Triune brain
Encyclopedia
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 Comparative evolutionary neuroanatomists currently regard its claims about brain evolution to be outdated.
, 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.
, 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.
, 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.
, Elizabeth Crosby and C. J. Herrick early in the twentieth century. 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 have invalidated the traditional neuroanatomical ideas upon which MacLean based his hypothesis.
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 anatomical evidence 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 neuroanatomical 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 popularity of the model can also be seen in the way it parallels recurring themes in popular culture and the arts. For example, some languages have phrases which refer to speaking from the "head", "heart", or "gut", or philosophically of the three virtues of "wisdom, benevolence and courage"--or psychologically of "thinking", "feeling", and "willing". In The Wizard of Oz
, for example, the quest for "a brain", "a heart", and "courage" play a central role. The three elements of the triune model map comfortably onto these more abstract conceptions.
In this sense, the triune brain (more properly, perhaps, the "triune mind") is seen as a highly simplified but powerful organizing theme. The statistician George E. P. Box
once quipped: "Essentially, all models are wrong, but some are useful."
Howard Bloom
, in his book The Lucifer Principle
, references the concept of the triune brain in his explanations of certain aspects of human behavior.
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of the vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
forebrain and behavior proposed by the American physician and neuroscientist
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...
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
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...
, the paleomammalian complex (limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...
), and the neomammalian complex (neocortex
Neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
), 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
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...
's Pulitzer prize
Pulitzer Prize
The Pulitzer Prize is a U.S. award for achievements in newspaper and online journalism, literature and musical composition. It was established by American publisher Joseph Pulitzer and is administered by Columbia University in New York City...
winning 1977 book The Dragons of Eden
The Dragons of Eden
The Dragons of Eden: Speculations on the Evolution of Human Intelligence is a Pulitzer Prize winning 1977 book by Carl Sagan. In it, he combines the fields of anthropology, evolutionary biology, psychology, and computer science to give a perspective of how human intelligence evolved.One of the main...
. Though embraced by some psychiatrists and at least one leading affective neuroscience
Affective neuroscience
Affective neuroscience is the study of the neural mechanisms of emotion. This interdisciplinary field combines neuroscience with the psychological study of personality, emotion, and mood.-Brain areas related to emotion:...
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
The reptilian complex, also known as the R-complex or "reptilian brain" was the name MacLean gave to the basal gangliaBasal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...
, 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 complex
The paleomammalian brain consists of the septumSeptum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...
, amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
, hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, hippocampal complex
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, and cingulate cortex
Cingulate cortex
The cingulate cortex is a part of the brain situated in the medial aspect of the cortex. It includes the cortex of the cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the cingulate sulcus...
. MacLean first introduced the term "limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...
" 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
The neomammalian complex consists of the cerebral neocortexNeocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
, 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.
Current status of the model
MacLean originally formulated the triune brain hypothesis in the 1960s, drawing on comparative neuroanatomical work done by Ludwig EdingerLudwig Edinger
Ludwig Edinger was an influential German anatomist and neurologist and co-founder of the University of Frankfurt. In 1914 he was also appointed the first German professor of neurology....
, Elizabeth Crosby and C. J. Herrick early in the twentieth century. 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 have invalidated the traditional neuroanatomical ideas upon which MacLean based his hypothesis.
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
Toolmaking
The term toolmaking may refer to:* The act of making tools of any kind, from the simplest handtools made of plant fiber or stone, to the most technologically advanced tools. This form of toolmaking is a topic of interest in anthropology. Some non-human animal species also use tools.* Tool and die...
of the New Caledonian crow
New Caledonian Crow
The New Caledonian Crow is a tool-using species of crow endemic to New Caledonia and the Loyalty Islands. These crows are some of the only non-primate animals known to invent new tools by modifying existing ones, and then passing these innovations on to other individuals in the cultural group...
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 anatomical evidence 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
Pallium (neuroanatomy)
In a neuroanatomy context, the word pallium refers to the layers of gray and white matter that cover the upper surface of the telencephalon in vertebrates. The non-pallial part of the telencephalon builds the subpallium. In basal vertebrates the pallium is a relatively simple three-layered...
areas. While these areas lack the characteristic six neocortical layers, and sometimes lack lamination entirely, they make neuroanatomical 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.
Continuing popular interest in the model
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.The popularity of the model can also be seen in the way it parallels recurring themes in popular culture and the arts. For example, some languages have phrases which refer to speaking from the "head", "heart", or "gut", or philosophically of the three virtues of "wisdom, benevolence and courage"--or psychologically of "thinking", "feeling", and "willing". In The Wizard of Oz
The Wizard of Oz (1939 film)
The Wizard of Oz is a 1939 American musical fantasy film produced by Metro-Goldwyn-Mayer. It was directed primarily by Victor Fleming. Noel Langley, Florence Ryerson and Edgar Allan Woolf received credit for the screenplay, but there were uncredited contributions by others. The lyrics for the songs...
, for example, the quest for "a brain", "a heart", and "courage" play a central role. The three elements of the triune model map comfortably onto these more abstract conceptions.
In this sense, the triune brain (more properly, perhaps, the "triune mind") is seen as a highly simplified but powerful organizing theme. The statistician George E. P. Box
George E. P. Box
- External links :* from a at NIST* * * * * *** For Box's PhD students see*...
once quipped: "Essentially, all models are wrong, but some are useful."
Howard Bloom
Howard Bloom
Howard Bloom is an American author. He was a publicist in the 1970s and 1980s for singers and bands such as Prince, Billy Joel, and Styx. In 1988 he became disabled with chronic fatigue syndrome...
, in his book The Lucifer Principle
The Lucifer Principle
The Lucifer Principle is a book by Howard Bloom. It sees a social group, not an individual, as a main subject of human evolution. It "explores the intricate relationships among genetics, human behavior, and culture" and argues that "evil is a by-product of nature's strategies for creation and that...
, references the concept of the triune brain in his explanations of certain aspects of human behavior.
See also
- The Dragons of EdenThe Dragons of EdenThe Dragons of Eden: Speculations on the Evolution of Human Intelligence is a Pulitzer Prize winning 1977 book by Carl Sagan. In it, he combines the fields of anthropology, evolutionary biology, psychology, and computer science to give a perspective of how human intelligence evolved.One of the main...
, Pulitzer prize winning 1977 book by Carl Sagan - The Triune Brain in Evolution: Role of Paleocerebral Functions, Paul D. MacLean, 1990, Springer.
- Principles of Brain Evolution, Georg F. Striedter, 2005, Sinauer associates.
- Comparative Vertebrate Neuroanatomy, Second Edition, Ann B. Butler and William Hodos, 2005, John Wiley and Sons.
Further reading
- MacLean, Paul D. "Brain Evolution Relating to Family, Play, and the Separation Call". Arch. Gen. Psychiatry 42: 405-417, 1985.