Finally, differences in excitation-inhibition ratio can also direct signal flow within and across cortical layers. Principal neurons in layer 2/3, for example, project their axons horizontally within their own layer, as well as vertically, towards layer 5. The activity of layer 2/3 principal neurons, however, generates an excitation-inhibition ratio that differs between layers: it favors inhibition within its own layer but is biased towards excitation in layer 5 (Adesnik and Scanziani, 2010).
