Texture discrimination in active touch: Effects of the extension of the exploration and their exploitation

The active control of exploratory movements is an integral part of active touch. We investigated how the spatio-temporal extension of an exploration affects texture discrimination via active touch and how participants spontaneously control the extension of their exploration. Participants stroke one to 8 times across a virtual grating. Half of the participants judged the grating according to ridge amplitude, the other half judged ridge period. In both tasks just noticeable differences (JNDs) decreased with an increasing number of strokes, down to a (presumably) minimum JND for stroke numbers of about 6-7 or higher (according to a model fit). This extends previous findings on the perceptual effects of extended stimulation to the domain of active touch. Further, for spontaneous exploration, 6-7 strokes may be regarded as being optimal, given that this extension results in minimal JNDs while using the smallest number of strokes necessary. Many participants tended to spontaneously use clearly more than this optimal number of strokes, before they had experience with the task. With experience, stroke numbers were, however, individually closer to and, on average slightly below this number. That is, participants seem to have adapted their exploration to the task. This supports the view that the active control of exploratory movements can be used in order to optimize performance-when participants have sufficient experience with the haptic task.