In the literature only a small subset of the broad range of scales of human haptic perception has been explored. In this experiment, haptic detection thresholds have been investigated over a wide range of spatial scales. Computer-controlled manufacturing techniques have made it possible to produce stimuli with sufficient accuracy to explore the gamut of haptic scales. We used stimuli with a convex or concave Gaussian-shaped profile running over the middle of a strip. The width of the Gaussian profile was varied between 150 microns and 240 mm. In this range of spatial scales a number of mechanisms, ranging from cutaneous mechanoreception to proprioception in fingers, arm and shoulder, contribute to haptic perception. The discrimination threshold between a flat surface and a Gaussian surface was investigated in a series of two alternative forced choice experiments on human subjects. The thresholds run from 1 micron for the narrowest Gaussian profiles to 8 mm for the broadest profile. The same thresholds were found for convex and concave shapes. Over the range of spatial scales, from 1 mm width on, the dependence of the detection threshold on the spatial width of the Gaussian profile was found to be a power function with an exponent of about 1.3.

Several studies have shown that people can catch a ball even if it is visible only during part of its flight. Here, we examine how well they can do so. We measured the movements of a ball and of the hands of both the thrower and the catcher during one-handed underarm throwing and catching. The catcher's sight was occluded for 250 ms at random moments. Participants could catch most balls without fumbling. They only really had difficulties if vision was occluded before the ball was released and was restored less than 200 ms before the catch. In such cases, it was impossible to accurately predict the ball's trajectory from motion of the ball and of the thrower's hand before the occlusion, and there was not enough time to adjust the catching movement after vision was restored. Even at these limits, people caught most balls quite adequately.

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