effect because it further reduces the amount by which the tip of the iceberg (the membrane potential tuning curve) sticks out of the water surface (the spike threshold) thus further sharpening the tuning of the spike output of the neuron (Fig. 4). Importantly, this effect of inhibition occurs no matter whether inhibition is un-tuned or as equally tuned as stimulus-driven excitation. Indeed, the increased firing rates and reduced stimulus selectivity in visual cortex following pharmacological blockade of inhibition could be explained by a simple spike threshold model in which excitation and inhibition are identically tuned (Katzner et al., 2011). Second, recent studies in auditory (Wu et al., 2008), olfactory cortex (Poo and Isaacson, 2009) and visual cortex (Liu et al., 2011), but see (Tan et al., 2011) of the rodent, reveal that in these model systems the tuning curves of inhibition are actually broader than those of excitation in individual cells (Fig. 3B). As a consequence, non-preferred stimuli generate an excitation inhibition ratio that favors inhibition relative to the preferred stimulus. Here, inhibition contributes to sharpening the tuning not only by exacerbating the iceberg effect, but also by actually narrowing the iceberg (Fig. 3B).
