Figure 1

<p>The six displays used in the experiments. Display (a) is the Hermann grid display <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000265#pone.0000265-Hermann1" target="_blank">[1]</a>. Most observers see illusory dark gray smudges at the grid intersections. The illusion is strongest when viewed on a computer monitor. Displays (b) and (c) are based on displays that were presented at the European Conference on Visual Perception (Geier, Sera, Bernath, 2005, <i>Perception</i> 34, supplement 54).