Through the recruitment of interneurons via feedforward and/or feedback excitatory projections, inhibition generated in cortical networks is somehow proportional to local and/or incoming excitation. This proportionality has been observed in several sensory cortical regions where changes in the intensity or other features of a sensory stimulus lead to concomitant changes in the strength of both cortical excitation and inhibition (Fig. 2A) (Anderson et al., 2000; Poo and Isaacson, 2009; Wehr and Zador, 2003; Wilent and Contreras, 2004; Zhang et al., 2003). In addition, during spontaneous cortical activity, increases in excitation are invariably accompanied by increases in inhibition (Fig. 2B) (Atallah and Scanziani, 2009; Haider et al., 2006; Okun and Lampl, 2008). Furthermore, acute experimental manipulations selectively decreasing either inhibition or excitation shift cortical activity to a hyper-excitable (epileptiform) or silent (comatose) state (Dudek and Sutula, 2007). Thus, not only does excitation and inhibition increase and decrease together during physiological cortical activity (van Vreeswijk and Sompolinsky, 1996), but interference of this relationship appears to be highly disruptive. Highlighting the importance of a proper relationship between excitation and inhibition, is the fact
