Three experiments establish the size-weight illusion as a primarily haptic phenomenon, despite its having been more traditionally considered an example of vision influencing haptic processing. Experiment 1 documents, across a broad range of stimulus weights and volumes, the existence of a purely haptic size-weight illusion, equal in strength to the traditional illusion. Experiment 2 demonstrates that haptic volume cues are both sufficient and necessary for a full-strength illusion. In contrast, visual volume cues are merely sufficient, and produce a relatively weaker effect. Experiment 3 establishes that congenitally blind subjects experience an effect as powerful as that of blindfolded sighted observers, thus demonstrating that visual imagery is also unnecessary for a robust size-weight illusion. The results are discussed in terms of their implications for both sensory and cognitive theories of the size-weight illusion. Applications of this work to a human factors design and to sensor-based systems for robotic manipulation are also briefly considered.In experimental psychology, weight perception can be traced back to the early experiments of Ernst Weber (1834/1978). His main interest was whether weight perception resulted more from cutaneous inputs alone or from the muscular sense associated with lifting an object. He found that weight discrimination is more exact if the object is actually lifted rather than simply placed on a hand passively resting on a table. Since these early experiments, weight perception has been and continues to be investigated in many psychophysical experiments (see recent review by Jones, 1986). Despite this extensive research, it remains a problem for perceptual theorists.Weight is an important dimension of an object, particularly if it has to be moved or manipulated by either a human or a robot. Its assessment presents an interesting paradox to a manipulator-in order to manipulate an object efficiently, its weight must be considered; however, to judge its weight, the object must be lifted. One cautious solution would be to try to lift the object with a small lifting force. If this force proved to be ineffective, it could be increased slight!y. If this too proved to be unsuccessful, it could be increased repeatedly until an effective force was found that just lifted the object.A more efficient solution to this paradox would be to use knowledge of particular objects and their properties derived from past experience for an initial weight estiThis research was supported by a contract to S. J. Ledennan by the Manufacturing Research Corporation of Ontario Centre of Excellence, and by a Postgraduate Level Scholarship from the Natural Sciences and Engineering Research Council of Canada to R. R. Ellis. The research fulfilled part of the requirements for a master's degree (Ellis, 1990). We would particularly like to express our appreciation to the Montreal Association for the Blind (Paul Barber was especially helpful in assisting us) and to the subjects who participated in Experiment 3. We also...

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Experiment 1 documents modality effects on the material-weight illusion for a low-mass object set (58.5 g). These modality effects indicate that the material-weight illusion is principally a haptically derived phenomenon: Haptically accessed material cues were both sufficient and necessary for fullstrength illusions, whereas visually accessed material cues were only sufficient to generate moderatestrength illusions. In contrast, when a high-mass object set (357 g) was presented under the same modality conditions, no illusions were generated. The mass-dependent characteristic of this illusion is considered to be a consequence of differing grip forces. Experiment 2 demonstrates that the enforcement of a firm grip abolishes the low-mass material-weight illusion. Experiment 3 documents that a firm grip also diminishes perceptual differentiation of actual mass differences. Several possible explanations of the consequences of increasing grip force are considered.Charpentier (1891) first demonstrated that the perceived weight of an object, commonly referred to as its "heaviness," depends not only on its physical mass but also on its size. The larger of two objects of equal mass was consistently reported as lighter. This phenomenon has come . to be known as the size-weight illusion. Finding a link between mass and size subsequently prompted other investigators to search for additional factors that might contribute to illusory differences in weight perception.For example, Dresslar (1894) documented a shapeweight illusion in which objects that were the same in mass, volume, and material but different in shape were judged to be different in weight. Unfortunately, he provided no metric for the variations in shape, rather only vaguely referring to a difference in "compactness."In 1898, Wolfe documented the material-weight illusion.) which is the subject ofthe present paper. Inthis illusion, objects with the same mass but fabricated from different surface materials are judged, on lifting, to weigh different amounts. The general pattern is for same-mass objects ofdenser materials (i.e., brass) to be judged lighter than same-mass less dense objects (i.e., wood). Ross (1969) explained this effect (together with the size-weight illusion) in terms of expectancies concerning the effects of density on weight perception. Harshfield and DeHardt (1970) showed that subjects' rankings of expected weight, assessed visually for five different same-size same-mass objects (steel, brass, aluminum, mahogany, and balsa wood), were in the reverse order ofrankings ofperceived weight after lifting. It should be noted, however, that this was a between- groups study in that the subjects who ranked the objects for expected weight did not also rank them for perceived weight via lifting. As such, it could not be determined whether or not perceived weight was in fact based on expectations related to the materials used and/or to direct sensory feedback from the stimulus objects (e.g., density) . Harshfield and DeHardt renamed the effect of material on pe...