The reader-centric definition of the cell assembly differs from representation-based descriptions (Abeles 1991; Braitenberg and Schuz, 1991; Gernstein et al., 1989; Hebb, 1949; Hopfield and Tank, 1986; Palm 1982; Wickelgren, 1999) in some key aspects. Hebb’s cell assembly is essentially a graph of synaptically interconnected excitatory neurons (Abeles, 1991; Hopfield and Tank, 1986; Palm, 1982; 1987; Wennekers et al., 2003). However, unless the active neurons produce an interpretable output, connectedness is not sufficient to define an assembly. For the reader-centric definition of the assembly, direct excitatory connections among assembly members are optional but not obligatory because what matters is that neurons of an upstream assembly fire within the integrating time window of the reader mechanism (Fig. 1C). For example, in a prominent model of assembly sequences (‘synfire chain’) what matters is that at least one neuron in the target layer responds to the inputs from the upstream layer, irrespective of whether neurons in the upstream layer are strongly connected or not (Abeles, 1991). Naturally, if the transiently formed assembly members are interconnected anatomically, their co-activation can strengthen their membership and
