Patterns were chosen to concentrate on form discrimination in
rigorous threshold procedures. They were pentagons pointing in
one of the four cardinal directions and thus differing only in the
form of their outline and not in area, contour length, or any other
major property (Fig. 1 upper). A second set required the binary
choice between a square and an octagon, with a difference again
only in shape and not area and very little in contour length
(Fig. 1, lower). Each run featured patterns belonging to only one
of these sets, but the selections within the set was always random.
Observers quickly learned to associate target direction or class
with computer keyboard arrow keys.