Learning, memory and the brain.

Human and non-human animals acquire information about the world through the process of learning, and store that information as memory. Yet central as the storage process is to adaptive behaviour, progress in understanding its neural bases has been slow and only recently efforts have shown clear signs of being successful. The knowledge that comes from this progress strongly suggests that different kinds of learning involve different neural circuits and accordingly involve different memory systems. Indeed, it is becoming increasingly clear that multiple memory systems may be a fundamental part of the design of the vertebrate brain. It has long been supposed that learning leads to the formation, or to the strengthening of particular pathways in the brain. Once formed, or strengthened in this way a pathway was viewed as a 'trace' or 'engram' 'representing' the particular experience or relationship which had been learned. There is substantial evidence that neural pathways, especially synaptic connectivity, can be modified by experience-as by rearing rats in an 'enriched' environment with other rats rather than rearing them in isolation--as well as by modifying the diet or by depriving young rats of their thyroid gland. This evidence demonstrates that the central nervous system is plastic, but provides no hint that such plasticity is involved in learning. The evidence that synaptic plasticity is indeed involved in learning and memory is relatively recent.