Figure 9.1 Nerve cell from the hippocampus, reconstituted by confocal microscopy. (Courtesy of P. Adams.)

system works (Hinton and Anderson, 1981). How elaborate is it? Estimates place the number of neurons in the central nervous system at between 1010 and 1011; on average, said Koch, "each cell in the brain is connected with between 1,000 and 10,000 others." A good working estimate of the number of these connections, called synapses, is a hundred million million, or 1014.

Adams, a biophysicist, designs models and conducts experiments to explore the details of how the basic electrical currency of the brain is minted in each individual neuron (Figure 9.1). He has no qualms referring to it as a bottom-up approach, since it has become highly relevant to computational neuroscience ever since it became appreciated "that neurons do not function merely as simple logical units" or on-off switches as in a digital computer. Rather, he explained, a large number of biophysical mechanisms interact in a highly complex manner that can be thought of as computing a nonlinear function of a neuron's inputs. Furthermore, depending on the state of the cell, the same neuron can compute several quite different functions of its inputs. The "state" Adams referred to can be regulated both by chemical modulators impinging on the cell as well as by the cell's past history.

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