The surface-weight illusion: On the contribution of grip force to perceived heaviness

Previous psychophysical studies have shown that an object, lifted with a pr ecision grip, is perceived as being heavier when its surface is smooth than when it is rough. Three experiments were conducted to assess whether this surface-weight illusion increases with object weight, as a simple fusion mo del suggests. Experiment 1 verified that grip force increases more steeply with object weight for smooth objects than for rough ones. In Experiment 2, subjects rated the weight of smooth and rough objects. Smooth objects were judged to be heavier than rough ones; however this effect did not increase with object weight. Experiment 3 employed a different psychophysical metho d and replicated this additive effect, which argues strongly against the si mple fusion model. The whole pattern of results is consistent with a weight ed fusion model in which the sensation of grip force contributes only parti ally to the perceived heaviness of a lifted object.