The timing of sensory-evoked inhibition relative to excitation is another factor that could sharpen the tuning of cortical neurons to preferred stimuli. As mentioned above, studies in auditory (Wehr and Zador, 2003; Wu et al., 2006), somatosensory (Wilent and Contreras, 2005), and visual cortex (Liu et al., 2010) indicate that, in response to impulse like stimuli, inhibition follows excitatory input with a brief (few ms) temporal delay (Fig. 3A, Fig. 5). This slight lag between excitation and inhibition enforces a brief window of opportunity for the integration of synaptic excitation and subsequent spike output (Fig. 5) thus making principal cells precise coincidence detectors of afferent input (Luna and Schoppa, 2008; Mittmann et al., 2005; Pouille and Scanziani, 2001). Some experimental observations suggest that the relative timing of excitatory and inhibitory synaptic input contributes to stimulus-selective firing. For example, in response to preferred directions of whisker deflection, excitation precedes inhibition in barrel cortex but the temporal delay between the two synaptic conductances is reduced in response to nonpreferred stimuli (Wilent and Contreras, 2005). Similarly, in neurons of auditory cortex that are
