ABSTRACT The authors report an experiment in which twenty-five participants discriminated force vectors presented along five directions (up, left, right, diagonally up left, diagonally up right). The force vectors were presented with a three degree-of-freedom forcefeedback device. A three-interval one-up three-down adaptive procedure was used. The five reference force-direction conditions were presented in randomly interleaved order. The results show an average force-direction discrimination threshold of 33° regardless of the reference-force direction. Position data recorded at a nominal sampling rate of 200 Hz revealed a 10.1 mm average displacement of the fingertip between the start and end positions in a trial. The average maximum deviation from the starting position within a trial was 21.3 mm. We conclude that the resolution with which people can discriminate force direction is not dependent on the direction of the force per se. These results are useful for designers of haptic virtual environments.

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(submitted 30-Sept-2005)

ABSTRACT The perceptual sensitivity of touch to orientation differences in adjacent segments of textures with different configurations was measured in two experiments. We found that sensitivity to the orientation difference was not only a function of the magnitude of that difference but of the reference orientation. In Experiment 1, we examined the exploratory patterns that were used to make these judgments and found that distinct exploratory patterns were used early but tended to converge on one dominant pattern. In Experiment 2, constraining exploration trajectories to previously unobserved patterns and halving exploration time only slightly lowered perceptual accuracy but did not alter the pattern of effects. That the configuration of the texture elements influenced accuracy more than did the exploratory procedure used has implications for how texture is encoded through the skin and the procedural knowledge underlying haptic texture exploration.

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(submitted 27-Feb-2005)

ABSTRACT When the human fingertip is pressed against a surface or bent, the hemodynamic state of the fingertip is altered in a way that is common to all people. Normal force, shear force, and finger extension/flexion all result in visibly distinct patterns of blood volume or perfusion beneath the fingernail. These patterns of blood perfusion can be used not only to monitor the state of the finger, but also to understand how the fingernail interacts with the bone and surrounding tissues when various forces or postures are applied. In this paper, photographic techniques are used to catalog the average patterns of fingernail coloration corresponding to various states of applied forces and postures across human subjects of a variety of size, gender, and skin color. Results indicate that there are at least seven different states of force and posture that yield distinct coloration patterns that are statistically significant and common to people in general.

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(submitted 1-Aug-2005)