Haptic Exploration of Spheres

Haptic exploration is a complex sensory/motor activity involving a high number of degrees of freedom, multiple sensing modalities, and integration of sensory information over space and time. Lederman and Klatzky created a taxonomy of “Exploratory Procedures” which are stereotyped hand and finger movements which people preferentially use to make specific sensory discriminations and object identifications. Their experiments were mostly aimed at either 1) identification of objects through haptic interaction and 2) assessment of a property such as temperature or roughness. Psychophysical literature relevant to the present study has been reviewed in detail in [1]. A review from the point of view of haptic interface design is available in [2]. The work reported here studies haptic perception of sphericity. If an object is placed in the fingers, how well can humans determine whether or not it is a sphere? Starting from nearly perfect spheres, such as for example, precision industrial balls, a variety of increasingly subtle distortions can be made. If a subject is to detect deviations from perfect sphericity, they may use a variety of queues, depending on how the reference spheres are distorted. These cues may include • Eccentricity (non-constant diameter) • Texture (local variations in radius) • Changes in curvature (the curvature is constant at all points on a perfect sphere) To define these experiments more precisely, we wish to distort some test spheres in precisely defined ways, ideally to determine a sensory threshold for a specific attribute. These cues are not variable independently. Since a sphere is defined as a surface of constant local curvature, any amount of eccentricity would also change local curvature. Texture also changes curvature. We will define texture to mean radius and curvature changes on a small scale compared to the sphere’s nominal radius which average out to zero within each small region.