BackgroundWhen scanning surfaces, humans perceive some of their physical attributes. These percepts are frequently accompanied by a sensation of (un)pleasantness. We therefore hypothesized that aspects of the mechanical activity induced by scanning surfaces with fingertips could be objectively associated with a pleasantness sensation. Previously, we developed a unidimensional measure of pleasantness, the Pleasant Touch Scale, quantifying the pleasantness level of 37 different materials. Findings of this study suggested that the sensation of pleasantness was influenced by the average magnitude of the frictional forces brought about by sliding the finger on the surface, and by the surface topography. In the present study, we correlated (i) characteristics of the fluctuations of frictional forces resulting from the interaction between the finger and the surface asperities as well as (ii) the average friction with the sensation of pleasantness.ResultsEight blindfolded participants tactually explored twelve materials of the Pleasant Touch Scale through lateral sliding movements of their index fingertip. During exploration, the normal and tangential interaction force components, fN and fT, as well as the fingertip trajectory were measured. The effect of the frictional force on pleasantness sensation was investigated through the analysis of the ratio fT to fN, i.e. the net coefficient of kinetic friction, μ. The influence of the surface topographies was investigated through analysis of rapid fT fluctuations in the spatial frequency domain. Results showed that high values of μ were anticorrelated with pleasantness. Furthermore, surfaces associated with fluctuations of fT having higher amplitudes in the low frequency range than in the high one were judged to be less pleasant than the surfaces yielding evenly distributed amplitudes throughout the whole spatial frequency domain.ConclusionCharacteristics of the frictional force fluctuations and of the net friction taking place during scanning can reliably be correlated with the pleasantness sensation of surfaces.

Predicting the consequences of one’s own movements can be challenging when confronted with completely novel environmental dynamics, such as microgravity in space. The absence of gravitational force disrupts internal models of the central nervous system (CNS) that have been tuned to the dynamics of a constant 1-g environment since birth. In the context of object manipulation, inadequate internal models produce prediction uncertainty evidenced by increases in the grip force (GF) safety margin that ensures a stable grip during unpredicted load perturbations. This margin decreases with practice in a novel environment. However, it is not clear how the CNS might react to a reduced, but non-zero, gravitational field, and if adaptation to reduced gravity might be beneficial for subsequent microgravity exposure. That is, we wondered if a transfer of learning can occur across various reduced-gravity environments. In this study, we investigated the kinematics and dynamics of vertical arm oscillations during parabolic flight maneuvers that simulate Mars gravity, Moon gravity, and microgravity, in that order. While the ratio of and the correlation between GF and load force (LF) evolved progressively with practice in Mars gravity, these parameters stabilized much quicker to subsequently presented Moon and microgravity conditions. These data suggest that prior short-term adaptation to one reduced-gravity field facilitates the CNS’s ability to update its internal model during exposure to other reduced gravity fields.

Haptic force estimation is a critical aspect of human dexterity. To manipulate objects or interact with a haptic interface successfully, the normal and tangential components of the contact forces produced by our fingertips must be carefully coordinated. The tight coupling between tangential and normal contact forces observed during voluntary movements indicates that the nervous system can estimate these forces with high accuracy. Here, we examined the influence of gravity on manual force production in an isometric task. We trained participants to produce isometric tangential forces with the thumb and index finger on a dynamometer held in precision grip. They were tasked with reproducing these forces in a normal gravitational environment (1 g) and during parabolic maneuvers creating phases of micro- (0 g) and hypergravity (1.6 g). The isometric task results showed that arm weight biases estimation of the forces exerted by the fingertips. Reproduced tangential forces were consistently larger downward than upward in 1 g. This asymmetry was reduced under microgravity and increased under hypergravity. Critically, the normal forces did not reflect this asymmetry, demonstrating that tangential forces were misestimated and pointing to a haptic illusion created by gravity. This gravitational effect on haptic force estimation may have implications for the design of prostheses incorporating force feedback, haptic devices for teleoperations, or haptic supports aimed at improving sensorimotor performance in space.

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