Triune Brain

quadrune brain

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. 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 consists of the reptilian complex, the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed as structures sequentially added to the forebrain in the course of 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.’ Though embraced by some psychiatrists and at least one leading affective neuroscience researcher, the model never won wide acceptance among comparative neurobiologists. Comparative evolutionary neuroanatomists currently regard its claims about brain evolution to be outdated.

The reptilian complex, also known as the R-complex or ‘reptilian 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 fact that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean contended that the reptilian complex was responsible for species typical instinctual behaviors involved in aggression, dominance, territoriality, and ritual displays.

The paleomammalian brain consists of the septum, amygdala, 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. Whatever the merits of the triune brain hypothesis, MacLean’s recognition of the limbic system as a major functional system in the brain has won wide acceptance 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’) and were responsible for the motivation and emotion involved in feeding, reproductive behavior, and parental behavior.

The neomammalian complex consists of the cerebral neocortex, a structure found uniquely in mammals. MacLean regarded its addition as the most recent step in the evolution of the human brain, conferring the ability for language, abstraction, planning, and perception.

Subsequent findings have invalidated the traditional neuroanatomical ideas upon which MacLean based his hypothesis, although the three functional domains described by the original triune model remain useful organizing themes and concepts.

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 fishes 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. Sauropsids were shown to possess forebrain roof structures similar in connectivity and function to the cerebral cortex (MacLean’s neomammalian complex) in mammals. Because these structures look different from the corresponding forebrain roof structures in mammals, they were originally mistaken for a part of the basal ganglia.

Recent behavioral studies likewise 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 African Gray Parrot.

Structures of the limbic system, which MacLean contended arose in early mammals, have now been shown to exist across the whole 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 date to a common vertebrate ancestor.

Finally, the third statement of the triune brain hypothesis, which is that the neocortex appears in modern mammals, is also clearly wrong. All recent studies based on paleontological data or comparative anatomy evidences strongly suggest that the neocortex was already present in the earliest emerging mammals.

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 nevertheless sometimes possess well developed pallial areas. While these areas lack the characteristic six neocortical layers, and sometimes lack lamination entirely, they make neuroantomical connections with other brain structures like those made by neocortex and mediate similar functions such as perception, learning and memory, decision making, motor control, conceptual thinking, and tool use. Scientifically, the triune brain hypothesis was based on what is now recognized as a faulty interpretation of the anatomical organization and evolution of the vertebrate brain. The idea holds little favor in current neuroscience.

The triune model continues to hold interest for some psychologists and members of the general public because of its focus on the recognizable differences between most reptiles, early mammals, and late mammals. Reasons for the success are its simplicity; the theory in this form recognizes three major evolutionary periods in the development of the brain that are characterized by three recognizably distinct ways of solving adaptive challenges).

Under this model, the ‘neocortex’ represents that cluster of brain structures involved in advanced cognition, including planning, modeling and simulation; the ‘reptilian brain’ refers to those brain structures related to territoriality, ritual behavior and other ‘reptile’ behaviors; and ‘limbic brain’ refers 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.

The three brains are said to act in coordination or competition in this variation of the model. While there is no scientific consensus on the applicability of the model at a level other than the three distinct evolutionarily distinct brain systems, some people find this to be a helpful model because of its broad explanatory value.

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