Figure 1.

Illustrating an Impossible maze trial. Counting from the left, the 21^st gate or wicket has narrowed, creating a constriction point that cannot be negotiated by the red cursor that the subject controlled. Impossible maze trials required a trial-decline response (?) to fend off a long penalty timeout. In contrast, on Possible maze trials, all 26 wickets were generously wide for passage by the cursor to the blue-circle goal position.