Mirela Kahrimanovic

Haptic perception of volume and surface area of 3-D objects

Haptic perception of volume (Experiment 1) and surface area (Experiment 2) was studied with tetrahedrons, cubes, and spheres as stimuli (2–14 cm3). The results of Experiment 1 showed that subjects perceived a tetrahedron to be larger in volume than either a cube or a sphere of the same physical volume and that they perceived a cube to be larger than a sphere. This pattern was independent of object size. The biases were smaller in conditions with mass information than in those without. The average biases in the different conditions ranged from 7% to 67%. Analyses revealed that the subjects apparently based their volume judgments on the surface area of objects. Experiment 2 showed that surface area itself could be perceived accurately, almost independently of the objects’ shape. Experiment 3 investigated volume perception of objects in the absence of surface area (wire-frame objects) and showed larger biases than those observed with solid objects. With wire-frame objects, the maximal distance between two vertex points was probably the dimension on which the volume judgment was based. In conclusion, haptic volume perception of geometric objects has to be inferred from other object properties, but surface area can be perceived unbiased.

Context effects in haptic perception of roughness

The influence of temporal and spatial context during haptic roughness perception was investigated in two experiments. Subjects examined embossed dot patterns of varying average dot distance. A two-alternative forced-choice procedure was used to measure discrimination thresholds and biases. In Experiment 1, subjects had to discriminate between two stimuli that were presented simultaneously to adjacent fingers, after adaptation of one of these fingers. The results showed that adaptation to a rough surface decreased the perceived roughness of a surface subsequently scanned with the adapted finger, whereas adaptation to a smooth surface increased the perceived roughness (i.e. contrast after effect). In Experiment 2, subjects discriminated between subsequent test stimuli, while the adjacent finger was stimulated simultaneously. The results showed that perceived roughness of the test stimulus shifted towards the roughness of the adjacent stimulus (i.e. assimilation effect). These contextual effects are explained by structures of cortical receptive fields. Analogies with comparable effects in the visual system are discussed.

Seeing and feeling volumes: The influence of shape on volume perception

The volume of common objects can be perceived visually, haptically or by a combination of both senses. The present study shows large effects of the objects shape on volume perception within all these modalities, with an average bias of 36%. In all conditions, the volume of a tetrahedron was overestimated compared to that of a cube or a sphere, and the volume of a cube was overestimated compared to that of a sphere. Additional analyses revealed that the biases could be explained by the dependence of the volume judgment on different geometric properties. During visual volume perception, the strategies depended on the objects that were compared and they were also subject-dependent. However, analysis of the haptic and bimodal data showed more consistent results and revealed that surface area of the stimuli influenced haptic as well as bimodal volume perception. This suggests that bimodal volume perception is more influenced by haptic input than by visual information.

Characterization of the Haptic Shape-Weight Illusion with 3D Objects

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.