Response confidence and the Ponzo illusion.
The number of longer line judgments and the ratings of confidence in these judgements were significantly higher when the comparison line was located near the apex of the Ponzo wedge but only confidence ratings were affected by the orientation of the wedge (ns = 24 men, 24 women). Differences between the two measures suggest that perceptual and nonperceptual processes may be involved in the illusion.
Diaz, L. F. and Delay, E. R., "Response confidence and the Ponzo illusion." (1992). Regis University Faculty Publications. 1219.