Anisotropy in the haptic perception of force direction and magnitude

A Hand-Held Device Presenting Haptic Directional Cues for the Visually Impaired

Haptic information is essential in everyday activities, especially for visually impaired people in terms of real-world navigation. Since human haptic sensory processing is nonlinear, asymmetric vibrations have been widely studied to create a pulling sensation for the delivery of directional haptic cues. However, the design of an input control signal that generates asymmetric vibrations has not yet been parameterised. In particular, it is unclear how to quantify the asymmetry of the output vibrations to create a better pulling sensation. To better understand the design of an input control signal that generates haptic directional cues, we evaluated the effect of the pulling sensations corresponding to the three adjustable parameters (i.e., delay time, ramp-down step length, and cut-off voltage) in a commonly applied step-ramp input signal. The results of a displacement measurement and a psychophysical experiment demonstrate that when the quantified asymmetry ratio is in a range of 0.3430-0.3508 with an optimised cut-off voltage for our hand-held device, the haptic directional cues are better perceived by participants. Additionally, the results also showed a superior performance in haptic delivery by shear forces than normal forces.

A haptic illusion created by gravity

Human dexterity requires very fine and efficient control of fingertip forces, which relies on the integration of cutaneous and proprioceptive feedback. Here, we examined the influence of gravity on isometric force control. We trained participants to reproduce isometric vertical forces on a dynamometer held between the thumb and the index finger in normal gravity and tested them during parabolic flight creating phases of microgravity and hypergravity, thereby strongly influencing the motor commands and the proprioceptive feedback. We found that gravity creates the illusion that upward forces are larger than downward forces of the same magnitude. The illusion increased under hypergravity and was abolished under microgravity. Gravity also affected the control of the grip force employed to secure the grasp. These findings suggest that gravity biases the haptic estimation of forces, which has implications for the design of haptic devices to be used during flight or space activities.

Human Stiffness Perception and Learning in Interacting With Compliant Environments

Humans are capable of adjusting their posture stably when interacting with a compliant surface. Their whole-body motion can be modulated in order to respond to the environment and reach to a stable state. In perceiving an uncertain external force, humans repetitively push it and learn how to produce a stable state. Research in human motor control has led to the hypothesis that the central nervous system integrates an internal model with sensory feedback in order to generate accurate movements. However, how the brain understands external force through exploration movements, and how humans accurately estimate a force from their experience of the force, is yet to be fully understood. To address these questions, we tested human behaviour in different stiffness profiles even though the force at the goal was the same. We generated one linear and two non-linear stiffness profiles, which required the same force at the target but different forces half-way to the target; we then measured the differences in the learning performance at the target and the differences in perception at the half-way point. Human subjects learned the stiffness profile through repetitive movements in reaching the target, and then indicated their estimation of half of the target value (position and force separately). This experimental design enabled us to probe how perception of the force experienced in different profiles affects the participants' estimations. We observed that the early parts of the learning curves were different for the three stiffness profiles. Secondly, the position estimates were accurate independent of the stiffness profile. The estimation in position was most likely influenced by the external environment rather than the profile itself. Interestingly, although visual information about the target had a large influence, we observed significant differences in accuracy of force estimation according to the stiffness profile.

Increased upper-limb sensory attenuation with age

The pressure of our own finger on the arm feels differently than the same pressure exerted by an external agent: the latter involves just touch, whereas the former involves a combination of touch and predictive output from the internal model of the body. This internal model predicts the movement of our own finger and hence the intensity of the sensation of the finger press is decreased. A decrease in intensity of the self-produced stimulus is called sensory attenuation. It has been reported that, due to decreased proprioception with age and an increased reliance on the prediction of the internal model, sensory attenuation is increased in older adults. In this study, we used a force-matching paradigm to test if sensory attenuation is also present over the arm and if aging increases sensory attenuation. We demonstrated that, while both young and older adults overestimate a self-produced force, older adults overestimate it even more showing an increased sensory attenuation. In addition, we also found that both younger and older adults self-produce higher forces when activating the homologous muscles of the upper limb. While this is traditionally viewed as evidence for an increased reliance on internal model function in older adults because of decreased proprioception, proprioception appeared unimpaired in our older participants. This begs the question of whether an age-related decrease in proprioception is really responsible for the increased sensory attenuation observed in older people.

Visuo-Haptic Discrimination of Viscoelastic Materials

In our daily lives, we interact with different types of deformable materials. Regarding their mechanical behavior, some of those materials lie in a range that is between purely elastic and purely viscous. This range of mechanical behavior is described as viscoelasticity. In certain types of haptic interactions, such as assessment of ripeness of fruit, firmness of cheese, and consistency of organ tissue, we rely heavily on our haptic perception of viscoelastic materials. The relationship between the mechanical behavior of viscoelastic materials and our perception of them has been investigated in the field of psychorheology. However, our knowledge on how we perceive viscoelastic materials is still quite limited though some research work has already been done on purely elastic and purely viscous materials. History- and frequency-dependent behavior of viscoelastic materials result in a complex time-dependent response, which requires relatively more sophisticated models to investigate their behavior than those of purely elastic and viscous materials. In this study, we model viscoelasticity using a "springpot" (i.e., fractional-order derivative element) and express its behavior in the frequency domain using two physical parameters-"magnitude" and "phase" of complex stiffness. In the frequency domain, we are able to devise signal detection experiments where we can investigate the perception of viscoelastic materials using the perceptual terms of "firmness" and "bounciness," corresponding to the physical parameters of "magnitude" and "phase." The results of our experiments show that the just-noticeable difference (JND) for bounciness increases linearly with increasing "phase," following Weber's law, while the JND for firmness is surprisingly independent of the level of "phase."

Quantification of Motion Palpation

CONTEXT

The palpation of motions is at the heart of the practice of foreign-trained osteopaths. When practicing osteopathic manual therapy (care provided by foreign-trained osteopaths) in the cranial field or osteopathic cranial manipulative medicine, the palpation of small motions (several tens of micrometers) is a key process. However, to the authors' knowledge, the smallest detectable motion has not been identified.

OBJECTIVE

To quantify motion detection capacity by passive palpation.

METHODS

Participants were instructed to hold a mechanical device containing a micrometric actuator between their hands and report when they felt motion while 6 series of 27 random motions were generated by the actuator. After each series, if a participant succeeded or failed to detect motion with a confidence level of greater than 98%, the motions in the next series were set to a smaller or larger magnitude, respectively. After 6 series, the individual motion detection capacity was recorded. Statistical significance was set at P=.02.

RESULTS

A total of 21 participants were selected, comprising 14 osteopaths and 7 nonosteopaths. The average performance of the sample was 148 μm. Thirteen participants (62%) perceived motions of 200 μm or less, and 7 participants (33%) detected motions of 50 μm or less with bare hands. Osteopathic training did not notably affect the performance. Osteopaths were twice as likely to claim detection of nonexisting motions than to miss existing ones, whereas nonosteopaths were equally subject to both types of errors.

CONCLUSION

The data show human passive palpatory sensitivity to be in the range of several tens of micrometers. This range is comparable to that reported for calvarial motion (10-50 μm).

Errors in visuo-haptic and haptic-haptic location matching are stable over long periods of time

People make systematic errors when they move their unseen dominant hand to a visual target (visuo-haptic matching) or to their other unseen hand (haptic-haptic matching). Why they make such errors is still unknown. A key question in determining the reason is to what extent individual participants' errors are stable over time. To examine this, we developed a method to quantify the consistency. With this method, we studied the stability of systematic matching errors across time intervals of at least a month. Within this time period, individual subjects' matches were as consistent as one could expect on the basis of the variability in the individual participants' performance within each session. Thus individual participants make quite different systematic errors, but in similar circumstances they make the same errors across long periods of time.

Haptic perception of force magnitude and its relation to postural arm dynamics in 3D

In a previous study, we found the perception of force magnitude to be anisotropic in the horizontal plane. In the current study, we investigated this anisotropy in three dimensional space. In addition, we tested our previous hypothesis that the perceptual anisotropy was directly related to anisotropies in arm dynamics. In experiment 1, static force magnitude perception was studied using a free magnitude estimation paradigm. This experiment revealed a significant and consistent anisotropy in force magnitude perception, with forces exerted perpendicular to the line between hand and shoulder being perceived as 50% larger than forces exerted along this line. In experiment 2, postural arm dynamics were measured using stochastic position perturbations exerted by a haptic device and quantified through system identification. By fitting a mass-damper-spring model to the data, the stiffness, damping and inertia parameters could be characterized in all the directions in which perception was also measured. These results show that none of the arm dynamics parameters were oriented either exactly perpendicular or parallel to the perceptual anisotropy. This means that endpoint stiffness, damping or inertia alone cannot explain the consistent anisotropy in force magnitude perception.

Unintentional changes in the apparent stiffness of the multi-joint limb

We explored the phenomenon of unintentional changes in the apparent stiffness of the human arm produced by transient changes in the external force. The subjects performed a positional task against a constant baseline force and were instructed not to react to changes in the force. A HapticMaster robot produced a smooth force increase (a perturbation) leading to a hand movement, followed by a dwell time. No visible hand drift was observed during the dwell time. After the robot force dropped to its initial baseline value, the hand moved toward the initial position but stopped short of it. Small perturbations were applied at different time intervals along different directions during the dwell time. Arm apparent stiffness distribution in a horizontal plane was approximated with an ellipse. The apparent stiffness magnitude along the main axis of the ellipse showed a non-monotonic increase with dwell time, while the apparent stiffness along the minor axis did not change significantly. We interpreted the early part of the changes in the apparent stiffness as due to peripheral muscle properties. The later part is interpreted as caused by a combination of two processes: a drift in the referent hand coordinate due to the hypothesized back-coupling between the referent and actual hand coordinates and an implicit instruction to keep the hand steady when no changes in robot-generated force took place. The data provide support for the idea of back-coupling between the referent and actual body configurations, which may be an important contributor to stability of motor actions.