Biology subjects

Fuchs, X.

Publications and source records attributed to Fuchs, X..

7 recordsLinked to original sources

bioRxiv2026

From Head to Toe: Efficient Somatosensory Mapping with Fast Stimulation and Multivariate Pattern Analysis

BackgroundSomatosensory evoked potentials (SEPs) measured with electroencephalography (EEG) are widely used to study cortical responses to touch but most research has limited the focus on few body parts, typically a finger, and applied time-consuming testing protocols. Multivariate pattern analysis (MVPA) provides a complementary approach that may increase sensitivity and allow faster stimulation, yet its relationship to classical SEP analysis in somatosensory research remains largely unexplored. MethodsFifteen participants received vibrotactile stimulation on the finger, hand, cheek, and foot while EEG was recorded. We compared a traditional "slow" stimulation protocol (800-1200 ms inter-stimulus intervals) with a "fast" protocol (300-500 ms). We compared temporal and topographical aspects between SEP and MVPA. ResultsBoth stimulation protocols produced highly similar SEP components (P100, N140, P200), topographies, and classification results, while the fast protocol reduced testing time by about 60%. SEPs revealed systematic body-part differences, with earlier components for cheek stimulation and delayed responses for the foot. Multivariate classification distinguished body parts with accuracies up to [~]50-55% (chance: 25%), peaking around 100 ms after stimulus onset. Classifier weight maps closely matched SEP topographies over centroparietal electrodes, indicating that classification relied on physiologically meaningful somatosensory signals. Classification accuracy peaked around 100 ms after stimulus onset, coinciding with the SEP P100 component, but declined gradually thereafter, suggesting that early somatosensory responses contain particularly informative multivariate patterns that generalize over time. ConclusionsFaster stimulation protocols substantially increase efficiency without compromising interpretability. Combining classical SEP analysis with multivariate classification provides complementary insights and offers a powerful framework for mapping somatosensory representations across the body.

neuroscience↗

bioRxiv2024

Limb, not touch location, is coded in 3D space

Reduced tactile-spatial accuracy with crossed versus uncrossed limbs is considered key evidence for automatic remapping of touch location from skin-based to 3D-spatial codes. Yet, typical tasks use only few stimuli, potentially confounding categorical choice of a touched limb with precise stimulus location. Over five experiments, participants judged whether two touches occurred on the same limb (anatomical) or on the same side of space (external-spatial). Crossing consistently impaired performance, but more under external-spatial than anatomical instructions - contrary to what automatic remapping predicts. Limb assignment errors occurred equally frequent for arm-leg as arm-arm pairs, implying confusion about limb identity, not spatial position. With mouse-based, precise localization, crossing again impaired limb assignment, mainly for stimuli distal to the crossing point, whereas precision was unaffected: localization scatter did not increase, and showed no bias toward the limb's usual side of space. Crossing effects therefore emerge from choosing a limb, not from localizing touch. We propose that they reflect limb assignment, with touch cueing the region of space it is habitually associated with, followed by selecting a limb whose habitual or current posture occupies that region, constrained by which limbs are task-relevant; precise localization is performed afterwards on the selected limb.

neuroscience↗

bioRxiv2023

The role of kinesthetic and visuospatial cues in pain-induced movement avoidance

BackgroundAvoidance of movements is an important factor in chronic pain. Previous experiments have investigated the involved learning mechanisms by pairing movements with painful stimuli but, usually, other visuospatial cues are concurrently presented during learning. Therefore, participants might primarily avoid these visuospatial rather than the movement-related cues, potentially invalidating related interpretations of pain-induced movement avoidance. Here, we separated kinesthetic from visuospatial cues to investigate their respective contribution to avoidance learning. MethodsParticipants used a hand-held robotic manipulandum and, during an acquisition phase, received painful stimuli when performing center-out movements. Pain stimuli could be avoided by choosing a curved rather than direct movement trajectories. To distinguish the contribution of kinesthetic vs. visuospatial cues we used two generalization contexts: either participants executed novel movements passing through the same location at which pain had previously been presented in the acquisition phase; or they executed the same pain-associated movements after having been reseated, so that the hand did not pass through the pain-associated location. ResultsAvoidance generalization was comparable in both contexts, and remarkably, highly correlated between them. Our findings suggest that both visuospatial and kinesthetic cues available during acquisition were associated with pain and led to avoidance. ConclusionsOur research corroborates the fear-avoidance pain model and previous studies findings that pain can become associated with movements. However, our study indicates that visuospatial cues also play a critical role. Future studies should distinguish movement-related and space-related associations in pain learning. SignificanceChronic pain is a significant health issue typically attributed to maladaptive learning of pain-movement associations and movement avoidance. We demonstrate that visual cues can play a similarly important role as movement cues in pain learning. This aspect has not previously been considered and has likely confounded previous research findings.

neuroscience↗

bioRxiv2022

Modulation of implicitly perceived hand size by visuotactile recalibration

When concurrent visual and tactile stimuli are repeatedly presented with a spatial offset, even unisensory tactile stimuli are afterwards perceived with a spatial bias towards the previously presented visual stimuli. This so-called visuotactile ventriloquism aftereffect reflects visuotactile recalibration. It is unknown whether this recalibration occurs within a bodily map and interacts with perceived features like shape and size of body parts. Here, we applied tactile stimuli to participants hidden left hand and simultaneously presented visual stimuli with spatial offsets that - if integrated with the tactile stimuli - implied an enlarged hand size. We either used a fixed spatial mapping between tactile and visual positions ("congruent"), or a scrambled ("incongruent") mapping. We assessed implicitly perceived hand size via two independent behavioral assessments: pointing movements to unisensory tactile stimuli and tactile distance judgments. Moreover, we assessed explicitly perceived change in hand size with perceptual self-reports. Especially after congruent recalibration, participants localized unimodal tactile stimuli as if they were aiming at an enlarged hand. They also reported tactile distance as shorter after congruent than incongruent recalibration. These modulations resemble those obtained after using tools that prolong the arm and extend reaching space; they suggest that recalibration affected a common, implicit hand representation that underlies both tasks. In contrast, explicit perceptual self-reports did not differ significantly between congruent and incongruent recalibration. Thus, simple visuotactile stimuli are sufficient to modify implicitly perceived body size, indicating a tight link of low-level multisensory processes such as the visuotactile ventriloquism aftereffect and body representation.

neuroscience↗

bioRxiv2021

Illusory tactile movement crosses arms and legs and is coded in external space

Humans often misjudge where on the body a touch occurred. Theoretical accounts have ascribed such misperceptions to local interactions in peripheral and primary somatosensory neurons, positing that spatial-perceptual mechanisms adhere to limb boundaries and skin layout. Yet, perception often reflects integration of sensory signals with prior experience. On their trajectories, objects often touch multiple limbs; therefore, body-environment interactions should manifest in perceptual mechanisms that reflect external space. Here, we demonstrate that humans perceived the cutaneous rabbit illusion - the percept of multiple identical stimuli as hopping across the skin - along the Euclidian trajectory between stimuli on two body parts and regularly mislocalized stimuli from one limb to the other. A Bayesian model based on Euclidian, as opposed to anatomical, distance faithfully reproduced key aspects of participants localization behavior. Our results suggest that prior experience of touch in space critically shapes tactile spatial perception and illusions beyond anatomical organization.

neuroscience↗

bioRxiv2019

Online sensory feedback during active search improves tactile localization

Many natural behaviors involve closed feedback loops in which ongoing sensory input refines motor behavior. Previous research on tactile localization, however, has implemented localization as open-loop behavior. For instance, participants indicate a touched position on a silhouette shape of the body or on an occluding board mounted above the hand. Such studies have suggested that humans often make large errors when localizing touch on the skin, or that \"perceptual body representations\" are distorted. However, by artificially preventing tactile feedback from the target body area, the natural action-perception loop is interrupted. Therefore, these localization approaches may underestimate individuals localization ability and draw erroneous conclusions about the role and precision of body representations. Here, we tested tactile localization in a natural setting, in which participants first received brief touches on their left forearm and then searched for the target location by moving the right index finger across the skin. Tactile search reduced localization error when the searching finger was far from, but not when it was near the target, resulting in a remaining error of 1-2 cm. Error reduction was absent when participants searched on an acrylic barrier mounted above the arm, suggesting that availability of tactile feedback on the target arm but not proprioceptive and motor signals of the searching arm determined precision, thus confirming the pivotal role of closed-loop sensory feedback for tactile localization. We suggest that actively produced online tactile feedback routinely refines coarse spatial body representations, similar to the refinement of sparse spatial representations in visual memory through consecutive saccades.

neuroscience↗

bioRxiv2019

Phantom limb pain intensity is associated with generalized hyperalgesia

After limb amputation, most amputees suffer from phantom limb pain (PLP). The mechanisms underlying this condition are complex and insufficiently understood. Altered somatosensory sensitivity (either heightened or lowered) might contribute to PLP. Recent studies have tested this assumption but mainly focused on the residual limb. However, altered somatosensation in PLP might be generalized. In this study, we applied quantitative sensory testing to 37 unilateral upper-limb amputees (23 with PLP, 14 without PLP) and 19 healthy controls. We assessed thresholds to heat pain (HPT), pressure pain (PPT), warmth detection (WDT), and two-point discrimination (2PDT) at the residual limb, a homologous point and the thenar of the intact limb, and both corners of the mouth. We did not find significant differences in any of the thresholds between the groups. However, higher PLP intensity was significantly related to lower HPT at all measured body sites except for the residual limb. At the residual limb, lower HPT were observed in more distal amputations and in amputees showing a higher degree of prosthesis use. Although WDT did by itself not significantly correlate with PLP intensity at any of the body sites, multiple regression analysis showed the highest multiple correlations with PLP intensity for a combination of high WDT and low HPT at the corners of the mouth. In this model, the combination of HPT and WDT shared 58% of the variance with PLP intensity. Other factors of potential importance, especially residual limb pain, were not significantly associated to any sensory threshold. We conclude that the intensity, but not the presence of PLP is positively associated with higher heat pain sensitivity. Since this association was observed at various, distributed body sites, we suggest that central mechanisms might be underlying such generalized hyperalgesia.

neuroscience↗