Segregation of excitatory principal cells into functional groups is made possible by inhibition, and this grouping-parsing function is perhaps the most fundamental task performed by the large family of interneuronal classes in the cortex (Freund and Buzsáki, 1996; Klausberger and Somogyi, 2008). As an illustration, consider a ring of excitatory neurons with just one inhibitory interneuron in the middle, reciprocally connected to the excitatory cells (Figure 9A). An external input to any of the neurons may activate a subset of the ring neurons, while silencing others. The interneuron-guided grouping (i.e., formation of a candidate assembly) depends on the location of the input in the ring and, critically, on the fine details of synaptic strengths (i.e., the structure of the synapsemble). With different initial conditions, the interneuron can be ‘enslaved’ to different constellations of excitatory neurons. This example also shows that there is a temporally exquisite relationship between the active assembly, interneurons, and the silenced population. The assembly forming/segregating ability of interneurons may be due to the efficient synapses formed between pyramidal cells and interneurons (Csicsvari et al., 1998; Galarreta and
