We investigated the perception of differences between directiondependent, movement-opposing forces. The magnitude of these forces changed in whenever the direction of motion reversed. They were felt by participants during an experiment that required them to scan a virtual surface, represented by a planar haptic interface, via left-right motions of the index finger. We found that individuals are surprisingly insensitive to changes in opposing force magnitude that are contingent on reversals in

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... rection of motion, despite large contrasts in force magnitude. Forces of 1 N failed consistently to be discriminated from forces of 0 N during sequential presentation at the highest speeds. As the mean scanning speed of the digit was reduced, the effect progressively vanished. The effect we observed is simple and robust enough to be demonstrated on virtually any haptic force-feedback interface. We suggest possible interpretations based on temporal information processing in the nervous system, on physiology and biomechanics, and through inferences that the nervous system may rely on to relate motor commands to sensory input during dynamic haptic interaction. The results obtained raise fundamental questions about the perceptual interpretation of kinesthetic stimuli involving rapid movement, and may also suggest a reconsideration of requirements for haptic interfaces.