SUMMARY

Synapse-specific increases and decreases in synaptic strength that depend on the activity of the presynaptic and postsynaptic neuron are central to modern theories of how networks in the brain operate in setting up sensory representations of the world, in memory, and in producing appropriate motor responses. The goals of this chapter are to show how different features of these synaptic modifications are crucial to the operation of different types of network, and to the operation of several different brain systems. The types of network considered will be three that are fundamental to brain function, namely pattern associators, autoassociators, and competitive networks. Each performs a different type of operation for the brain. Then the ways in which these types of synaptic modification are implicated in the operation of the hippocampus and related cortical areas in memory (see the section The Primate Hippocampus), and the cerebral neocortex in visual object recognition (see the section Synaptic Modification Rules) and shortterm memory (see the section Short-Term Memory) will be described. The points made apply to any synapse-specific modification process in the brain, regardless of whether that process happens to be long-term potentiation/depression (LTP/LTD). More formal descriptions of the operation of some of the networks introduced here are provided by Rolls and Treves (1998), and by Hertz et al. (1991). Ways in which these architectures may be specified genetically are suggested by Rolls and Stringer (2000b).

Pattern Associators

A fundamental operation of most nervous systems is to learn to associate a first stimulus with a second that occurs at about the same time, and to retrieve the second stimulus when the first is presented.