When performing aiming movements we are typically accurate even when features of a target or its context result in us perceiving the movement reuirements to be different to the actual requirements. However, some research suggests that our action system can be fooled. In the current study, 12 participants performed aiming movements with a stylus on a digitizer to round or elliptical targets, with or without a Ponzo Illusion as a background context. The movements were performed to remembered targets, with the target and background either disappearing immediately prior to the movement commencing, or after a 2000ms delay. In accordance with Fitts' Law, the participants decreased their movement time to larger targets. They also decreased their MT for targets in the illusion condition, suggesting that they perceived the target to be larger than it was (i.e. the Ponzo Illusion) and acted accordingly. Participants used greater movement amplitude in the illusion condition. This suggests in the illusion condition they perceived the targets to be further away than they were. The 2000ms delay did not change the pattern of results. It was concluded that the Ponzo Illusion resulted in the participants planning and performing movements according to the perceived rather than the actual target characteristics.
Longstaff, MG & Isaac, M 2011, 'The Ponzo Illusion affects movement characteristics in memory guided target aiming movements', in E Grassi & JL Contreras-Vidal (eds), Proceedings of the 15th International Graphonomics Society Conference, Hotel Live Aqua, Cancun, Mexico, 12-15 June, International Graphonomics Society, pp. 221-224.