2023-04-25 16:03:09
Our brain, this fabulous object. Millions of years of evolution have carved mazes of neural pathways into this material “with the consistency of lukewarm porridge”, according to mathematician Alan Turing. But how can this soft matter secrete thoughts and emotions, drive complex behaviors, command movements which themselves modulate our mental states – and vice versa?
Plato (c. 427 BC − c. 347 BC) and Descartes (1596-1650) on one side, Spinoza (1632-1677) on the other. For the first, the spirit and the body were two substances of radically different natures. For Spinoza they are “one and the same thing expressed in two different ways”.
The Dutch thinker was right. A study published in the journal Nature, April 19, provides new evidence, supporting brain imaging. The authors precisely mapped our primary motor cortex, that narrow strip that runs from temple to temple on the surface of the brain and drives our movements. By revealing the complexity of its organization and the neural connections of the cortex, this work shows that “the action and control of the body are fused into a common circuit”, summarize the researchers at Washington University, in St. Louis, Missouri. A discovery which, according to them, “might help explain why mental states and body movements interact so often”.
This new map of the cortex leads to redrawing a strange creature lying on the surface of our brain (we host one in each hemisphere, lying on the primary motor cortex). A deformed little man, in fact. It features, from the temple to the middle of the brain, a hanging tongue, an enormous head, a monstrous mouth, oversized fingers and hand, a stunted arm, shoulder and trunk, and a spindly leg with a big foot.
A comprehensive view of the nervous system
This iconic character, the “Penfield homunculus”, appears in countless medical textbooks. It takes its name from the Canadian neurosurgeon Wilder Penfield (1891-1976), who drew a portrait of it from 1937. A feat at the time: in his brain-operated but conscious patients, he noted the movements induced by stimulation electricity of precise points of the cortex. He deduced that such a region controls the movement of the thumb, another of the hand, another of the toes, etc. If the tongue or the hands occupy disproportionate spaces, it is because their ultra-precise movements (to speak or grasp an object) mobilize more cerebral resources.
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