Size-Mass Illusion Figure 3. Heaviness ratings and peak velocities of the objects. Ratings and velocities are shown for each trial with (A) and without (B) visual feedback of the objects’ trajectories. The lines represent polynomials that were fitted to the data solely as a guide for the eye. doi:10.1371/journal.pone.0042518.g003 however, perceived differently through inertial forces than through gravitational forces [15]. An explanation is that the perceptual system treats gravitational and inertial mass differently and thereby violates the equivalence principle [16]. Here we have shown that this violation is limited as the size–weight illusion is based on a prior for mass, regardless whether mass is perceived through gravitation or inertia. Therefore, we conclude that the References 1. Charpentier A (1891) Analyse experimentale: De quelques elements de la sensation de poids. Archives de Physiologie Normale et Pathologique 3: 122– 135. 2. Ellis RR, Lederman SJ (1993) The role of haptic versus visual volume cues in the size weight illusion. Perception & Psychophysics 53: 315–324. 3. Buckingham G, Goodale MA (2010) Lifting without Seeing: The Role of Vision in Perceiving and Acting upon the Size Weight Illusion. PLOS ONE 5 e9709. 4. Ellis R, Lederman S (1999) The material-weight illusion revisited. Perception & Psychophysics 61: 1564–1576. 5. Buckingham G, Cant JS, Goodale MA (2009) Living in A Material World: How Visual Cues to Material Properties Affect the Way That We Lift Objects and Perceive Their Weight. Journal of Neurophysiology 102: 3111–3118. 6. Flanagan JR, Bittner JP, Johansson RS (2008) Experience Can Change Distinct Size-Weight Priors Engaged in Lifting Objects and Judging their Weights. Current Biology 8: 1742–1747. 7. Ernst MO, Banks MS (2002) Humans integrate visual and haptic information in a statistically optimal fashion. Nature 415: 429–433. 8. Ernst MO (2007) Learning to integrate arbitrary signals from vision and touch. Journal of Vision 7. 9. Weiss Y, Simoncelli E, Adelson E (2002) Motion illusions as optimal percepts. Nature Neuroscience 5: 598–604. illusion is based on a prior for the mass of an object, not a prior for forces acting on the object. Author Contributions Conceived and designed the experiments: MAP JBJS. Performed the experiments: MAP. Analyzed the data: MAP. Wrote the paper: MAP. 10. Brayanov JB, Smith MA (2010) Bayesian and ‘‘Anti-Bayesian’’ Biases in Sensory Integration for Action and Perception in the Size-Weight Illusion. Journal of Neurophysiology 103: 1518–1531. 11. Gordon AM, Forssberg H, Johansson RS, Westling G (1991) Visual force cues in the programming of manipulative forces during precision grip. Experimental Brain Research 83: 477–482. 12. Flanagan JR, Beltzner MA (2000) Independence of perceptual and sensorimotor predictions in the size-weight illusion. Nature Neuroscience 3: 737–741. 13. Grandy M, Westwood D (2006) Opposite perceptual and sensorimotor responses to a size-weight illusion. Journal of Neurophysiology 95: 3887–3892. 14. Ross HE, Brodie EE, Benson AJ (1984) Mass discrimination during prolonged weightlessness. Science 225: 219–221. 15. Ross HE, Brodie EE (1987) Weber fractions for weight and mass as a function of stimulus-intensity. Quarterly Journal of Experimental Psychology 39: 77–88. 16. Bergmann Tiest WM, Kappers AML (2010) Haptic perception of gravitational and inertial mass. Attention, Perception & Psychophysics 72: 1144–1154. 17. Zwislocki JJ, Goodman DA (1980) Absolute scaling of sensory magnitudes - a validation. Perception & Psychophysics 28: 28–38. 18. Buckingham G, Goodale MA (2010) The influence of competing perceptual and motor priors in the context of the size-weight illusion. Experimental Brain Research 205: 283–288. PLoS ONE | www.plosone.org 5 August 2012 | Volume 7 | Issue 8 | e42518