of neurons in behaving animals (Abeles 1991; Strangman et al., 1996; Edelman, 1987; Hebb, 1949; Palm, 1982). However, the recent rapid progress in large-scale recording of individual neurons in multiple brain regions (Buzsáki, 2004; Buzsáki et al., 1992; Eichenbaum and Davis, 1998; Nicolelis 1999; Wilson and McNaughton, 1993) and the initial attempts to track down and experimentally define putative cell assemblies (Harris et al., 2003; Harris, 2005; Truccolo et al., 2010) led to the recognition of another level of difficulties of a more conceptual nature.
