In this study we investigate the haptic perception of object size. We report the results from four psychophysical experiments. In the first, we ask subjects to discriminate the size of objects that vary in surface curvature and compliance while changing contact force. We show that objects exhibit size constancy such that perception of object size using haptics does not change with changes in contact force. Based on these results, we hypothesize that size perception depends on the degree of spread between the digits at initial contact with objects. In the second experiment, we test this hypothesis by having subjects continuously contact an object that changes dynamically in size. We show that size perception takes into account the compliance of the object. In the third and fourth experiments we attempt to separate the individual contributions of proprioceptive and cutaneous input. In the third, we test the ability of subjects to perceive object size after altering the sensitivity of cutaneous receptors with adapting vibratory stimuli. The results from this experiment suggest that initial contact is signaled by the cutaneous slowly adapting type 1 afferents (SA1) and/or the rapidly adapting afferents (RA). In the last experiment, we block cutaneous input at the site of contact by anesthetizing the digital nerves and show that proprioceptive information alone provides only a rough estimate of object size. We conclude that the perception of object size depends on inputs from SA1 and possibly RA afferents, combined with inputs from proprioceptive afferents that signal the spread between digits.
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