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There are currently three primary models of how neurons function, each with its uses and variations, according to James McClelland, a professor of psychology and computer science at Carnegie Mellon University and codirector of the Center for Neural Basis of Cognition. The first and simplest is the integrate-and-fire model, which is based on the idea that the neuron adds and subtracts excitatory and inhibitory inputs until it reaches a threshold, at which point it fires a single impulse or action potential. Another model is the sigmoid transfer function, in which the neuron adds up excitatory and inhibitory inputs (as in the integrate-and-fire model) but treats the output as a continuous quantity. Finally, in the sigma-pi unit model, a neuron's output is equal to the sum of many products, each consisting of a multiplication of several inputs.