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. Author manuscript; available in PMC: 2019 Apr 1.
Published in final edited form as: Psychon Bull Rev. 2018 Apr;25(2):827–845. doi: 10.3758/s13423-017-1368-7

Figure 4. Pilot study results (N = 45).

Figure 4

The Ponzo illusion influenced both perception and grasping, and these effects were correlated, but only grasping seemed to become resistant to the illusion over 20 trials.

Center: We found significant illusion effects on both manual size estimates (ΔMSE) and maximum grip apertures (ΔMGA).

Left: Effects on perception and on grasping were positively correlated. Each dot is a participant. Participants who reliably reported the illusion via MSEs also tended to grasp according to the illusory sizes.

Right: Evolution of ΔMSE and ΔMGA over twenty trials, shown alongside average results from the individual model fits.

Top-right: The perceptual effect appears to have increased over the course of the experiment, perhaps because participants exaggerated or categorized their MSE responses with repeated trials. The model accurately estimated the mean perceptual effect from the MGA data.

Bottom-right: Grasping was influenced by the illusion in the early trials but this effect gradually diminished over time (n.s.). The model, which was fit to each participant’s MGA time series by estimating size distortion and error correction rate parameters, roughly captures this pattern.