The Shallows: What the Internet Is Doing to Our Brains

by Nicholas Carr

The anatomical alterations in the slug’s relatively simple memory circuits were extensive. In one case, the researchers found that, before a long-term memory was consolidated, a particular sensory neuron had some thirteen hundred synaptic connections to about twenty-five other neurons. Only about forty percent of those connections were active—in other words, sending signals through the production of neurotransmitters. After the long-term memory had been formed, the number of synaptic connections had more than doubled, to about twenty-seven hundred, and the proportion that were active had increased from forty percent to sixty percent. The new synapses remained in place as long as the memory persisted. When the memory was allowed to fade—by discontinuing the repetition of the experience—the number of synapses eventually dropped to about fifteen hundred. The fact that, even after a memory is forgotten, the number of synapses remains a bit higher than it had been originally helps explain why it’s easier to learn something a second time.