Despite the expected critical role of synapsembles in neural syntax, experimental evidence supporting the role of synapsembles in combining and separating neuronal assemblies is scarce, largely because of the lack of tools to directly measure synaptic connectivity in the behaving animal. An indirect measure of short-term plasticity can be obtained by examining the fine-timescale spike transmission probabilities between simultaneously recorded neurons (Baeg et al., 2007; Constantinidis and Goldman-Rakic, 2002; Fujisawa et al., 2008; Hirabayashi and Miyashita; 2005). Even with this indirect method, only connections between principal cells and interneurons can be studied reliably with current methods (Fig. 8A). As Figure 8 illustrates, synaptic efficacy (defined operationally as the magnitude of excess coincidental spikes at <3 msec latencies between the pre- and postsynaptic neuron; Fujisawa et al., 2008) between connected pairs is not constant but varies both as a function of the animal’s position in the maze (Fig. 8B) and as a function of left versus right trajectories (Fig. 8C). Remarkably, the temporal span of the effective spike transmission between pyramidal cell-interneuron pairs is comparable to the activity lifetime of the
