Introducing the brain: examples of plasticity (Introduction)

by David Turell , Tuesday, March 24, 2020, 23:16 (1703 days ago) @ dhw

Research shows all animal brains show plasticity. This article describes some more of ours:

https://www.quantamagazine.org/the-brain-reshapes-our-malleable-senses-to-fit-the-world...

"One way to think of this is as nature’s way to prevent cortical idleness. If an area of cortex is no longer receiving inputs from its natural place, it would be wasteful for that area of cortex to be forever inactive. Instead, after a while, its function is given over to undamaged inputs. In the more general case, you can easily imagine this mechanism as a way of dealing with small strokes. (Neuropathologists tell us that we all incur these small losses of brain tissue during the course of our lives.) Imagine that you have a tiny cortical stroke, affecting only a very small blood vessel, and that the region of brain it feeds dies. It would be wasteful of precious cortical resources for areas of the brain that used to receive input from the region that is now damaged by the stroke to be forever silent.

"Instead, the brain makes the best of a bad situation by giving those brain areas over to their neighbor.

***

"The senses adapt to various types of neural damage, which are pretty crude events on the big scale of neural life. But there are also subtler reorganizations that occur naturally and happen to all of us.

"One of the striking indications of brain plasticity came from scanning the brain activity of people who had been blind from birth. When blind volunteers used their fingers to read Braille while in the scanner, the brain areas usually occupied by processing visual input — again, the primary visual cortex — were activated. Somehow, the processing of tactile information had taken over the unused visual center.

"Another dramatic example came from a study of violinists. To play the violin, you make large, relatively crude motions with one arm as the bow sweeps up and down across the strings. With the other hand you make a series of very subtle movements, depressing the strings at varying, tightly defined locations up and down the violin’s fingerboard — very quickly if you are a good violinist, astonishingly quickly if you’re a star. This is a remarkable task for the speed and precision it requires. Professional violinists practice these movements for hours each day.

"This has a consequence on the physical arrangement of the connections in their brains, because movements of the fingers are controlled by a specific brain area. In professional violinists, the area expands, even pushing aside functions from neighboring brain tissue. But this occurs only for the hand that fingers the strings. The same regions on the other side of the brain, which control the other hand, have no expansion because the required movements of that hand are relatively crude.

***

"The neuroscientist Donald Hebb predicted that vision is to a major extent learned. Complex perceptions are formed through experience, by association, because objects in the world occur in clusters of individual features. He believed that this had to happen early in life, before the brain became unable to form the necessary new assemblies. His basic idea was right: Much of vision does depend on visual experience. But his conclusion that this had to happen at a young age seems to be only partly true.

"The evidence comes from experiments in which individuals blind from birth were later given sight...n a brilliant combination of humanitarianism and science, Sinha organized a program to search for these children and transport them to New Delhi, where surgeons in a modern hospital replaced their lenses with clear synthetic ones — the same cataract operation carried out for many aging individuals.

***

"Yet their vision seems never to have become perfect. Their visual acuity remained below normal, even after months of training. One patient commented that he could read headlines in the newspaper but not the finest print. Some had trouble with specific visual tasks, such as separating two forms that overlap each other.

So it seems that much vision can be restored, but that the plasticity of the visual system is not limitless. Further evidence of this comes from the behavior of the cortical regions in primates’ inferior temporal lobe termed “face patches” because they respond only to faces as a visual stimulus.

Comment: It is best to learn vision from birth. The plasticity examples are fascinating, but the article is really about how vision is learned from birth.