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 for so-called feedback or recurrent inhibition (Fig. 1A). Of course, not all cortical excitation received by inhibitory interneurons is locally generated. Cortical cells receive excitatory inputs via long-range axons originating from subcortical nuclei, as well as from different cortical regions and different cortical layers. These excitatory afferent inputs diverge onto both principal cells and interneurons, generating feedforward inhibitory circuits (Fig. 1B) (Buzsaki, 1984). Interestingly, the same afferent fibers make stronger excitatory connections onto interneurons than principal cells ensuring that even minimal levels of afferent input generate inhibition in cortical circuits (Cruikshank et al., 2007; Gabernet et al., 2005; Glickfeld and Scanziani, 2006; Helmstaedter et al., 2008; Hull et al., 2009; Stokes and Isaacson, 2010). Together, these two simple inhibitory circuits, feedback and feedforward, represent fundamental building blocks of cortical architecture and account for the fact that cortical excitation and inhibition are inseparable (van Vreeswijk and Sompolinsky, 1996). GABAergic interneurons will be recruited no matter whether excitation is generated locally or
