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Fig. 8.3. Effect of afferent normalization on V1 neuron tuning. The differences in V1 population activities shown in Figure 8.2 are due to changes in how individual neurons respond at different contrasts. These plots show orientation tuning curves of the neuron at the center of the cortex, which prefers stimuli oriented at 60^o. Each curve shows the peak settled responses of this neuron to sine gratings whose orientations are indicated in the x-axis and contrast specified in the legend at right. In each case, the sine grating phase was used that resulted in the largest response. (a) Without afferent normalization, the neuron becomes less selective for orientation as contrast increases. Given enough contrast (above 50%), the neuron responds at full strength to inputs of all orientations, and thus no longer provides information about the input orientation. (b) With normalization, the tuning curve is the same over a wide range of contrasts, allowing the neuron to respond only to inputs that match its orientation preference. The curves are similar at 20% contrast (solid line), but the neuron now responds selectively to other contrasts as well. Afferent normalization is therefore crucial for preserving orientation selectivity over a wide range of contrasts.