Manipulating the strength of the Ponzo and horizontal-vertical illusions through extraction of local cue information.

Previous research has shown that by placing nontargets around the endpoints of the shaft of the Mueller-Lyer illusion it is possible to markedly affect its normal strength. Using the same technique with the Ponzo illusion and the horizontal-vertical illusion, the current study demonstrated a similar ability to affect these other illusions of length. These findings provide varying amounts of support or challenges for different theories of illusion formation.