Synaptic excitation and inhibition are inseparable events. Even the simplest sensory stimulus, like a whisker deflection (Swadlow, 2003; Wilent and Contreras, 2005) a brief tone (Tan et al., 2004; Wehr and Zador, 2003; Wu et al., 2008), an odor (Poo and Isaacson, 2009) or an oriented bar in the visual field (Anderson et al., 2000; Monier et al., 2003) lead to the concomitant occurrence of synaptic excitation and inhibition in sensory cortices. This co-occurrence of excitation and inhibition is not limited to activity generated by sensory stimuli but holds true also for spontaneous cortical activity. During spontaneous cortical oscillations (Atallah and Scanziani, 2009) or ”up and down states” (Haider et al., 2006; Okun and Lampl, 2008), for example, excitation and inhibition wax and wane together.
