Comparing the Role of Lateral Force During Active and Passive Touch: Lateral Force and its Correlates are Inherently Ambiguous Cues for Shape Perception under Passive Touch Conditions

Lateral forces are important cues for shape perception through active touch [1,2]. They can elicit illusory haptic shapes, or perceptually transform the shape of physical objects [1]. Here, it is shown that lateral force, and correlates such as acceleration and velocity, are inherently ambiguous as shape cues under passive touch conditions. The lateral forces arising when passively touching objects with different shapes can be equal. Lateral force ambiguity can also happen during active touch, when an object moves relative to a subject's limb. Lateral force ambiguity may be used to explore two important, unsolved questions: whether active and passive touch percepts are equivalent [4], and whether they arise from the same brain mechanisms [4]. A method is proposed, and used, to generate geometryless, force-matched stimuli to compare the role of lateral force during active and passive touch, while minimizing other cues. Such comparisons have been extremely hard to make because of the difficulty to meaningfully match the many factors involved [4-8].