Figure 1.

A schematic illustration of the sequence of events. Each block of trials began with a diagonal arrow that indicated the locations the participant was expected to attend. The color of this arrow was either green (indicating a block of easy judgments) or red (indicating a series of difficult judgments, as shown here). Subsequently, the participant observed a series of blue diamonds. The participant was requested to make a decision regarding whether the top or bottom gap was larger by pressing an upper or lower button. Note that each diamond included two pairs of gaps: an easy pair and a difficult pair, so the visual stimuli were identical on the easy and difficult blocks, with only the initial diagonal arrow indicating whether the participant was comparing the easy or difficult pair. Occasionally, a bright fuchsia-colored circle interrupted the stream of diamonds. The participant was asked to ignore these stimuli.