Do neuronal assemblies more resemble quartets, chamber orchestras or large philharmonic orchestras? In Hebb’s cell assemblies, membership is defined by connectedness through excitatory synapses (Figure 1A). However, as discussed above, neither a sufficient nor total number of assembly members can be determined without knowing the timeframe and the goal. Since the reader-centric definition of the assembly depends on classifier mechanisms, the question of assembly size should also be approached from this perspective. As discussed above (Figure 2), if the goal of an assembly is to discharge a downstream pyramidal cell in vivo, the number of neurons whose spikes can be integrated in approximately 20 msec (i.e., one gamma cycle) can quantitatively define the size of the effective assembly. Since approximately 1% of hippocampal pyramidal cells fire in a 20-msec time window during theta-related behaviors (Csicsvari et al., 1998; 1999), and 15,000 to 30,000 CA3 pyramidal cells converge on a CA1 pyramidal neuron (Li et al., 1994; Megias et al., 2001), these relationships indicate that, on average, 150 to 300 CA3 pyramidal cells firing within a gamma cycle comprise an assembly
