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By Root 1196 0

taking place (as thought occurs) in the “wetware”—the hundred billion or more neurons that make up the human brain. Cognitive neuroscientists—some were neurobiologists who had studied psychology, others were psychologists who had studied neurobiology—were interested in such phenomena as the flow of sodium and other ions into and out of the axon (main stem) of a neuron as electrical impulses pass along it; the molecular structure of the neurotransmitters (the chemicals produced in the synapses, the junctions at which the impulses are passed to other neurons); the bursts of neurotransmitter molecules that leap across the microscopic synaptic gaps from neuron to neuron carrying messages of excitation or inhibition; and the neuronal routes and networks activated by different kinds of stimuli and mental activities.

Behavioral neuroscientists (as they were then known), often white-coated, spent much of their time in operating rooms and laboratories, where, among other things, they surgically destroyed specific portions of animals’ brains to learn what aspects of behavior those parts control; they interviewed and tested people who had suffered brain damage; they measured and recorded the spikes of activity of single neurons and the overall patterns of brain excitation (“brain waves”) during various mental activities; they administered drugs that increase or decrease the production of particular neurotransmitters to determine what functions these perform; and they did chemical analyses of the brain tissue of laboratory animals and human cadavers to see what neurotransmitters were in either short supply or excess in individuals whose behavior is abnormal in some respect.

A good deal of their work, as we have already seen, involved testing patients with cerebral damage (most often strokes), pinpointing the affected brain area and identifying it as the cause of the patient’s diminished or lost perceptual and mental abilities. But much other neuroscientific research, though arguably valuable, had its comical overtones. One investigator implanted sixteen microelectrodes into the muscles of a male grasshopper in order to record the electrical impulses of its neurons during courtship. Others inserted microelectrodes into the left front leg of a cockroach and the foot of a snail to measure the neural impulses that produce movement toward some goal; the investigators regarded this as research on “motivated behavior.”14

Of all cognitive processes, especially in more advanced species, memory is the most basic, and for decades cognitive neuroscientists sought to identify how and where memory exists at the cellular level. A few examples of the ways in which they did so:

—As long ago as 1949, Donald Hebb, a Canadian psychologist, hypothesized that memories are stored by the modification of the synapses connecting neurons (an idea not unlike Descartes’s). The repeated activation of a synapse in a learning experience, he said, somehow strengthens the synapse and links the two neurons into a circuit or “memory trace.”15 Hebb’s hypothesis was more or less confirmed in 1973 when a British neurophysiologist, Timothy Bliss, and a colleague, Terje Lømo, measured the voltage in one neural pathway in the brain of a rabbit, then sent repeated bursts of electricity down the path, and afterward found that the pathway carried a higher voltage than before. The synapses had been strengthened by the electrical impulses. The implication was that that is what happens in learning.16 (Later research, as we will see, has added many details and complexities to the explanation.) —Also in the early 1970s, an American psychologist, William Greenough, raised rats in two environments, one containing toys, mazes, and other stimulating devices, the other without any. The rats in the stimulating environment developed heavier areas of cerebral cortex; the neurons in those areas had grown more dendrites and thus more synapses than those of rats in the dull environment. Later, by means of electron microscopy, Greenough and a colleague actually counted 20 to 25 percent more