Figure 1. Vernier Stimuli in Position Noise.

The Vernier stimulus was comprised of two horizontal segments separated by a small gap (N0). Each segment consisted of ten discrete white rectangular dots against a black background. Noise was introduced by manipulating the position of each dot vertically with a Gaussian probability function; its standard deviation determined the vertical spread of the dots (N2–N8). The visual task was to identify the misaligned pair among the three choices (upper, middle or lower). In the figure, all the lower pairs shown are misaligned - the right segments are higher than those on the left.