Categorical perception of tactile distance

The tactile surface forms a continuous sheet covering the body. And yet, the perceived distance between two touches varies across stimulation sites. Perceived tactile distance is larger when stimuli cross over the wrist, compared to when both fall on either the hand or the forearm. This effect could reflect a categorical distortion of tactile space across body-part boundaries (in which stimuli crossing the wrist boundary are perceptually elongated) or may simply reflect a localised increased in acuity surrounding anatomical landmarks (in which stimuli near the wrist are perceptually elongated). We tested these two interpretations, by comparing a well-documented bias to perceive mediolateral tactile distances across the forearm/hand as larger than proximodistal ones along the forearm/hand at three different sites (hand, wrist, and forearm). According to the ‘categorical’ interpretation, tactile distances should be elongated selectively in the proximodistal axis thus reducing the anisotropy. According to the ‘localised acuity’ interpretation, distances will be perceptually elongated in the vicinity of the wrist regardless of orientation, leading to increased overall size without affecting anisotropy. Consistent with the categorical account, we found a reduction in the magnitude of anisotropy at the wrist, with no evidence of a corresponding specialized increase in precision. These findings demonstrate that we reference touch to a representation of the body that is categorically segmented into discrete parts, which consequently influences the perception of tactile distance

Mapping quantal touch using 7 Tesla functional magnetic resonance imaging and single-unit intraneural microstimulation.

Using ultra-high field 7 Tesla (7T) functional magnetic resonance imaging (fMRI), we map the cortical and perceptual responses elicited by intraneural microstimulation (INMS) of single mechanoreceptive afferent units in the median nerve, in humans. Activations are compared to those produced by applying vibrotactile stimulation to the unit's receptive field, and unit-type perceptual reports are analyzed. We show that INMS and vibrotactile stimulation engage overlapping areas within the topographically appropriate digit representation in the primary somatosensory cortex. Additional brain regions in bilateral secondary somatosensory cortex, premotor cortex, primary motor cortex, insula and posterior parietal cortex, as well as in contralateral prefrontal cortex are also shown to be activated in response to INMS. The combination of INMS and 7T fMRI opens up an unprecedented opportunity to bridge the gap between first-order mechanoreceptive afferent input codes and their spatial, dynamic and perceptual representations in human cortex

Edge orientation signals in tactile afferents of macaques

The orientation of edges indented into the skin has been shown to be encoded in the responses of neurons in primary somatosensory cortex in a manner that draws remarkable analogies to their counterparts in primary visual cortex. According to the classical view, orientation tuning arises from the integration of untuned input from thalamic neurons with aligned but spatially displaced receptive fields (RFs). In a recent microneurography study with human subjects, the precise temporal structure of the responses of individual mechanoreceptive afferents to scanned edges was found to carry information about their orientation. This putative mechanism could in principle contribute to or complement the classical rate-based code for orientation. In the present study, we further examine orientation information carried by mechanoreceptive afferents of Rhesus monkeys. To this end, we record the activity evoked in cutaneous mechanoreceptive afferents when edges are indented into or scanned across the skin. First, we confirm that information about the edge orientation can be extracted from the temporal patterning in afferent responses of monkeys, as is the case in humans. Second, we find that while the coarse temporal profile of the response can be predicted linearly from the layout of the RF, the fine temporal profile cannot. Finally, we show that orientation signals in tactile afferents are often highly dependent on stimulus features other than orientation, which complicates putative decoding strategies. We discuss the challenges associated with establishing a neural code at the somatosensory periphery, where afferents are exquisitely sensitive and nearly deterministic

Differential effects of radiant and mechanically applied thermal stimuli on human C-tactile afferent firing patterns

International audienceC-tactile (CT) afferents respond to gentle tactile stimulation, but only a handful of studies in humans and animals have investigated whether their firing is modified by temperature. We describe the effects of radiant thermal stimuli, and of stationary and very slowly moving mechanothermal stimuli, on CT afferent responses. We find that CT afferents are primarily mechanoreceptors, as they fired little during radiant thermal stimuli, but they exhibited different patterns of firing during combined mechano-cool stimulation compared with warming. CTs fired optimally to gentle, very slowly moving, or stationary mechanothermal stimuli delivered at neutral temperature (~32°C, normal skin temperature), but they responded with fewer spikes (median 67% decrease) and at significantly lower rates (47% decrease) during warm (~42°C) tactile stimuli. During cool tactile stimuli (~18°C), their mean instantaneous firing frequency significantly decreased by 35%, but they often fired a barrage of afterdischarge spikes at a low frequency (~5 Hz) that outlasted the mechanical stimulus. These effects were observed under a variety of stimulus conditions, including during stationary and slowly moving touch (0.1 cm/s), and we complemented these tactile approaches using a combined electrical-thermal stimulation experiment where we found a suppression of spiking during warming. Overall, CT afferents are exquisitely sensitive to tactile events, and we show that their firing is modulated with touch temperatures above and below neutral skin temperature. Warm touch consistently decreased their propensity to fire, whereas cool touch produced lower firing rates but afterdischarge spiking

Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress

The differences in the degree of sympathetic nerve activity (SNA) over cutaneous blood vessels, although known to be more prominent in the periphery than the core vasculature, has not been thoroughly investigated quantitatively. Hence, two studies were carried out to investigate the differences in SNA between the periphery and the core during the cold pressor test (CPT) (right-hand immersion in ice water) and cold exposure (whole body exposed to cold air) using photoplethysmography (PPG). Two methods utilizing PPG, namely differential multi-site PTT measurements and low-frequency spectral analysis were explored for quantitative determination of SNA. Each study involved 12 healthy volunteers, and PPG signals were acquired from the right index finger (RIF), left index finger (LIF) (periphery) and the ear canal (core). During CPT, Pulse Transit Time (PTT) was measured to the respective locations and the mean percentage change in PTT during ice immersion at each location was used as an indicator for the extent of SNA. During cold exposure, the low-frequency spectral analysis was performed on the acquired raw PPGs to extract the power of the sympathetic [low-frequency (LF): 0.04–0.15 Hz] and parasympathetic components [high-frequency (HF): 0.15–0.4 Hz]. The ratio of LF/HF components was then used to quantify the differences in the influence of SNA on the peripheral and core circulation. PTT measured from the EC, and the LIF has dropped by 5 and 7%, respectively during ice immersion. The RIF PTT, on the other hand, has dropped significantly (P < 0.05) by 12%. During the cold exposure, the LF/HF power ratio at the finger has increased to 86.4 during the cold exposure from 19.2 at the baseline (statistically significant P = 0.002). While the ear canal LF/HF ratio has decreased to 1.38 during the cold exposure from 1.62 at baseline (P = 0.781). From these observations, it is evident that differential PTT measurements or low-frequency analysis can be used to quantify SNA. The results also demonstrate the effectiveness of the central auto-regulation during both short and long-term stress stimulus as compared to the periphery

Intraneural stimulation elicits discrimination of textural features by artificial fingertip in intact and amputee humans.

Restoration of touch after hand amputation is a desirable feature of ideal prostheses. Here, we show that texture discrimination can be artificially provided in human subjects by implementing a neuromorphic real-time mechano-neuro-transduction (MNT), which emulates to some extent the firing dynamics of SA1 cutaneous afferents. The MNT process was used to modulate the temporal pattern of electrical spikes delivered to the human median nerve via percutaneous microstimulation in four intact subjects and via implanted intrafascicular stimulation in one transradial amputee. Both approaches allowed the subjects to reliably discriminate spatial coarseness of surfaces as confirmed also by a hybrid neural model of the median nerve. Moreover, MNT-evoked EEG activity showed physiologically plausible responses that were superimposable in time and topography to the ones elicited by a natural mechanical tactile stimulation. These findings can open up novel opportunities for sensory restoration in the next generation of neuro-prosthetic hands

Roughness Encoding in Human and Biomimetic Artificial Touch: Spatiotemporal Frequency Modulation and Structural Anisotropy of Fingerprints

The influence of fingerprints and their curvature in tactile sensing performance is investigated by comparative analysis of different design parameters in a biomimetic artificial fingertip, having straight or curved fingerprints. The strength in the encoding of the principal spatial period of ridged tactile stimuli (gratings) is evaluated by indenting and sliding the surfaces at controlled normal contact force and tangential sliding velocity, as a function of fingertip rotation along the indentation axis. Curved fingerprints guaranteed higher directional isotropy than straight fingerprints in the encoding of the principal frequency resulting from the ratio between the sliding velocity and the spatial periodicity of the grating. In parallel, human microneurography experiments were performed and a selection of results is included in this work in order to support the significance of the biorobotic study with the artificial tactile system

Welcoming back my arm: Affective touch increases body ownership following right hemisphere stroke

© The Author(s) (2020). Published by Oxford University Press on behalf of the Guarantors of Brain.Right-hemisphere stroke can impair the ability to recognize one's contralesional body parts as belonging to one's self. The study of this so-called 'disturbed sense of limb ownership' can provide unique insights into the neurocognitive mechanisms of body ownership. In this study, we address a hypothesis built upon experimental studies on body ownership in healthy volunteers. These studies have shown that affective (pleasant) touch, an interoceptive modality associated with unmyelinated, slow-conducting C-tactile afferents, has a unique role in the sense of body ownership. In this study, we systematically investigated whether affective touch stimulation could increase body ownership in patients with a disturbed sense of limb ownership following right-hemisphere stroke. An initial feasibility study in 16 adult patients with acute stroke enabled us to optimize and calibrate an affective touch protocol to be administered by the bedside. The main experiment, conducted with a different sample of 26 right hemisphere patients, assessed changes in limb ownership elicited following self- (patient) versus other- (experimenter) generated tactile stimulation, using a velocity known to optimally activate C-tactile fibres (i.e. 3 cm/s), and a second velocity that is suboptimal for C-tactile activation (i.e. 18 cm/s). We further examined the specificity and mechanism of observed changes in limb ownership in secondary analyses looking at (i) the influence of perceived intensity and pleasantness of touch, (ii) touch laterality and (iii) level of disturbed sense of limb ownership on ownership change and (iv) changes in unilateral neglect arising from touch. Findings indicated a significant increase in limb ownership following experimenter-administered, C-tactile-optimal touch. Voxel-based lesion-symptom mapping identified damage to the right insula and, more substantially, the right corpus callosum, associated with a failure to increase body ownership following experimenter-administered, affective touch. Our findings suggest that affective touch can increase the sense of body-part ownership following right-hemisphere stroke, potentially due to its unique role in the multisensory integration processes that underlie the sense of body ownership.Peer reviewedFinal Accepted Versio

A Better Touch: C-tactile Fibers Related Activity is Associated to Pain Reduction During Temporal Summation of Second Pain

C-tactile (CT) fibers, responsible for the so-called “affective” touch (AT), have drawn a fair amount of attention within the scientific community for their marked social dimension. However, while the pain-relieving potential of discriminative touch (DT) has been documented, proofs of the analgesic properties of AT are still scarce. Additionally, no study has so far tested its possible pain-relieving effect on a clinically-relevant model. Temporal summation of second pain (TSSP), otherwise referred to as “wind-up,” relies on repetitive stimulation of C-nociceptors and it is thought to reflect central sensitization, a process linked to many chronic pain conditions. In the present experimental, within participants, design we induced TSSP through trains of ascending and descending repetitive heat stimulation. Forty-two healthy participants’ pain was measured during 2 different tactile stimulations (stroking velocities AT: 10 cm/s; DT: 0.3 cm/s) or without concomitant tactile input. Since measures of pleasantness of the tactile stimulation have been found to strongly correlate with C-tactile fibers’ firing rate, these, together with participants’ body awareness, were also taken into account. Our results show that AT brought about a decrease of our participants’ pain as opposed to both DT and no touch, while DT did not produce any significant pain reduction. Thus, only AT successfully modulated wind-up. As expected, AT was perceived as more pleasant than DT, while a clear relationship between body awareness and pain was found only during DT. Targeting CT fibers could pave the way to new treatments for chronic pain conditions whose aetiology depend on abnormal C-nociceptors’ physiology. Perspective: This study extends previous findings on the analgesic potential of affective touch, documenting a clear pain reduction during temporal summation of second pain (TSSP). Since TSSP is thought to reflect central sensitization, the psychophysiological mechanisms of affective touch could be exploited for new chronic pain treatments

On the functional anatomy of the urge-for-action

Several common neuropsychiatric disorders (e.g., obsessive-compulsive disorder, Tourette syndrome (TS), autistic spectrum disorder) are associated with unpleasant bodily sensations that are perceived as an urge for action. Similarly, many of our everyday behaviors are also characterized by bodily sensations that we experience as urges for action. Where do these urges originate? In this paper, we consider the nature and the functional anatomy of “urges-for-action,” both in the context of everyday behaviors such as yawning, swallowing, and micturition, and in relation to clinical disorders in which the urge-for-action is considered pathological and substantially interferes with activities of daily living (e.g., TS). We review previous frameworks for thinking about behavioral urges and demonstrate that there is considerable overlap between the functional anatomy of urges associated with everyday behaviors such as swallowing, yawning, and micturition, and those urges associated with the generation of tics in TS. Specifically, we show that the limbic sensory and motor regions—insula and mid-cingulate cortex—are common to all of these behaviors, and we argue that this “motivation-for-action” network should be considered distinct from an “intentional action” network, associated with regions of premotor and parietal cortex, which may be responsible for the perception of “willed intention” during the execution of goal-directed actions