Academic journal article Attention, Perception and Psychophysics

Haptic Perception of Volume and Surface Area of 3-D Objects

Academic journal article Attention, Perception and Psychophysics

Haptic Perception of Volume and Surface Area of 3-D Objects

Article excerpt

Haptic perception of volume (Experiment 1) and surface area (Experiment 2) was studied with tetrahedrons, cubes, and spheres as stimuli (2-14 cm^sup 3^). 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.

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The most well-known example of a study on the perception of volume is probably Piaget's (1968) experiment on conservation of liquid volume with young children. In his experiments, liquid was poured from a tall cylinder into a shorter, wider one, so that the height of the liquid in the second cylinder was smaller. A young child who observes this action will most likely indicate that there is more liquid in the tall skinny cylinder. According to Piaget, this happens because the child tends to focus on just one dimension of the cylinder-that is, the height-in order to make the volume judgment. Studies performed with adult participants have also suggested that visual volume perception depends on the longest linear dimension of objects (Frayman & Dawson, 1981; Holmberg, 1975; Lauer, 1929; Raghubir & Krishna, 1999; Stanek, 1968, 1969; Wansink & Van Ittersum, 2003). This phenomenon has been termed the elongation bias.

Krishna (2006) showed that the extent and direction of this elongation bias is affected by sensory modality. She used two transparent plastic glasses, each with a volume of about 200 cm3, with one glass taller than the other. Adult participants had to judge the volume of these glasses by use of visual cues alone, bimodal visual and haptic cues, and haptic cues alone. With visual cues alone and with bimodal cues, the elongation bias was obtained. With haptic cues alone, a reversal of the elongation bias occurred. Krishna suggested that this difference between the modalities occurs because the salience of a dimension may depend on sensory input. The height is salient for the visual sense, but the width for the haptic sense.

Objects can, however, differ along more geometric dimensions than only the height-to-width ratio. When a long and a short cylinder or a cube and an elongated rectangular cuboid are compared by touch, the most obvious difference between the compared objects is indeed the heightto- width ratio. On the other hand, a cube and a sphere differ along more geometric properties that can be salient for the haptic sense-for example, curvature, edges, and number of flat surfaces. The important question we will address in this article is, can we generalize the influence of the longest linear dimension on haptic volume perception to objects that differ along more geometric dimensions?

In the first experiment, blindfolded subjects handled tetrahedrons, spheres, and cubes of varying volume and matched them according to their perceived volume. The stimuli were perceived by enclosure, since that has been shown to be the stereotypical exploratory procedure for haptic volume judgment (Lederman & Klatzky, 1987). …

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