Introducing the shape-length illusion

The size of an object can be perceived by pinching it between thumb and index finger. Here we show a new haptic illusion in which the length of objects with large local curvature is overestimated. Participants pinched ellipses between the thumb and the index finger and matched the size of the major axis to a set of rectangles which carry no curvature information. Ellipses with high local curvature at the contact areas were consistently rated to be larger than those with smaller curvature. This illusion shows that local shape information can influence judgment the size of an object.