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Triune brain

From Wikipedia, the free 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 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. The basal ganglia is 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, in some circles of evolutionary and developmental neuroscience, the concept of the triune brain has been subject to criticism and is regarded as a myth. Due to its longevity, the triune brain idea has also been called "one of the most successful and widespread errors in all of science." as the hypothesis is no longer espoused by the majority of comparative neuroscientists in the post-2000 era. The theory has been embraced by some psychiatrists and at least one leading affective neuroscience researcher.

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[edit]

Reptilian complex

The 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 behaviors involved in aggression, dominance, territoriality, and ritual displays.

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 behavior, and parental behavior.

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.

Status of the model[edit]

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. 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 Dr. Terrence Deacon, have refined the traditional neuroanatomical ideas upon which MacLean based his hypothesis. Deacon believed that the evolutionary addition of different parts of the brain is not realistic, and that all parts of the brain were already existing. He concluded that they were just further developed upon as the homosapien species evolved and gained life experiences.

The basal ganglia, which are structures that make up MacLean's reptilian complex, were shown to take up a much smaller portion of the forebrains of reptiles and birds, than previously supposed. They were also shown to exist in amphibians and fish as well as mammals and sauropsids.

Some 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. 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 fish as well.

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). The telencephalon of birds and mammals makes neuroanatomical connections with other telencephalic structures like those made by the neocortex. It mediates similar functions such as perception, learning and memory, decision making, motor control, and conceptual thinking.

Lay interest[edit]

The triune model of the mammalian brain is seen as an oversimplified organizing theme by some in the field of comparative neuroscience. It continues to hold public interest because of its simplicity.

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.

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. The prefrontal cortex, she believes, is the key to our future if we can harness its power.