The upper left panel shows the full complement of boundary dots for one of the shapes to be identified. A sampling of these dots is shown in the upper right panel as filled circles, this being an example of a display set. To pick the display set for a given subject, the sampling began at a randomly selected starting point, shown by the arrow, and included every Nth dot, counting clockwise from this location. [See text for discussion of how N was determined for each shape.] The display set was then divided into subsets, one containing the odd dots from the counting process, and the other containing the even dots. These are shown in the lower left and right panels, respectively. The dots in each subset were displayed as a group, varying the time interval between each subset as a function of room illumination.