Bisecting the Ponzo illusion.

A modification of the Ponzo illusion, in which the judged lines are centered in different sized obliques, was presented to matched normal and mentally retarded males (ns = 8) under conditions of equal and unequal retinal sizes. Magnitude of illusion was affected by IQ (WISC) and by physical proximity rather than retinal contour. The results seem to imply a central rather than peripheral explanation for the illusion.