Both feedforward and feedback inhibitory circuits can generate these rapid sequences of excitation and inhibition. In feedforward circuits, since afferent inputs contact both principal cells and interneurons, the onset of excitation recorded in principal neurons will precede the onset of inhibition by a monosynaptic delay (that can be as brief as one ms) (Gabernet et al., 2005; Pouille and Scanziani, 2001; Stokes and Isaacson, 2010). Feedback circuits also provide inhibition that follows excitation because the firing of local principal neurons will be followed by the recruitment of GABAergic interneurons.
