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Tactual Exploration

Brain-machine interfaces use neuronal activity recorded from the brain to establish direct communication with external actuators, such as prosthetic arms. It is hoped that brain-machine interfaces can be used to restore the normal sensorimotor functions of the limbs, but so far they have lacked tactile sensation. Here we report the operation of a brain-machine-brain interface (BMBI) that both controls the exploratory reaching movements of an actuator and allows signalling of artificial tactile feedback through intracortical microstimulation (ICMS) of the primary somatosensory cortex. Monkeys performed an active exploration task in which an actuator (a computer cursor or a virtual-reality arm) was moved using a BMBI that derived motor commands from neuronal ensemble activity recorded in the primary motor cortex. ICMS feedback occurred whenever the actuator touched virtual objects. Temporal patterns of ICMS encoded the artificial tactile properties of each object. Neuronal recordings and ICMS epochs were temporally multiplexed to avoid interference. Two monkeys operated this BMBI to search for and distinguish one of three visually identical objects, using the virtual-reality arm to identify the unique artificial texture associated with each. These results suggest that clinical motor neuroprostheses might benefit from the addition of ICMS feedback to generate artificial somatic perceptions associated with mechanical, robotic or even virtual prostheses.

tactual exploration

Eight 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.

In a recent study [25], we used the Rasch model to build a Pleasant Touch Scale. This model allowed us, on an objective basis, to construct a scale for use in future studies about the sensation of pleasantness in touch (see Introduction S1 for details on the Rasch model). The Pleasant Touch Scale considered 37 samples of materials that were classified along a single underlying scale according to their level of pleasantness. The establishment of this scale involved 198 participants and accounted for their individual scoring tendencies. This way, the scale became independent from the fact that the participants scored the same samples differently, that is, with different degrees of leniency (e.g. a less lenient participant had a higher probably to perceive a same material as less pleasant than a more lenient subject). The results of this study showed that materials having an irregular surface topography (e.g. sandpaper) or eliciting high friction during exploration (e.g. wax), had a lower pleasantness level than materials having a more regular surface topography (e.g. paper) or materials being more slippery (e.g. marble). Moreover, an analysis of invariance highlighted the fact that the pleasantness levels of most surfaces were dependent on the participants' fingertip moisture levels. Taken together, the results brought us to formulate the hypothesis that surface topography and frictional properties might strongly be implicated in the sensation of pleasantness during active touch exploration. These indications were objective in the sense that they were based on unidimensional, linear and invariant pleasantness measures of the materials forming the Pleasant Touch Scale.

The aim of the present study was to objectively relate a given sample, characterized by its frictional properties, to objective measures of pleasantness (determined through the Rasch model). This study provides evidence that the evaluation of the pleasantness of a texture is correlated with both the net value of friction force and the fluctuations of friction force (reflecting the surface topography, the material with which the sample was made as well as the sample's microstructural properties) created during tactile exploration of the surface.

Participants washed and dried their hands. The fiducial marker was fixed to the nail of the participants' right index fingers. Participants were blindfolded and the moisture level of their right index fingertip was measured using the Corneometer CM 825. The materials were mounted by the experimenter on the measurement device in a randomized order. For each trial, the participants were instructed to position their right index fingertip just above the selected material. On cue, they explored the sample through a lateral sliding movement (from left to right) with a spontaneous exploration force and speed. The participants explored each sample through ten successive sliding movements. During each exploration, the high-frequency fluctuations of tangential force component were recorded, along with the net interaction force, and the fingertip position. After each exploration, the participants were asked to rate the pleasantness of the samples on the basis of a 3-level scale as very pleasant (scored 2), pleasant (scored 1), or unpleasant (scored 0). For each sample, the fingertip moisture level was again recorded immediately after the last exploration trial.

All analyses focused on 20 mm (i.e. between 40 and 60 mm of each material) of the active steady-state fingertip slip phase. During this phase, force data were numerically low-pass filtered (butterworth 4th order filter) at 800 Hz and the fingertip position signal was differentiated with respect to time to estimate the exploration velocity. The software package Matlab (version 7.10) was used to process force as well as fingertip position data.

We firstly computed the mean velocity, v, the mean tangential force component, fT, as well as the mean normal force component, fN, per sample exploration and per participant. The values of the dynamic coefficient of friction, μ, were determined by dividing fT (of each sample exploration of each participant) by fN (of each sample exploration of each participant). Secondly, we computed the average values for all parameters over the ten explorations. To investigate the effect of surface topography, the analysis focused on the rapid fluctuations of the friction force, i.e. fT. Past virtual reality studies have shown that participants can identify complex textured surfaces on the basis of the tangential skin displacement only [5]. Since the finger oscillations resulting from scanning a surface depend on the surface's topography, it can be hypothesized that the fluctuations of the tangential force provide key information regarding the nature of the scanned surface. In the present study, the raw measurements of friction force were sampled in the temporal domain. Yet, we experience surfaces as spatially stable objects and not as time-encoded signals. In the aforementioned study [5], we showed that while people can identify surfaces based on spatially-encoded friction force fluctuations they are unable to do so when the same forces are encoded temporally.

We first wondered whether participants spontaneously adapted their exploration kinematics according to the surfaces being explored. The results of the corresponding RM-ANOVAs showed that participants neither significantly adapted their exploration velocity, v, nor significantly modified the normal force, fN, according to the surface being scanned (Table 2). Each participant adopted a preferred exploration strategy which was by-and-large the same for all surfaces being explored. The results of the Spearman correlation analysis were in line with this observation. Neither v nor fN was significantly correlated with the pleasantness levels of the surfaces (Table 2).

We have described some physical factors that are correlated with the sensation of pleasantness during active surface exploration with the fingertip. These factors are the average coefficient of friction (μ), the average magnitude of the tangential interaction force component (fT) as well as the offset (β) and the decay coefficient (α) of friction-induced vibrations in the spatial domain.

The purpose of this study was to examine how contact forces normal to the skin surface and shear forces tangential to the skin surface are deployed during tactile exploration of a smooth surface in search of a tactile target. Six naive subjects participated in two experiments. In the first experiment, the subjects were asked to explore a series of unseen smooth plastic surfaces by using the index finger to search for either a raised or recessed target. The raised targets were squares with a height of 280 µm above the background surface and that varied in side lengths from 0.2 mm to 8.0 mm. A second series of smooth plastic surfaces consisted of small recessed squares (side lengths: 2.0, 3.0, 4.0 and 8.0 mm) that were etched to a depth of 620 µm. Although made of an identical material, the plastic substrate had a lower coefficient of friction against the skin because only the recessed square had been subjected to the electrolytic etching process. The surfaces were mounted on a six-axes force and torque sensor connected to a laboratory computer. From the three axes of linear force, the computer was able to calculate the instantaneous position of the index finger and the instantaneous tangential force throughout the exploratory period. When exploring for the raised squares, the subjects maintained a relatively constant, average normal force of about 0.49 N with an average exploration speed of 8.6 cm/s. In contrast, all subjects used a significantly higher average normal force (0.64 N) and slightly slower mean exploration speed (7.67 cm/s) when searching for the small recessed squares. This appeared to be an attempt to maximize the amount of skin penetrating the recessed squares to improve the probability of target detection. In a second experiment, subjects were requested to search for an identical set of raised squares but with the fingertip having been coated with sucrose to impede the scanning movement by increasing the friction. Overall, the subjects maintained the same constant normal force that they used on the uncoated surface. However, they increased the tangential force significantly. The similarity of the search strategy employed by all subjects supports the hypothesis that shear forces on the skin provide a significant stimulus to mechanoreceptors in the skin during tactile exploration. Taken together, these data suggest that, in active tactile exploration with the fingertip, the tangential finger speed, the normal contact force, and the tangential shear force are adjusted optimally depending on the surface friction and whether the target is a raised asperity or a recessed indentation. 041b061a72


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