  Our aim is to provide effective interaction with virtual objects, despite the
lack of co-location of virtual and real-world contacts, while taking advantage
of relatively large skin area and ease of mounting on the forearm. We performed
two human participant studies to determine the effects of haptic feedback in
the normal and shear directions during virtual manipulation using haptic
devices worn near the wrist. In the first study, participants performed
significantly better while discriminating stiffness values of virtual objects
when the feedback consisted of normal displacements compared to shear
displacements. Participants also commented that they could detect normal cues
much easier than shear, which motivated us to perform a second study to find
the point of subjective equality (PSE) between normal and shear stimuli. Our
results show that shear stimuli require a larger actuator displacement but less
force than normal stimuli to achieve perceptual equality for our haptic
bracelets. We found that normal and shear stimuli cannot be equalized through
skin displacement nor the interaction forces across all users. Rather, a
calibration method is needed to find the point of equality for each user where
normal and shear stimuli create the same intensity on the user's skin.
