The rate at which the firing of a neuron increases in response to increasing excitatory input, i.e. the slope of the input-output relationship, is called gain and is a property that describes how neurons integrate incoming signals. This slope is not fixed but can be modulated, a phenomenon that goes under the name of gain control (Carvalho and Buonomano, 2009; Chance et al., 2002; Mitchell and Silver, 2003; Shu et al., 2003). Changes in gain are often referred to as multiplicative (or divisive) because for a pure change in slope the firing probability of the neuron is affected by the same factor across a wide range of inputs. Neurons in the visual cortex offer a classical example of gain modulation, where two independent properties of a visual stimulus, contrast and orientation, interact in a multiplicative manner in generating spike output (Anderson et al., 2000; Carandini and Heeger, 1994; Miller, 2003; Sclar and Freeman, 1982). Specifically, increasing the contrast of the stimulus increases the spike output of the neuron by a given factor, no matter what the orientation of the stimulus
