Developmental susceptibility to the horizontal-vertical illusion

Developmental susceptibility to four forms of the horizontal-vertical (HV) illusion was examined in subjects ranging in age from 3 to 20 years. For each form of the HV illusion, one standard and six comparison stimuli were developed, with subjects first inspecting the standard and then selecting the matching comparator. The magnitude of the illusion, estimated from the difference between the size of the standard and that of the selected comparator, was found to be similar to values reported previously and determined through the use of a wide range of stimuli, procedures, and subject groups. Susceptibility to the HV illusion was greatest for the youngest subjects. It decreased for older subjects in accordance with the development of spatial coordinates, as suggested by Piaget.