The influence of hand posture on tactile processing: Evidence from a 7T functional magnetic resonance imaging study.

Although behavioral evidence has shown that postural changes influence the ability to localize or detect tactile stimuli, little is known regarding the brain areas that modulate these effects. This 7T functional magnetic resonance imaging (fMRI) study explores the effects of touch of the hand as a function of hand location (right or left side of the body) and hand configuration (open or closed). We predicted that changes in hand configuration would be represented in contralateral primary somatosensory cortex (S1) and the anterior intraparietal area (aIPS), whereas change in position of the hand would be associated with alterations in activation in the superior parietal lobule. Multivoxel pattern analysis and a region of interest approach partially supported our predictions. Decoding accuracy for hand location was above chance level in superior parietal lobule (SPL) and in the anterior intraparietal (aIPS) area; above chance classification of hand configuration was observed in SPL and S1. This evidence confirmed the role of the parietal cortex in postural effects on touch and the possible role of S1 in coding the body form representation of the hand.

Examining constraints on embodiment using the Anne Boleyn illusion.

Using a mirror box, the concurrent stroking of the lateral side of the fifth finger behind the mirror along with stroking the empty space next to the mirror-reflected hand's fifth finger results in a strong sense of having a sixth finger-the Anne Boleyn illusion. We used this illusion to understand what constraints illusory embodiment. In Experiment 1, we manipulated the anatomical constraints, posture, and stroking of the sixth finger, along with other variants. Given evidence from other body illusions, we predicted no illusory embodiment in conditions in which the sixth finger was created in a manner incompatible with a typical hand, when the mirror and viewed hands were in different posture, and when stroking differed. Surprisingly, the illusion was persistent in most variants, including those with curved fingers, elongated fingers, and even with mismatches between the posture of the viewed and hidden hand. In Experiment 2, we manipulated the orientation, shape, and length of the illusory sixth finger, presenting more extreme versions of the illusion. The illusion was significantly diminished only when the sixth finger was far from the hand, or in a very implausible posture. This evidence suggests that body representations are extremely flexible and allow for embodiment of empty space in conditions not seen in other body illusions. We suggest that bottom-up information from concurrent visuotactile input, combined with reduced constraints provided by the "blank canvas" of empty space, results in a particularly robust illusion. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Body image and perception among adults with and without phantom limb pain.

BACKGROUND: Following lower-limb amputation, phantom limb pain (i.e., pain perceived as coming from the amputated portion of the limb) is common. Phantom limb pain may be associated with impaired body image and perception, which may be targets for rehabilitative intervention. OBJECTIVE: To compare measures of body image and perception between adults with and without phantom limb pain post amputation and evaluate associations between measures of body image and perception and phantom limb pain. DESIGN: Survey. SETTING: Online, remote assessment. PARTICIPANTS: Seventy-two adults ≥1 year post unilateral lower-limb loss (n = 42 with phantom limb pain, n = 30 without phantom limb pain or pain in the remaining portion of the limb). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Self-reported outcome measures assessing body image (i.e., Amputee Body Image Scale-Revised), perceptual disturbances associated with the phantom limb (i.e., a modified Bath Complex Regional Pain Syndrome Body Perception Disturbance Scale), and prosthesis satisfaction (i.e., Trinity Amputation and Prosthesis Experience Scale) were administered; participants with phantom limb pain reported pain interference via the Brief Pain Inventory-Short Form. Between-group comparisons of self-reported outcome measure scores were conducted using Mann Whitney U or chi-square tests, as appropriate (a = .05). RESULTS: Compared to peers without phantom limb pain, adults with phantom limb pain reported more negative body image; increased phantom limb ownership, attention, and awareness; and reduced prosthesis satisfaction and embodiment (U = 175.50-364.00, p < .001 to .034). Disturbances in phantom limb perception (i.e., size, weight, pressure, temperature) were similar between groups (p = .086 to >.999). More negative body image was associated with increased phantom limb pain interference (τb  = .25, p = .026). CONCLUSIONS: Adults with phantom limb pain demonstrate more negative body image and hypervigilance of the phantom limb as compared to peers with nonpainful phantom sensations. Mind-body treatments that target impaired body image and perception may be critical interventions for adults with phantom limb pain.

Can they touch? A novel mental motor imagery task for the assessment of back pain.

Introduction: As motor imagery is informed by the anticipated sensory consequences of action, including pain, we reasoned that motor imagery could provide a useful indicator of chronic back pain. We tested the hypothesis that mental motor imagery regarding body movements can provide a reliable assessment of low back pain. Methods: Eighty-five subjects with back pain and forty-five age-matched controls were shown two names of body parts and asked to indicate if they could imagine moving so that the named body parts touched. Three types of imagined movements were interrogated: movements of arms, movements of legs and movements requiring flexion and/or rotation of the low back. Results: Accuracy and reaction times were measured. Subjects with back pain were less likely to indicate that they could touch body parts than age-matched controls. The effect was observed only for those movements that required movement of the low back or legs, suggesting that the effect was not attributable to task difficulty or non-specific effects. There was an effect of pain severity. Compared to subjects with mild pain, subjects with severe pain were significantly less likely to indicate that they could move so that named body parts touched. There was a correlation between pain ratings and impaired performance for stimuli that involved the lower but not upper body. Discussion: As the Can They Touch task is quick, easy to administer and does not require an explicit judgment of pain severity, it may provide useful information to supplement the assessment of subjects with chronic pain.

Understanding components of embodiment: Evidence from the mirror box illusion.

Past studies have examined embodiment in the rubber hand illusion, using principal components analysis (PCA) to identify factors from questionnaire responses during synchronous and asynchronous stroking. To better understand the phenomenology of embodiment, we used PCA in the mirror box illusion to examine performance across conditions that varied in movement synchrony to examine multisensory integration and movement type to vary the amount of multisensory congruence. We found three dissociable components in all conditions: embodiment, deafference and attentiveness. We also examined how these embodiment ratings varied across the four conditions. As hypothesized, embodiment ratings were highest for synchronous movement, with feelings of deafference highest for asynchronous movement. Furthermore, there was a movement by timing interaction, such that sliding resulted in greater differences in synchronous versus asynchronous ratings than tapping. These results suggest that embodiment or deafference can be changed as a function of the amount of multisensory congruence.

Body representation among adults with phantom limb pain: Results from a foot identification task.

BACKGROUND: Impaired body representation (i.e. disrupted body awareness or perception) may be a critical, but understudied, factor underlying phantom limb pain (PLP). This cross-sectional study investigated whether adults with lower-limb loss (LLL) and PLP demonstrate impaired body representation as compared to Pain-Free peers with and without LLL. METHODS: Participants (n = 41 adults with PLP, n = 27 Pain-Free peers with LLL, n = 39 Controls with intact limbs) completed an online foot identification task. Participants judged whether randomized images depicted left or right feet (i.e. left-right discrimination) as quickly as possible without limb movement. Using two Generalized Estimating Equations, effects of group, image characteristics (i.e. side, foot type, view, angle) and trial block (i.e. 1-4) were evaluated, with task response time and accuracy as dependent variables (a ≤ 0.050). RESULTS: Adults with PLP demonstrated slower and less accurate performance as compared to Controls with intact limbs (p = 0.018) but performed similarly to Pain-Free peers with LLL (p = 0.394). Significant three-way interactions of group, view and angle indicated between-group differences were greatest for dorsal-view images, but smaller and angle-dependent for plantar-view images. While all groups demonstrated significant response time improvements across blocks, improvements were greatest among adults with PLP, who also reported significant reductions in pain intensity. CONCLUSIONS: Adults with PLP demonstrate body representation impairments as compared to Controls with intact limbs. Body representation impairments, however, may not be unique to PLP, given similar performance between adults with and without PLP following LLL. SIGNIFICANCE: Following lower-limb loss, adults with phantom limb pain (PLP) demonstrate impaired body representation as compared to Controls with intact limbs, evidenced by slower response times and reduced accuracy when completing a task requiring mental rotation. Importantly, 80% of participants with pre-task PLP reported reduced pain intensity during the task, providing compelling evidence for future investigations into whether imagery-based, mind-body interventions have positive effects on PLP.

Examining central biases in somatosensory localization: Evidence from brain-damaged individuals.

How does the brain localize touch under conditions of uncertainty caused by brain damage? By testing single cases, previous work found mislocalization of touch toward the center of the hand. We investigated whether such central bias changes as a function of uncertainty in somatosensory system. Fifty-one brain-damaged individuals were presented with a tactile detection task to establish their tactile threshold, and a tactile localization task in which they localized suprathreshold stimuli presented at different locations on the hand. We predicted that with increased somatosensory uncertainty, indexed by higher detection thresholds, participants would more likely to localize the stimuli toward the center of the hand. Consistent with this prediction, participants' localization errors were biased towards the center of the hand and, importantly, this bias increased as detection threshold increased. These findings provide evidence that instead of showing random errors, uncertainty leads to systematic localization errors toward the center of the hand or the center of the stimulus distribution, which overlapped in the present study. We discuss these findings under different frameworks as potential mechanisms to explain biases in tactile localization subsequent to brain damage.

Mechanical Pain Sensitivity in Postamputation Pain.

OBJECTIVES: Postamputation, clinical markers of pain-related peripheral and central nervous system hypersensitivity remain understudied. This study aimed to identify whether adults with postamputation pain demonstrate greater pain sensitivity in primary (ie, amputated region) and secondary (ie, nonamputated region) sites, as compared with pain-free peers and controls with intact limbs. METHODS: Ninety-four participants postunilateral, transtibial amputation (59 with pain, 35 pain-free) and 39 controls underwent pain-pressure threshold (PPT) testing at 10 sites. Pain-pressure thresholds were normalized to sex-specific control data using Z score conversions. Normalized primary-site and secondary-site PPTs were compared between groups using multivariate analysis of variance (P<0.050). RESULTS: Compared with pain-free peers, adults with postamputation pain demonstrated reduced normalized PPTs across primary and secondary sites (mean difference=0.61-0.74, P=0.001 to 0.007). Compared with controls, adults with postamputation pain demonstrated reduced normalized PPTs (mean difference=0.52, P=0.026) only at primary sites. DISCUSSION: Adults with postamputation pain demonstrate greater amputated region pain sensitivity as compared with pain-free peers or controls with intact limbs, indicating peripheral sensitization persists even after limb healing. Secondary-site pain sensitivity was similar between controls and adults with postamputation pain, suggesting central nervous system hypersensitivity may not be ubiquitous with postamputation pain. Studies are needed to investigate mechanisms underlying pain sensitivity differences between adults with and without postamputation pain.

Visuoproprioceptive conflict in hand position biases tactile localization on the hand surface.

The location of touch can be represented in a somatotopic reference frame and, combined with proprioceptive information, in an external reference frame. There is evidence that body position influences where individuals feel touch on the skin surface, indicating that proprioceptive information affects tactile localization in a somatotopic reference frame. In conditions with visual and proprioceptive mismatch of body position, where do individuals feel touch on the body? We used the mirror box illusion to address this question. Participants placed 1 hand on each side of a mirror aligned with the body midline, such that the hand reflection in the mirror looked like the hand hidden behind the mirror. The illusion creates a spatial mismatch between the actual hidden hand position and where the participant perceives their hand to be (the mirror image location). Across three experiments, localization judgments on the hidden hand were consistently and systematically biased toward the actual hand position relative to the viewed hand position. These findings provide evidence that proprioceptive estimates of limb position influence tactile localization and are discussed in relation to two models of tactile localization. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Intact tactile detection yet biased tactile localization in a hand-centered frame of reference: Evidence from a dissociation.

We examined the performance of an individual with subcortical damage, but an intact somatosensory thalamocortical pathway, to examine the functional architecture of tactile detection and tactile localization processes. Consistent with the intact somatosensory thalamocortical pathway, tactile detection on the contralesional hand was well within the normal range. Despite intact detection, the individual demonstrated substantial localization biases. Across all localization experiments, he consistently localized tactile stimuli to the left side in space relative to the long axis of his hand. This was observed when the contralesional hand was palm up, palm down, rotated 90° relative to the trunk, and when making verbal responses. Furthermore, control experiments demonstrated that this response pattern was unlikely a motor response error. These findings indicate that tactile localization on the body is influenced by proprioceptive information specifically in a hand-centered frame of reference. Furthermore, this also provides evidence that aspects of tactile localization are mediated by pathways outside of the primary somatosensory thalamocortical pathway.

Impairments in action and perception after right intraparietal damage.

We examined visually-guided reaching and perception in an individual who underwent resection of a small tumor in right intraparietal sulcus (pIPS). In the first experiment, she reached to targets presented on a touch screen. Vision was occluded from reach onset on half of the trials, whereas on the other half she had vision during the entire reach. For visually-guided reaching, she demonstrated significantly more reach errors for targets left of fixation versus right of fixation. However, there were no hemispatial differences when reaching without vision. Furthermore, her performance was consistent for reaches with either hand, providing evidence that pIPS encodes location based on an eye-centered reference frame. Second, previous studies reported that optic ataxics are more accurate when reaching to remembered versus visible target locations. We repeated the first experiment, adding a five second delay between stimulus presentation and reach initiation. In contrast to prior reports, she was less accurate in delayed versus immediate reaching. Finally, we examined whether a small pIPS resection would disrupt visuospatial processing in a simple perceptual task. We presented two small circles in succession in either the same location or offset at varying distances, and asked whether the two circles were presented in the same or different position. She was significantly more impaired left of fixation compared to right of fixation, providing evidence for a perceptual deficit after a dorsal stream lesion.

External coding and salience in the tactile Simon effect.

Previous studies have demonstrated a tactile Simon effect in which stimulus codes are generated based on the stimulated hand, not on limb position in external space (the somatotopic Simon effect). However, given evidence from visual Simon effect studies demonstrating that multiple stimulus codes can be generated for a single stimulus, we examined whether multiple stimulus codes can be generated for tactile stimuli as well. In our first experiment using four stimulators (two on each side of the hand), we found novel evidence for a hand-centered Simon effect, along with the typical somatotopic Simon effect. Next, we examined whether the potential salience of these somatotopic codes could be reduced, by testing only one hand with two stimulators attached. In Experiments 2-4, we found a strong hand-centered Simon effect with a diminished somatotopic Simon effect, providing evidence that stimulus salience can change the weighting of somatosensory stimulus coding. Finally, we also found novel evidence that the hand-centered Simon effect is coded in external, not somatotopic, coordinates. Furthermore, the diminished somatotopic Simon effect when testing on one hand only provides evidence that salience is an important factor in modulating the tactile Simon effect.

Towards a unified perspective of object shape and motion processing in human dorsal cortex.

Although object-related areas were discovered in human parietal cortex a decade ago, surprisingly little is known about the nature and purpose of these representations, and how they differ from those in the ventral processing stream. In this article, we review evidence for the unique contribution of object areas of dorsal cortex to three-dimensional (3-D) shape representation, the localization of objects in space, and in guiding reaching and grasping actions. We also highlight the role of dorsal cortex in form-motion interaction and spatiotemporal integration, possible functional relationships between 3-D shape and motion processing, and how these processes operate together in the service of supporting goal-directed actions with objects. Fundamental differences between the nature of object representations in the dorsal versus ventral processing streams are considered, with an emphasis on how and why dorsal cortex supports veridical (rather than invariant) representations of objects to guide goal-directed hand actions in dynamic visual environments.

Magnifying the View of the Hand Changes Its Cortical Representation. A Transcranial Magnetic Stimulation Study.

Changes in the perceived size of a body part using magnifying lenses influence tactile perception and pain. We investigated whether the visual magnification of one's hand also influences the motor system, as indexed by transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs). In Experiment 1, MEPs were measured while participants gazed at their hand with and without magnification of the hand. MEPs were significantly larger when participants gazed at a magnified image of their hand. In Experiment 2, we demonstrated that this effect is specific to the hand that is visually magnified. TMS of the left motor cortex did not induce an increase of MEPs when participants looked at their magnified left hand. Experiment 3 was performed to determine if magnification altered the topography of the cortical representation of the hand. To that end, a 3 × 5 grid centered on the cortical hot spot (cortical location at which a motor threshold is obtained with the lowest level of stimulation) was overlaid on the participant's MRI image, and all 15 sites in the grid were stimulated with and without magnification of the hand. We confirmed the increase in the MEPs at the hot spot with magnification and demonstrated that MEPs significantly increased with magnification at sites up to 16.5 mm from the cortical hot spot. In Experiment 4, we used paired-pulse TMS to measure short-interval intracortical inhibition and intracortical facilitation. Magnification was associated with an increase in short-interval intracortical inhibition. These experiments demonstrate that the visual magnification of one's hand induces changes in motor cortex excitability and generates a rapid remapping of the cortical representation of the hand that may, at least in part, be mediated by changes in short-interval intracortical inhibition.

The role of the right superior temporal gyrus in stimulus-centered spatial processing.

Although emerging neuropsychological evidence supports the involvement of temporal areas, and in particular the right superior temporal gyrus (STG), in allocentric neglect deficits, the role of STG in healthy spatial processing remains elusive. While several functional brain imaging studies have demonstrated involvement of the STG in tasks involving explicit stimulus-centered judgments, prior rTMS studies targeting the right STG did not find the expected neglect-like rightward bias in size judgments using the conventional landmark task. The objective of the current study was to investigate whether disruption of the right STG using inhibitory repetitive transcranial magnetic stimulation (rTMS) could impact stimulus-centered, allocentric spatial processing in healthy individuals. A lateralized version of the landmark task was developed to accentuate the dissociation between viewer-centered and stimulus-centered reference frames. We predicted that inhibiting activity in the right STG would decrease accuracy because of induced rightward bias centered on the line stimulus irrespective of its viewer-centered or egocentric locations. Eleven healthy, right-handed adults underwent the lateralized landmark task. After viewing each stimulus, participants had to judge whether the line was bisected, or whether the left (left-long trials) or the right segment (right-long trials) of the line was longer. Participants repeated the task before (pre-rTMS) and after (post-rTMS) receiving 20 min of 1 Hz rTMS over the right STG, the right supramarginal gyrus (SMG), and the vertex (a control site) during three separate visits. Linear mixed models for binomial data were generated with either accuracy or judgment errors as dependent variables, to compare 1) performance across trial types (bisection, non-bisection), and 2) pre- vs. post-rTMS performance between the vertex and the STG and the vertex and the SMG. Line eccentricity (z = 4.31, p < 0.0001) and line bisection (z = 5.49, p < 0.0001) were significant predictors of accuracy. In the models comparing the effects of rTMS, a significant two-way interaction with STG (z = -3.09, p = 0.002) revealed a decrease in accuracy of 9.5% and an increase in errors of the right-long type by 10.7% on bisection trials, in both left and right viewer-centered locations. No significant changes in leftward errors were found. These findings suggested an induced stimulus-centered rightward bias in our participants after STG stimulation. Notably, accuracy or errors were not influenced by SMG stimulation compared to vertex. In line with our predictions, the findings provide compelling evidence for right STG's involvement in healthy stimulus-centered spatial processing.

Integrating multisensory information across external and motor-based frames of reference.

In the mirror box illusion, participants often report that their hand is located where they see it, even when the position of the reflected hand differs from the actual position of their hand. This illusory shift (an index of multisensory integration) is stronger when the two hands engage in synchronous bimanual movement, in which visual and proprioceptive information is congruent in both motor-based (i.e. coordinate centered on the effector) and external (i.e. coordinates centered on elements external to the effector) frames of reference. To investigate the separate contributions of external and motor-based congruence in multisensory integration, we instructed participants to make synchronous or asynchronous tapping movements in either the same (i.e. both hands palms up) or opposing (palm up, palm down) postures. When in opposing postures, externally congruent movements were incongruent in a motor-based frame of reference, and vice versa. Across three experiments, participants reported more illusory shift and stronger ownership of the viewed hand in the mirror for external versus motor-based congruence trials regardless of motor outflow or motor effort, indicating that information from an externally-based representation is more strongly weighted in multisensory integration. These findings provide evidence that not only information across sensory modalities, but also information regarding crossmodal congruence represented in different spatial frames of reference, is differentially weighted in multisensory integration. We discuss how our findings can be incorporated into current computational models on multisensory integration.

No evidential value in samples of transcranial direct current stimulation (tDCS) studies of cognition and working memory in healthy populations.

A substantial number of studies have been published over the last decade, claiming that transcranial direct current stimulation (tDCS) can influence performance on cognitive tasks. However, there is some skepticism regarding the efficacy of tDCS, and evidence from meta-analyses are mixed. One major weakness of these meta-analyses is that they only examine outcomes in published studies. Given biases towards publishing positive results in the scientific literature, there may be a substantial "file-drawer" of unpublished negative results in the tDCS literature. Furthermore, multiple researcher degrees of freedom can also inflate published p-values. Recently, Simonsohn, Nelson and Simmons (2014) created a novel meta-analytic tool that examines the distribution of significant p-values in a literature, and compares it to expected distributions with different effect sizes. Using this tool, one can assess whether the selected studies have evidential value. Therefore, we examined a random selection of studies that used tDCS to alter performance on cognitive tasks, and tDCS studies on working memory in a recently published meta-analysis (Mancuso et al., 2016). Using a p-curve analysis, we found no evidence that the tDCS studies had evidential value (33% power or greater), with the estimate of statistical power of these studies being approximately 14% for the cognitive studies, and 5% (what would be expected from randomly generated data) for the working memory studies. It is likely that previous tDCS studies are substantially underpowered, and we provide suggestions for future research to increase the evidential value of future tDCS studies.

Influence of the Body Schema on Multisensory Integration: Evidence from the Mirror Box Illusion.

When placing one hand on each side of a mirror and making synchronous bimanual movements, the mirror-reflected hand feels like one's own hand that is hidden behind the mirror. We developed a novel mirror box illusion to investigate whether motoric, but not spatial, visuomotor congruence is sufficient for inducing multisensory integration, and importantly, if biomechanical constraints encoded in the body schema influence multisensory integration. Participants placed their hands in a mirror box in opposite postures (palm up, palm down), creating a conflict between visual and proprioceptive feedback for the hand behind the mirror. After synchronous bimanual hand movements in which the viewed and felt movements were motorically congruent but spatially in the opposite direction, participants felt that the hand behind the mirror rotated or completely flipped towards matching the hand reflection (illusory displacement), indicating facilitation of multisensory integration by motoric visuomotor congruence alone. Some wrist rotations are more difficult due to biomechanical constraints. We predicted that these biomechanical constraints would influence illusion effectiveness, even though the illusion does not involve actual limb movement. As predicted, illusory displacement increased as biomechanical constraints and angular disparity decreased, providing evidence that biomechanical constraints are processed in multisensory integration.