Task-induced steering of system linearity.

Symbols plot linearity index (see Methods) at two higher (y axis) versus two lower (x axis) contrast levels separately for each observer and SNR value, from energy-extraction (black) and polarity-discrimination (red) tasks in A, and from peripheral (red) versus foveal (black) conditions in B (both conditions involved the same bright-bar detection task). Error bars are omitted to avoid clutter. Contour lines indicate data spread (from thick to thin, lines refer to 4 different percentage heights of 99/95/70/30 along the surface detailed in the caption to Fig 2). Bottom and right (left in B) histograms plot distributions of x and y values; top-left (bottom-right in B) histograms plot distributions for values projected onto positive diagonal line. Orange arrows point from average coordinates of black data cloud to average coordinates of red data cloud. Red cloud is shifted upwards to the right of black cloud in A, indicating greater linearity at both high and low contrast. Red cloud is shifted upwards to the left of black cloud in B, indicating no change in overall linearity between the two conditions, but a greater effect of contrast in the periphery compared with the fovea.