Brain Plasticity

From Nicholas Carr’s The Shallows: What The Internet Is Doing To Our Brains, a segment on how our experiences shape our brain:

One of the simplest yet most powerful demonstrations of how synaptic connections change came in a series of experiments that the biologist Eric Kandel performed in the early 1970s on a type of large sea slug called Aplysia (Sea creatures make particularly good subjects for neurological tests because they tend to have simple nervous systems and large nerve cells.)  Kandel, who would earn a Nobel Prize for his work, found that if you touch a slug’s gill, even very lightly, the gill will immediately and reflexively recoil. But if you touch the gill repeatedly, without causing any harm to the animal, the recoiling instinct will steadily diminish. The slug will become habituated to the touch and learn to ignore it. By monitoring slugs’ nervous systems, Kandel discovered that “this learned change in behavior was paralleled by a progressive weakening of the synaptic connections” between the sensory neurons that “feel” the touch and the motor neurons that tell the gill to retract. In a slug’s ordinary state, about ninety percent of the sensory neurons in its gill have connections to motor neurons. But after its gill is touched just forty times, only ten percent of the sensory cells maintain links to the motor cells. The research “showed dramatically,” Kandel wrote, that “synapses can undergo large and enduring changes in strength after only a relatively small amount of training.”

The plasticity of our synapses brings into harmony two philosophies of mind that have for centuries stood in conflict: empiricism and rationalism. In the view of empiricists, like John Locke, the mind we are born with is a blank slate, a “tabula rasa.” What we know comes entirely through our experiences, through what we learn as we live. To put into more familiar terms, we are products of nature, not nurture. In the view of rationalists, like Immanuel Kant, we are born with built-in mental “templates” that determine how we perceive and make sense of the world. All our experiences are filtered through these inborn templates. Nature predominates.

The Aplysia experiments revealed, as Kandel reports, “that both views had merit—in fact they complemented each other.” Our genes “specify” many of “the connections among neurons—that is, which neurons form synaptic connections with which other neurons and when.” Those genetically determined connections form Kant’s innate templates, the basic architecture of the brain. But our experiences regulate the strength, or “long-term effectiveness,” of the connections, allowing as Locke had argued, the ongoing reshaping of the mind and “the expression of new patterns of behavior.” The opposing philosophies of the empiricist and the rationalist find their common ground in the synapse. The New York University neuroscientist Joseph LeDoux explains in his book Synaptic Self that nature and nurture “actually speak the same language. They both ultimately achieve their mental and behavioral effects by shaping the synaptic organization of the brain.”

The brain is not the machine we once thought it to be. Through different regions are associated with different mental functions, the cellular components do not form permanent structures or play rigid roles. They’re flexible. They change with experience, circumstance, and need. Some of the most extensive and remarkable changes take place in response to damage to the nervous system. Experiments show, for instance, that if a person is struck blind, the part of the brain that had been dedicated to processing visual stimuli—the visual cortex—doesn’t just go dark. It is quickly taken over by circuits used for audio processing. And if the person learns to read Braille, the visual cortex will be redeployed for processing information delivered through the sense of touch. “Neurons seem to ‘want’ to receive input,” explains Nancy Kanwisher of MIT’s McGovern Institute for Brain Research: “When their visual input disappears, they start responding to the next best thing.” Thanks to the ready adaptability of neurons, the senses of hearing and touch can grow sharper to mitigate the effects of the loss of sight. Similar alterations happen in the brains of people who go deaf: their other senses strengthen to help make up for the loss of hearing. The area in the brain that processes peripheral vision, for example, grows larger, enabling them to see what they once would have heard.


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