brain
- Limbic system
- Neocortex
Each layer represents a different stage in the evolution of the brain, corresponding to advances in cognitive and behavioural capabilities. While the theory may not capture every complexity of the brain’s structure and function, it provides a foundation for understanding the intricate interconnection between the brain’s evolution, its organization, and human behaviour.
In recent years, however, the Triune brain hypothesis has been subject to reevaluation by various neuroscientists, arguing that this model oversimplifies both the evolutionary process and the structure of the brain. Human brain evolution expert Terrence Deacon, for example, contends that MacLean’s model fails to account for the intricate relationships between different brain networks, which often overlap and interact in complex ways.
Additionally, critics of MacLean’s brain theory assert that it doesn’t accurately reflect the true nature of vertebrate brains’ evolution. The evolution of the brain’s structure does not appear to have followed a linear path, but rather has been the result of numerous, cooperative adaptations in response to alterations in the environment and way of life.
So, although the Triune brain model has historical significance and provides a memorable framework for understanding certain aspects of brain evolution, modern neuroscience research shows that it might not be the most accurate or comprehensive way to explain the organization and development of vertebrate brains. Instead, researchers are now exploring alternative models that address the complex interplay of brain networks and acknowledge the multi-dimensional nature of brain evolution.
Evolutionary Perspectives
From an evolutionary point of view, the development of vertebrate brains can be traced back to the emergence of early vertebrates within the animal kingdom. The brains of these primitive creatures underwent a series of complex changes over millions of years, culminating in the advanced brains observed in modern vertebrates.
American physician and neuroscientist Paul MacLean proposed the concept of the ‘Triune brain’ as a way to explain the evolution and organization of the human brain. The model was based on comparative neuroanatomical research done in the early twentieth century by Ludwig Edinger, Elizabeth C. Crosby, and Charles Judson Herrick.
The Triune brain theory gained popularity partly due to its inclusion in Carl Sagan’s book Dragons of Eden, which won a Pulitzer Prize in 1978. Sagan’s work brought MacLean’s concept to a wider audience and sparked ongoing debates around the theory.
Triune Brain Theory Critiques
The availability of a number of innovative neuroanatomical techniques for charting the circuitry of animal brains fueled a resurgence of interest in comparative neuroanatomy in the 1980s. According to Terrence Deacon, further studies have refined the classic neuroanatomical principles on which MacLean’s hypothesis was built.
For example, the basal ganglia (structures derived from the floor of the forebrain that comprise MacLean’s reptilian complex) were discovered to occupy a much smaller portion of the forebrains of reptiles and birds (collectively known as sauropsids) than previously thought and to exist in amphibians and fish as well as mammals and sauropsids.
Furthermore, recent behavioural findings do not support MacLean’s reptilian complex’s notion of sauropsid behaviour as stereotyped and ritualistic. Birds have been found to exhibit highly sophisticated cognitive capacities, such as the New Caledonian crow’s toolmaking and the grey parrot’s language-like classification ability.
Credit: Progress in Neurobiology, 2018, 171, pp.114-124. ⟨10.1016/j.pneurobio.2018.08.003⟩.
Structures of the limbic system, which MacLean thought arose in early mammals, have since been found in a variety of current animals. Parental care of offspring is a “paleomammalian” feature found in birds and some fishes. Thus, these systems, like the basal ganglia, apparently evolved from a common vertebrate ancestor.
Finally, current paleontological and comparative anatomical investigations strongly indicate that the neocortex was present in the earliest developing mammals. Furthermore, while non-mammals lack a true neocortex (a structure comprising part of the forebrain roof, or pallium, consisting of six distinct layers of neurons), they do have pallial regions, and some parts of the pallium are considered homologous to the mammalian neocortex.
While these areas lack the typical six neocortical layers, birds and reptiles often have three layers in the dorsal pallium (the mammalian neocortex’s analog).
Reptilian Brain Characteristics
The reptilian brain, also known as the reptilian complex or R-complex, is the most primitive part of the triune brain. It includes structures such as the brainstem, pons, and basal ganglia. These parts of the brain are responsible for our most basic functions, such as heart rate, respiration, and temperature regulation. The reptilian brain operates on an unconscious level, meaning it is not directly involved in our conscious awareness.
The reptilian brain, MacLean proposed, is associated with instinctive behaviors and drives, such as those related to survival, territoriality, and aggression. The reptilian complex controls the fight or flight response, which is a crucial component of our stress response system. It is important to note that these instinctual responses are not unique to reptiles but are present across the animal kingdom, including in mammals and humans.
It was believed that this part of the brain reflects an ancestral relationship to reptiles, hence the name. While it serves crucial functions in the most basic aspects of our physiology, it has been largely conserved throughout evolution. As newer and more sophisticated structures developed in the brains of mammals and humans, the R-complex remained as a foundational component for our nervous system, ensuring essential functions were maintained for survival.
Limbic System Functionality
The limbic system, according to this model, is a complex set of brain structures that plays a crucial role in various aspects of human behavior and cognitive function.
Located at the border of the cortex and the midbrain, it includes structures such as the amygdala, hippocampus, and cingulate cortex. Called originally by MacLean the visceral brain (and renamed the limbic system in 1952), it broadly encompasses three essential functions: emotions and memory, social behaviour, and development over time.
Emotions and Memory
The limbic system is responsible for processing and regulating emotions, with the amygdala playing a vital role in our affective or emotional responses. The amygdala assesses incoming sensory information and generates appropriate emotional reactions, such as fear or pleasure.
The hippocampus, another critical component of the limbic system, is involved in the formation and consolidation of new memories. It facilitates the transfer of information from short-term to long-term memory and also helps in spatial navigation. The cingulate cortex plays a role in connecting the emotional and cognitive aspects of the brain, enabling the integration of emotional and sensory experiences into conscious awareness.
Social Behaviour
The limbic system is also essential for the regulation of social behaviours. The prefrontal cortex and mammalian brain play a crucial role in controlling impulse control and decision-making. These regions of the brain work together to facilitate complex social interactions, such as empathy, communication, and cooperation.
The amygdala contributes to social behaviour by responding to various social cues. Its activation generates emotional reactions, influencing how we perceive and respond to others. The hippocampus plays a part in social cognition by helping us remember social information, such as faces and names.
Development Over Time
Over time, the limbic system evolves and changes its functionality in response to various factors, such as age, experience, and environmental stimuli. The pallium is believed to be an ancestral structure that gave rise to the mammalian limbic system, evolving to form the distinct circuits that govern emotions, memory, and social behaviour.
As humans develop from infancy through adulthood, the limbic system undergoes significant changes. For instance, the amygdala and hippocampus form many new connections during childhood and adolescence.
The prefrontal cortex continues to develop until early adulthood, refining emotional regulation, judgment, and decision-making processes. These developmental changes contribute to the maturation of cognitive and emotional skills that humans rely upon throughout their lives.
Neocortex and Higher Order Cognition
The neocortex, also known as the neopallium, isocortex, or six-layered cortex, is a collection of layers in the mammalian cerebral cortex that are involved in higher-order brain functions such as sensory perception, cognition, motor command generation, spatial reasoning, and language. It is an essential part of the triune model and plays a crucial role in higher-order cognitive processes.
The neocortex is responsible for facilitating an individual’s ability to reason and use logic in decision-making processes. It is the most developed component in the human brain and is responsible for processing complex information and generating conscious thoughts.
The neocortex enables humans to engage in various cognitive processes, such as problem-solving, analyzing, and synthesizing data, which helps individuals arrive at logical conclusions.
Creativity and Planning
Not only does the neocortex handle reason and logic, but it also plays a vital role in creativity and planning. The neocortex is involved in advanced cognition, helping individuals come up with innovative ideas, solve complex problems, and engage in long-term planning, both in professional and personal contexts.
Furthermore, the neocortex is responsible for the development and execution of complex strategies that require working memory and the ability to manipulate and evaluate multiple variables simultaneously.
Neural Plasticity and Adaptation
The neocortex also demonstrates a remarkable ability for neural plasticity and adaptation. Neural plasticity refers to the brain’s ability to reorganize itself and form new neural connections throughout a person’s life.
The adaptive nature of the neocortex enables individuals to adjust to new experiences, learn new skills, and adapt to changes in their environment.
This capacity for neural plasticity is essential for human survival because it ensures that the brain can effectively respond to different challenges and adapt to evolving conditions. In summary, the neocortex is a crucial component of the triune brain, responsible for facilitating higher-order cognitive processes, such as reason, logic, creativity, planning, and neural adaptation, which are integral to human beings’ everyday lives.
Interplay Between Brain Regions
The theory considers diverse cognitive behaviours to be caused by three distinct entities rather than one. The reptile complex is supposed to control all intuitive and impulsive actions, whereas the neomammalian complex is in charge of keeping primal impulses in check.
Controlling the urge to eat is one example. If one is hungry, it appears that the reptile complex is instructing the body to eat. Individuals, on the other hand, have the logical decision not to eat when they are hungry, and this rational reasoning is considered to be governed by the neomammalian complex.
According to the model, these two (or three, depending on the situation) structures are always fighting for control of the body.
Some key interactions between brain regions include:
- Emotion and cognition: The limbic system and neocortex work together to facilitate decision-making by integrating emotional responses and rational thought. For example, the amygdala processes emotions like fear, while the prefrontal cortex helps to evaluate and regulate those emotional reactions.
- Memory consolidation: The hippocampus, which is part of the limbic system, is vital for forming and organizing long-term memories. It works closely with the neocortex to transfer and store context-rich memories, supporting higher cognitive functions.
- Attentional processes: The interplay between the default mode network (DMN) and attention networks in the brain is essential for shifting focus between internal and external stimuli. This dynamic balance between introspection and externally directed cognition is critical for adaptive behaviour and self-reflection.
References:
- Butler, Ann & Hodos, William. (2005). Comparative Vertebrate Neuroanatomy: Evolution and Adaptation, Second Edition. John Wiley & Sons, 10.1002/0471733849.ch19.
- 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
- Cory, G.A. (1999). MacLean’s Triune Brain Concept: In Praise and Appraisal. In: The Reciprocal Modular Brain in Economics and Politics. Springer, Boston, MA. doi: 10.1007/978-1-4615-4747-1_3
- Cory Jr., G.A. (2000), From Maclean’s Triune Brain Concept to the Conflict Systems Neurobehavioral Model: The Subjective Basis of Moral and Spiritual Consciousness. Zygon, 35: 385-414.doi: 10.1111/0591-2385.00283
- Heimer L, Van Hoesen GW. The limbic lobe and its output channels: implications for emotional functions and adaptive behavior. Neurosci Biobehav Rev. (2006) 30:126–47. doi: 10.1016/j.neubiorev.2005.06.006
- LeDoux J. Rethinking the emotional brain. Neuron. (2012) 73:653–76. doi: 10.1016/j.neuron.2012.02.004
- Reiner, A. The Triune Brain in Evolution. Role in Paleocerebral Functions. Science, vol. 250,4978 (1990): 303-5. doi:10.1126/science.250.4978.303-a
- Steffen PR, Hedges D and Matheson R (2022) The Brain Is Adaptive Not Triune: How the Brain Responds to Threat, Challenge, and Change. Front. Psychiatry 13:802606. doi: 10.3389/fpsyt.2022.802606
- Striedter, G. F. (2005). Principles of brain evolution. Sinauer Associates. ISBN: 978-0878938209
- Striedter, Georg & Northcutt, R.. (2019). Brains Through Time: A Natural History of Vertebrates. doi: 10.1093/oso/9780195125689.001.0001.
Top image credit: Lchunhori CC-BY