Chapters 6 and 7 describe the mechanisms underlying induction and expression of plasticity in the barrel cortex. In using the term plasticity, synaptic plasticity is meant implicitly, including not only the second-to-second changes that ensue during in vitro forms of plasticity such as LTP and long-term depression (LTD) but also the growth of pre- and postsynaptic elements of the synapse that occur over longer time scales. This chapter deals with the cellular evidence on the nature of synaptic plasticity mechanisms, while Chapter 7 deals with the systems level evidence for plasticity induced in the whole animal. The extent to which all plasticity mechanisms in the cortex are underpinned by synaptic plasticity is, of course, a subject of much debate and an issue we take up in the following pages.

Historically, synaptic plasticity has been studied extensively in hippocampus, visual cortex and barrel cortex. In some respects, plasticity studies in barrel cortex have caught up with those parallel investigations in the visual system and in some areas overtaken them. There are a number of reasons why this has happened, including the ease of manipulation of the peripheral receptors in the whisker system, the introduction of transgenics and knockouts in rodents and the invention of the thalamocortical slice preparation (Agmon and Connors, 1991). Although synaptic plasticity is not as well understood in the barrel cortex as in the hippocampus, even at this early stage it is clear that some differences exist between the two systems.