Inhibition in the cortex is generated by neurons that release the transmitter GABA. These neurons comprise approximately 20% of the cortical neuronal population (Meinecke and Peters, 1987) and, in contrast to their counterpart, the excitatory glutamatergic principal cells, don't generally form long range projections with their axon; hence the name local circuit interneurons. The interactions between GABAergic interneurons and glutamatergic principal cells are reciprocal: interneurons inhibit principal cells and are excited by them. In fact the connectivity between these two neuronal classes is quite high: individual interneurons can inhibit >50% of principal cells located within ~100 µm and receive excitatory input from a large fraction of them (Ali et al., 1999; Fino and Yuste, 2011; Glickfeld et al., 2008; Holmgren et al., 2003; Kapfer et al., 2007; Packer and Yuste, 2011; Silberberg and Markram, 2007; Stokes and Isaacson, 2010; Yoshimura and Callaway, 2005). Thus, not only are GABAergic interneurons excited in proportion to the level of local network activity, but they directly influence it through their inhibitory feedback. This simple connectivity pattern is ubiquitous in cortex and forms the basis
