The Atheist's Guide to Reality_ Enjoying Life Without Illusions - Alex Rosenberg [84]
Twenty years after he figured out learning and habit formation in the sea slug and won the Nobel Prize for it, Kandel turned his attention to how the mammalian brain learns and stores information. His model was the rat. In his new experiments, the rat had to learn where the life raft was in a deep-water tank. Kandel showed that the rat acquires and stores the location of the raft in the tank via the same changes in the neurons of its brain as those by which the sea slug learns and remembers its new habit. Of course, in the sea slug, the changes take place in a few neurons, and in the rat they take place in thousands. But in each animal, what happens is pretty much the same. The process involves the same neurotransmitters; the same calcium, sodium, chloride, and potassium molecules; and the same genes switching on and off in the neuron’s nucleus to build synapses of the same structure and composition. Just many more of them.
Sea slugs learn to do something and rats learn the location of the life raft. What they learn and how they learn it differ only by degree. One involves changing a few neural connections; the other involves changing many neural connections. But the neurons all change in the same way. What may look like the rat’s learning some fact about the world—where the life raft is in the tank—turns out to be just the rat’s acquiring a new set of neural circuits. The part of its brain that first stores information—the hippocampus—does so by rearranging input/output circuits that now respond to input stimuli in a new way.
The next step in the research program that started with sea slug learning was to figure out what happens to the neurons in the human hippocampus. This is the part of the brain that, as in the rat’s brain, first stores information that usually gets expressed as thoughts about stuff—like where and when you were born, what your mom looks like, and what the capital of France is. Surprise, surprise! What goes on in the human hippocampus is the same as what goes on, neuron by neuron, between the neurons in the rat’s hippocampus when it stores the location of the life raft in the tank. It’s also the same as what goes on among the sea slug’s neurons when they store its response to a mild electrical stimulus. All three involve the same changes in existing synapses and the same somatic gene expression to build new synapses. The big difference is one of degree, the number of neurons involved. In the human hippocampus (and the rest of the cortex, too), there are vastly more neurons to be changed, even more than in the rat hippocampus. But the basic process is the same.
Neuroscientists have already begun to discover how information is distributed within large sets of neuron circuits in our brain. The large sets are composed of vast numbers of small sets of neurons with extremely specialized abilities. These small sets of neural circuits have specific response patterns because they are very highly tuned input/output circuits. Here is a typical example of such a circuit: By the time your brain has fully developed, there are actually a set of neurons whose synapses have been wired together so that the only thing they do is respond to the visual input of your mother’s face, with some neural output that leads to mother-recognizing behavior like your saying “Hi, Mom” when you see her face.
How can neuroscientists know this? Simply because they can knock out those neurons temporarily with strong but localized magnetic fields, and the only thing that will change is that you won’t recognize your mother’s face. You will recognize her voice on the phone, her smell, her handwriting. But when