Pattern separation and pattern completion in the hippocampal system. Introduction to the Special Issue.

For a memory system such as the hippocampus in which asso-ciatively modifiable recurrent collateral connections between a population of neurons such as the CA3 pyramidal cells are believed, it is important that the memory representations to be stored are as orthogonal or uncorrelated as possible, to maximize the storage capacity. Only if the sets of neuronal firing, each representing a different memory, are relatively uncorrelated with each other will the number of memories that can be stored approach the theoretical limit for an autoassociation or attractor network (Rolls, 2012a; Treves & Rolls, 1991). In this situation, there is great interest in how the hip-pocampal system may perform pattern separation of the patterns to be stored in CA3 to make them relatively uncorrelated (Kesner & Rolls, 2015; Treves & Rolls, 1992). At the same time, an important property of a memory system is that it can complete a whole memory when just a partial cue for retrieval is provided. An example is the retrieval of a whole episo-dic memory from one part of it, for example who was present at a particular place and time (Rolls, 2010). Pattern completion is an inherent property of an autoassociation or attractor network such as the CA3 recurrent collateral system is believed to be (Rolls, 2008, 2016a), and there is also therefore great interest in the mechanisms for pattern completion in the hippocampal system. The concepts of pattern separation and pattern completion also apply to other systems with recurrent collateral associatively mod-ifiable connections, such as the local recurrent collateral networks that are a major architectural feature of the neocortex and olfac-tory cortex. There is therefore great interest in how pattern separation , pattern completion, and the concept of attractor networks may apply to these other cortical systems (Rolls, 2016a). In this context, the papers in this Special Issue consider the roles of pattern separation and pattern completion in many neural systems , including the hippocampal system illustrated in Fig. 1 with the forward projections from the cortex to the hippocampus (blue), and the backprojections from the hippocampus to the neocortex (green). Edmund Rolls provides some of the foundations, theoretical and empirical, for understanding pattern separation and pattern completion in the hippocampus and neocortex (Rolls, 2016b). Raymond Kesner shows how some of these concepts have been tested in the rat, with evidence from the effects of selective damage to different parts of the hippocampal system, and …