Within individual neurons the ratio between incoming excitation and inhibition can change rapidly, on a millisecond basis. In principal neurons of the auditory cortex, for example, brief tones lead to an increase in synaptic excitation that is followed within a couple of milliseconds by a surge in inhibition (Wehr and Zador, 2003; Wu et al., 2008). Similarly, whisker deflections lead to a rapid sequence of excitation followed by inhibition in neurons of the somatosensory "barrel" cortex (Fig. 3A) (Swadlow, 2002; Wilent and Contreras, 2005). Also in the visual cortex, visual stimulation with a light flash triggers excitatory and inhibitory conductances that are staggered by a few milliseconds (Liu et al., 2010). Hence, in these cortical areas, in response to impulse-like sensory stimuli, the ratio between excitation and inhibition is initially tilted towards excitation, and subsequently shifts towards inhibition. These rapid changes in the ratio between excitation and inhibition can have important consequences in tuning cortical neurons to specific stimuli and in shaping their activity pattern in time (see below).
