Academic journal article Perception and Psychophysics

Manual Exploration and the Perception of Slipperiness

Academic journal article Perception and Psychophysics

Manual Exploration and the Perception of Slipperiness

Article excerpt

In this article, we report on two experiments that examined the haptic perception of slipperiness. The first experiment aimed to determine whether the type of finger motion across a surface influenced the ability to accurately judge the frictional coefficient (or slipperiness) of that surface. Results showed that when using static contact, participants were not as good at distinguishing between various surfaces, compared with when their finger moved across the surface. This raises the issue of how humans are able to generate the appropriate forces in response to friction during grasping (which involves static finger contact). In a second study, participants lifted objects with surfaces of varying coefficients of friction. The participants were able to accurately perceive the slipperiness of the surfaces that were lifted; however, the grasping forces were not scaled appropriately for the friction. That is, there was a dissociation between haptic perception and motor output.

It has been proposed that human afferent visual input is separated into two fundamentally distinct pathways that allow for a division of labor in terms of how sensory information is processed (Milner & Goodale, 1995). The first pathway, the dorsal stream, is responsible for the visual control of skilled action. Conversely, the ventral stream is important in the visual identification of objects: Transformations made in this stream allow for the creation of cognitive representations of the intrinsic and extrinsic characteristics of viewed objects (Milner & Goodale, 1995). This dual-channel processing of visual input has been the topic of much research in the field of motor control; however, it is unclear whether or not a similar action-perception dissociation exists in other sensory modalities. Goodale and colleagues have provided both structural and behavioral evidence that the haptic system shares properties of the visual system, in terms of separate action-perception pathways (James, Humphrey, Gati, Servos, Menon, & Goodale, 2002; Westwood & Goodale, 2003). More recently Prather, Votaw, and Sathian (2004) have confirmed the notion of separate dorsal and ventral pathways for the processing of haptic input, depending on whether this input is to be used for action or perception.

There is other evidence that supports the notion of a haptic perception-action dissociation. The size-weight illusion is a robust phenomenon discovered over a century ago, in which two objects of various sizes are manipulated so that they have identical masses. When individuals are asked to lift these objects while using their sense of vision, they consistently report that the small object feels heavier than the large object (Charpentier, 1891). However, Gordon and his group have shown that grasping forces used when lifting a large box are greater than those used when lifting a smaller box, even though individuals perceive the small box to be heavier than the large box, thus eliciting a dissociation between the action and perception components of the task (Gordon, Forssberg, Johansson, & Westling, 199 Ia, 199Ib). In a similar study, Ellis and Lederman (1993) showed that haptic input alone was capable of producing this illusory effect to a greater degree than a solely visual condition. When lifting objects in a congruent manner that vary in both size and mass, larger objects are perceived as heavier, and more force is used to grasp them (Johansson & Westling, 1990). With the size-weight illusion, the heavier objects are perceived as being lighter, even though more force is used to grasp them. These data could be interpreted to suggest that there is a dissociation between the haptic perception of mass, and the action or motor output required to lift a particular mass.

The relationship between size and weight has been extended to show that the effects of object mass and size on weight perception follow from invariants of rotational dynamics. …

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