Haptic Illusions: Biases in the perception of volume, weight and roughness

The present thesis investigated the perception of volume, weight and roughness when exploring 3-dimensional objects by touch and/of vision, and examined whether these percepts were influenced by specific object properties (e.g shape, material). In perception research, the term bias has been used to indicate the occurrence of these systematic influences. Our experiments were conducted with healthy subjects, who were blindfolded during haptic conditions (i.e. exploration by touch only). Strong influences were found of the objects’ shape on the volume judgment of small objects during unimanual haptic conditions, visual and bimodal (i.e. both vision and touch) conditions, as well as on the bimanual haptic volume judgment of large objects. The direction of these biases was the same; a tetrahedron was perceived as larger in volume than a cube or a sphere of the same physical volume, and a cube was perceived as larger than a sphere. Our analyses suggested that the volume was not perceived directly but was based on other object dimensions that were salient during the exploration. For example, the haptic and bimodal biases for the small objects could be explained by assuming the volume judgment is based on the objects’ total surface area. In addition, the haptic volume percept was influenced also by the objects’ material properties. A cube with a smooth surface was perceived as larger than an equally sized cube with a rough surface, and a cube with a larger thermal conductivity was perceived as larger than a cube with a smaller thermal conductivity. The accuracy of the haptic system to discriminate the volume of same-shaped 3-D objects was also investigated. The results showed that subjects could discriminate objects with a volume difference of at least 11 %. My study also showed that the shape of objects had an influence on weight perception: perceptually larger objects were perceived as lighter, in both haptic and bimodal conditions. The observed biases were large, but large individual differences in the magnitude of the biases were found. Finally, we showed that prolonged exploration of a rough surface resulted in a decrease of the perceived roughness of a subsequently scanned surface, whereas prolonged exploration of a smooth surface resulted in an increase of the perceived roughness. In addition, perceived roughness of a surface explored with one finger shifted towards the roughness of the surface scanned with an adjacent finger. These results provided information about the way roughness information is processed in the brain. The studies presented in this thesis demonstrate clearly how different object dimensions influence our percepts. Investigation of these illusions provides knowledge about the accuracy and the abilities of the haptic system. In addition to the scientific relevance, these findings may also be important for specific applications. For example, the finding that shape has an influence on volume perception may be relevant for package designers and also in the field of remote handling. In order to avoid or at least to decrease the occurrence of these misperceptions, designers should be aware of the illusions described here.