How then could inhibition sharpen the tuning of cortical neurons to the preferred stimuli? This can happen in several ways. First, it is important to note that the tuning curve determined through the spike output of a neuron is not equal to the tuning curve determined by recording the membrane potential of that neuron. Because only the strongest excitatory input received by a neuron sufficiently depolarizes the membrane to reach threshold for spike generation, (i.e. the "tip" of the tuning curve of the membrane potential), the spike output of the neuron is more sharply tuned than the underlying membrane potential (Fig. 4), a phenomenon appropriately called "iceberg effect" (Carandini and Ferster, 2000; Rose and Blakemore, 1974). In other words, the non-linearity of spike rate versus membrane potential sharpens the tuning of a neuron. The addition of inhibition exacerbates the iceberg 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
