This is an instalment from the predecessor of the 5 Questions column: QED. Each instalment centred on a single question.
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Take a reptilian brain and a mammalian brain, then add a layer of cerebral cortex. Voilà: the human brain. Or is that rather too simple?
‘Typical reptilian brain behaviour’ or ‘take control of your mammalian brain’. Sentences like these fill many a self-help book. The idea: the human brain consists of three parts. The reptilian brain for primary instincts and maintaining basic bodily functions, the mammalian brain for more complex functions and emotions, and the human brain for conscious thought. Neuroscientist Paul MacLean introduced this triune brain in 1969, after which it became popular worldwide. But how much of it actually holds up?
Stacking
‘From an evolutionary perspective or from the standpoint of developmental biology, the idea that you would stack different types of brain on top of one another is of course nonsensical. All structures develop from the same neural tube’, says Menno Witter, Professor of Neuroscience at the Norwegian University of Science and Technology. Moreover, reptiles sit on an entirely different branch of the evolutionary tree from mammals, so no – there is no remnant of a reptilian brain inside the human brain.
Collaboration
‘It is a far too simplistic picture. The human brain is not a stack of puzzle pieces you can click together. It is a complex network in which all manner of structures work together. From animal experiments and imaging studies in humans, we know, for example, that all sorts of regions of the cerebral cortex – supposedly the rational human brain – are active when people view emotionally evocative images or hear an emotionally charged story’, Witter explains.
Counterpart
Witter is even opposed to the use of the more scientific term limbic system – of which an early version of the mammalian brain was said to be the counterpart. ‘That label arises purely from anatomy: it refers to structures in the limbus, or border, between the brainstem and the cerebral cortex. The word “system” implies that you can interpret this functionally. For primary motor and sensory systems such as the visual system, that works well enough, but beyond that there are no good arguments for saying that a specific brain network does only this or only that. Specific networks are more strongly involved in certain functions, certainly, but a function always depends on multiple networks.’
‘A function always depends on multiple networks’
The concept of a triune brain is not only incorrect, in Witter’s view. It is also harmful: ‘That hierarchy can give people with, say, an anxiety disorder the impression that they are suffering from a lower, unimportant piece of brain tissue. That does not do justice to the complexity of such a disorder.’ It is not only patients who are misled in this way: in Current Directions in Psychological Science (2020), American researchers call on psychologists to abandon the image of a triune brain. ‘If you assume a closed emotional brain system, it also seems logical that you need to manipulate it with treatments. Whereas far more brain structures may be involved. I find it astonishing and somewhat disappointing that this concept still appears in clinical psychology and psychiatry textbooks’, says Witter.
Survival systems
The human brain does, incidentally, have a dedicated system for the classic fight, flight, and freeze responses. ‘We do indeed have systems for rapid reactions. Humans are good at reasoning, which makes us slow. We would never survive if we always relied on that. These rapid survival systems are present in all vertebrates, simply because you cannot exist without them. But we can suppress that kind of reaction when necessary’, Witter explains. ‘The human brain has far more to offer than a small piece sitting on top of a reptilian brain. The triune brain was an appealing hypothesis because it was so simple, but the time has come to accept that this reductionism has clear drawbacks.’
Awkward questions from pupils? Send your questions to redactie@bionieuws.nl.