Differences in the timing of excitation and inhibition in response to impulse like stimuli are not the only way in which the ratio of these two opposing conductances is relevant for cortical processing. In some model sensory systems the ratio between excitation and inhibition in a given cortical neuron also depends on the property of the sensory stimulus, like its frequency (for auditory stimuli (Wu et al., 2008)), its position in space or orientation (for visual stimuli (Liu et al., 2011) but see (Tan et al., 2011)), or its chemical composition (for olfactory stimuli (Poo and Isaacson, 2009)). As will be described in more detail below, in these specific systems, sensory stimuli that are optimal for firing a cortical neuron (the "preferred" stimulus) generate an excitation-inhibition ratio that can be different than the ratio generated by sub-optimal stimuli. Thus, in some systems the excitation-inhibition ratio can contribute to shaping the response of a cortical neuron to distinct stimuli. As a consequence, because neighboring principal neurons in several cortical sensory areas are not necessarily tuned to the same stimuli (i.e. the
