The central issue in morphology of learning and memory is how such constructs are stored in the nervous system. The basic shape of neurons and synapses is illustrated in Figure 1. In the late nineteenth century, Santiago Ramón y Cajal (Spanish histologist, 1852-1934) suggested that learning might involve changes in the synaptic connections through which neurons communicate. Such synaptic change could take at least four possible forms. First, forming new synapses or removing existing synapses could alter the pattern of functional connections. Second, selectively strengthening or weakening some synapses could alter the pattern of functional connections. Very strong evidence exists for both of these possibilities during learning, and in models of learning such as long-term potentiation (LTP). Third, by forming new neurons (neurogenesis), neuronal circuits could be modified; there is increasing evidence that neuro-genesis occurs in the mature brain. Fourth, changes could occur in...