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. In addition, the results highlight the importance of analyzing illusion data using comparative difference scores and examining separate overestimation and underestimation components in each illusion.

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