Anne Klöcker

Physical Factors Influencing Pleasant Touch during Tactile Exploration

Background When 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. Results 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. Conclusion Characteristics of the frictional force fluctuations and of the net friction taking place during scanning can reliably be correlated with the pleasantness sensation of surfaces.

Rasch-built measure of pleasant touch through active fingertip exploration.

Background: Evidence suggests that somatic sensation has a modality for pleasant touch. Objective: To investigate pleasant touch at the fingertip level (i.e., glabrous skin site) through the elaboration of a linear unidimensional scale that measures (i) various materials according to the level of pleasantness they elicit through active fingertip explorations and (ii) subjects according to their pleasantness leniency levels. Subjects: We enrolled 198 healthy subjects without any neurological disease. Methods: Blindfolded subjects actively explored 48 materials with their index fingertips and reported the perceived pleasantness of each on a 4-level scale. The fingertip moisture levels on each subject were measured before the experimental session. Data were analyzed using the Rasch model. Results: We elaborated unidimensional linear scale that included 37 materials according to their pleasantness of touch. The pleasantness level of 21 materials was perceived differently, depending on the fingertip moisture levels of the subjects. Conclusion: Based on our findings, we formulated a Pleasant Touch Scale. Fingertip moisture levels appeared to be a major factor for (un)pleasant feelings during active exploration.

Nociceptive Local Field Potentials Recorded from the Human Insula Are Not Specific for Nociception

The insula, particularly its posterior portion, is often regarded as a primary cortex for pain. However, this interpretation is largely based on reverse inference, and a specific involvement of the insula in pain has never been demonstrated. Taking advantage of the high spatiotemporal resolution of direct intracerebral recordings, we investigated whether the human insula exhibits local field potentials (LFPs) specific for pain. Forty-seven insular sites were investigated. Participants received brief stimuli belonging to four different modalities (nociceptive, vibrotactile, auditory, and visual). Both nociceptive stimuli and non-nociceptive vibrotactile, auditory, and visual stimuli elicited consistent LFPs in the posterior and anterior insula, with matching spatial distributions. Furthermore, a blind source separation procedure showed that nociceptive LFPs are largely explained by multimodal neural activity also contributing to non-nociceptive LFPs. By revealing that LFPs elicited by nociceptive stimuli reflect activity unrelated to nociception and pain, our results confute the widespread assumption that these brain responses are a signature for pain perception and its modulation.

Gamma-band oscillations preferential for nociception can be recorded in the human insula

Abstract Transient nociceptive stimuli elicit robust phase‐locked local field potentials (LFPs) in the human insula. However, these responses are not preferential for nociception, as they are also elicited by transient non‐nociceptive vibrotactile, auditory, and visual stimuli. Here, we investigated whether another feature of insular activity, namely gamma‐band oscillations (GBOs), is preferentially observed in response to nociceptive stimuli. Although nociception‐evoked GBOs have never been explored in the insula, previous scalp electroencephalography and magnetoencephalography studies suggest that nociceptive stimuli elicit GBOs in other areas such as the primary somatosensory and prefrontal cortices, and that this activity could be closely related to pain perception. Furthermore, tracing studies showed that the insula is a primary target of spinothalamic input. Using depth electrodes implanted in 9 patients investigated for epilepsy, we acquired insular responses to brief thermonociceptive stimuli and similarly arousing non‐nociceptive vibrotactile, auditory, and visual stimuli (59 insular sites). As compared with non‐nociceptive stimuli, nociceptive stimuli elicited a markedly stronger enhancement of GBOs (150‐300 ms poststimulus) at all insular sites, suggesting that this feature of insular activity is preferential for thermonociception. Although this activity was also present in temporal and frontal regions, its magnitude was significantly greater in the insula as compared with these other regions.

Peripheral vs. central determinants of vibrotactile adaptation

Long-lasting mechanical vibrations applied to the skin induce a reversible decrease in the perception of vibration at the stimulated skin site. This phenomenon of vibrotactile adaptation has been studied extensively, yet there is still no clear consensus on the mechanisms leading to vibrotactile adaptation. In particular, the respective contributions of 1) changes affecting mechanical skin impedance, 2) peripheral processes, and 3) central processes are largely unknown. Here we used direct electrical stimulation of nerve fibers to bypass mechanical transduction processes and thereby explore the possible contribution of central vs. peripheral processes to vibrotactile adaptation. Three experiments were conducted. In the first, adaptation was induced with mechanical vibration of the fingertip (51- or 251-Hz vibration delivered for 8 min, at 40× detection threshold). In the second, we attempted to induce adaptation with transcutaneous electrical stimulation of the median nerve (51- or 251-Hz constant-current pulses delivered for 8 min, at 1.5× detection threshold). Vibrotactile detection thresholds were measured before and after adaptation. Mechanical stimulation induced a clear increase of vibrotactile detection thresholds. In contrast, thresholds were unaffected by electrical stimulation. In the third experiment, we assessed the effect of mechanical adaptation on the detection thresholds to transcutaneous electrical nerve stimuli, measured before and after adaptation. Electrical detection thresholds were unaffected by the mechanical adaptation. Taken together, our results suggest that vibrotactile adaptation is predominantly the consequence of peripheral mechanoreceptor processes and/or changes in biomechanical properties of the skin.

Habituation of phase-locked local field potentials and gamma-band oscillations recorded from the human insula

Salient nociceptive and non-nociceptive stimuli elicit low-frequency local field potentials (LFPs) in the human insula. Nociceptive stimuli also elicit insular gamma-band oscillations (GBOs), possibly preferential for thermonociception, which have been suggested to reflect the intensity of perceived pain. To shed light on the functional significance of these two responses, we investigated whether they would be modulated by stimulation intensity and temporal expectation – two factors contributing to stimulus saliency. Insular activity was recorded from 8 depth electrodes (41 contacts) implanted in the left insula of 6 patients investigated for epilepsy. Thermonociceptive, vibrotactile, and auditory stimuli were delivered using two intensities. To investigate the effects of temporal expectation, the stimuli were delivered in trains of three identical stimuli (S1-S2-S3) separated by a constant 1-s interval. Stimulation intensity affected intensity of perception, the magnitude of low-frequency LFPs, and the magnitude of nociceptive GBOs. Stimulus repetition did not affect perception. In contrast, both low-frequency LFPs and nociceptive GBOs showed a marked habituation of the responses to S2 and S3 as compared to S1 and, hence, a dissociation with intensity of perception. Most importantly, although insular nociceptive GBOs appear to be preferential for thermonociception, they cannot be considered as a correlate of perceived pain.

Nociceptive local field potentials recorded from the human insula are not specific for nociception

Introduction. Insular lesions can alter pain perception, and direct electrical stimulation of the insula can generate pain-related sensations. Furthermore, direct intracerebral recordings have shown that nociceptive stimulation can elicit robust local field potentials (LFPs) in the insula, interpreted as reflecting activity specifically involved in the encoding of pain and temperature sensations. Aims. Taking advantage of the high spatial resolution of direct intracerebral recordings performed in humans, our aim was to assess whether the insula exhibits responses that are specific to nociceptive stimulation. Methods. Six patients were investigated using depth electrodes implanted at different locations, comprising the anterior and posterior insula, for a total of 62 insular sites. Participants received brief stimuli belonging to each of the following four modalities: nociceptive laser stimuli, non-nociceptive tactile stimuli, auditory stimuli, and visual stimuli. The stimuli were delivered in blocks, both on the right and on the left side of the body. Results. All four types of stimuli elicited consistent LFPs in the posterior and anterior insula, appearing as large biphasic waves. The spatial distribution of the responses elicited by nociceptive stimulation at the different insular contacts was indistinguishable from the spatial distribution of the responses elicited by non-nociceptive tactile, auditory and visual stimulation. Conclusions Our results indicate that, in both the posterior and the anterior insula, LFPs elicited by transient nociceptive stimuli reflect cortical activities that are unspecific for pain. Importantly, this conclusion is not incompatible with the possible involvement of the insula in pain perception.