(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 is. As a consequence, the stimulus selective output of a neuron for a particular orientation remains the same at each contrast. This illustrates that changes in gain, while modulating the responsiveness of a neuron to a stimulus, do not affect the representation of that stimulus in the cortex. Gain modulation in cortex is a very general phenomenon that is proposed to play a role at every level of sensory processing, including modulation of visual responses by gaze direction (Andersen and Mountcastle, 1983) and attention (Williford and Maunsell, 2006).
