A particular type of haptic perception concerns the activity of the hand when it grasps an object at one end and rotates it in space. The lifting of the object necessitates the production of forces to oppose the resistance caused by the object's mass, and these forces modify the state of the muscles, tendons and articulations of the active limb. Turvey, Solomon and their associates (Solomon & Turvey 1988; Turvey 1996; Turvey & Carello 1995) showed that several properties, such as the length and orientation of a bar, may be perceived via these wielding movements and they analyzed the information provided by kinetics, that is by the association of forces and movements. Referring to Gibson (1966), they looked for the invariant which would account for this perception, and identified it in the moment of inertia. The moment of inertia is a measure of an object resistance to its rotation and it decreases as the axis of rotation approaches the center of the mass. As the rotational forces on an object can be exerted in several possible directions, there are several moments of inertia, the combination of which, under certain conditions, allows an "inertia tensor" to be calculated. This tensor quantifies the rotational inertia of an object (Solomon 1988; Solomon & Turvey 1988) and determines the observer's perceptual estimations. A long series of experiments led by Turvey and Solomon validate this point of view. In the simplest one, for example, a bar held at one end seems longer if a weight is added to the other end, and this overestimation decreases when the weight is placed closer to the held end, because the value of the inertia tensor in the wrist is changed in this condition.
Wielding is a very particular form of haptic perception which will not be studied in the present book. In this perception, the kinesthetic element dominates and the cutaneous element is greatly reduced. In addition, it requires that an object be small enough to be lifted by one hand. It is likely that this form of perception is used in daily life without awareness.
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