Spatial histograms of activity at different amplitudes. Absorbances were measured at each amplitude along the red line shown in the OIS image at top left. Each plotted value represents an average of pixels spanning 100 μm above and below the line and a distance of 40 μm along the line (bin size was 40 × 200 μm). The control condition is plotted on each graph to indicate "background" levels absorbance. Dashed lines on plots indicate where stimulus-evoked activity crosses background absorbance levels (indicating the boundaries of above background absorbance). Histograms indicate no significant change in cortical territory displaying above background absorbance with respect to stimulus amplitude. Higher amplitude stimulation produces regions of below-background absorbance directly outside of the regions of above-background absorbance.