There's more to touch than meets the eye: The salience of object attributes for haptics with and without vision.

Abstract

SUMMARY The availability and salience of object attributes under haptic exploration, with and without vision, were assessed by two tasks in which subjects sorted objects that varied factorially in size, shape, texture, and hardness. In the directed-discrimination task, subjeets were instructed to sort along a particular dimension. Although levels on all dimensions were easily discriminated, shape was relatively less so for haptic explorers without vision, as was hardness for those using vision and haptics. Size was least discriminable for both groups. In thefiee-sorting task, subjects were to sort objects by similarity. Three groups used haptic exploration only; these were differentiated by the experimenters' definition of object similarity: unbiased haptics (no particular definition of similarity), haptically biased hapties (similarity = objects feel similar), haptics plus visual imagery (similarity = objects' visual inmges are similar). A fourth group used vision as well as haptics, with instructions like those of the unbiased haptics group. Dimensional salience was measured by the extent to which levels on a dimension were differentiated in free sorting (more differentiation indicating higher salience). The unbiased haptics and haptically biased haptics groups were highly similar; both found the substance dimensions (hardness and texture) relatively salient. The haptics plus visual imagery group showed shape to be overwhelmingly salient, even more so when they were instructed to use two hands, but less so when they had just seen the objects. The haptics plus vision group showed salience to be more evenly distributed over the dimensions. Exploratory hand movements were videotaped and scored into four categories of exploratory procedure (I.,derman & Klatzky, 1987): lateral motion, pressure, contour following, and enclosure (related to texture, hardness, shape, and size, respectively). The distribution of exploratory procedures was found to be directly related to both the designated dimension in the directed-discrimination task, and the salient dimension in the free-sorting task. The results support our contention that the haptic and visual systems have distinct encoding pathways, with haptics oriented toward the encoding of substance rather than shape. This may reflect a direct influence of haptic exploratory procedures: The procedures that are executed under unbiased haptic encoding are those that are generally found to be rapid and accurate (high ease of encoding), and the execution of these procedures determines which object properties become salient.