Synaptic inhibition also helps in solving an important problem relating to dynamic range: how neuronal populations are recruited as the number of active excitatory afferents changes (Pouille et al., 2009; Shadlen and Newsome, 1998). The problem results from two basic connectivity properties of excitatory afferents in cortex; namely, high divergence (each afferent excites many neurons) and weak synapses (the activity of a single afferent is insufficient to depolarize a neuron above spike threshold). Because neurons need the concomitant activity of several afferents to reach spike threshold, yet these afferents diverge onto many neurons, small increases in the number of active excitatory afferents can lead to an explosive, almost all or none recruitment of the entire population. This strongly limits the range or combinations of afferent inputs that can be differentially represented by the firing of neuronal populations. With inhibition increasing concomitantly with the number of active afferents (for example through the progressive recruitment of feedforward inhibitory neurons), on the other hand, the recruitment of the neuronal population occurs in a progressive manner over a much wider range of inputs (Liu
