The present study shows an effect of 3D shape on perceived weight of objects. This effect could be explained partly by the size-weight and the shape-size illusions, suggesting that the perceived size is not the only factor responsible for the shape-weight illusion.

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.

In the present study, blindfolded subjects had to explore differently shaped objects with two hands and to judge their volume. The results showed a significant effect of the shape of objects on their perceived volume. Additional analysis showed that this effect could not be explained by the subjects' tendency to base the volume judgment on a specific object dimension other than the volume itself. This contrasts with the results from previous studies, which used cylindrical objects or objects that could fit in one hand, in which the effect of shape on volume perception could be explained by the height/width ratio or the surface area of objects, respectively.

Haptic matching of the orientation of bars separated by a horizontal distance leads to large systematic deviations (eg Kappers and Koenderink, 1999 Perception 28 781–795). A bar on the right side has to be rotated clockwise in order to be perceived as parallel to a bar at the left side. This finding leads to the following intriguing question which we investigated in this study: Will a bar moving from left to right in a fixed orientation be perceived as rotating counterclockwise? Blindfolded subjects had to touch a bar that moved from left to right or from right to left while it was rotating clockwise or counterclockwise with di erent speeds or did not rotate. For each trial they had to decide whether the rotation was clockwise or counterclockwise. From psychometric curves fitted to the data, we could determine that the results were consistent with the findings in the static case: A bar moving from left to right has to rotate clockwise in order to be perceived as non-rotating (and vice versa). In other words, a translating bar causes the illusory perception of a rotation.