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 (de Almeida et al., 2010); a number similar to the estimate in HVC of the zebra finch (Hahnloser et al., 2002). Under special conditions, when the inputs converge on the same dendritic branch and fire synchronously in <6 msec, as few as 20 neurons may be sufficient to initiate a forward-propagating dendritic spike (Losonczy and Magee, 2006). These conditions may be present in the hippocampus during sharp wave ripples (Csicsvari et al., 2000) and in the geniculo-cortical system during visual transmission (Wang et al., 2010).
