Somatosensation in the Brain: A Theoretical Re-evaluation and a New Model

The Correlation Between Ankle Somatosensory Acuity and Sensory Organisation in Postural Stability

The sensory organisation test (SOT) and active movement extent discrimination assessment (AMEDA) are commonly used tools to assess postural stability and somatosensory acuity. Research on the relationship between these assessments is limited. This study aimed to explore the relationship between ankle somatosensation and postural stability in healthy adults. Participants completed one assessment of ankle somatosensory acuity (AMEDA) and one assessment of postural stability (SOT). Ankle somatosensory acuity was assessed on the non-dominant foot and measured their ability to detected small changes in joint movement within the inversion/eversion plane. The SOT involved both feet upon the testing platform and six "conditions" which distorted the sensory systems and assessed the ability to use visual, somatosensory, and vestibular feedback to maintain postural control. A Spearman's Rank-Order Correlation was run to assess the relationship between AMEDA and SOT measures. We hypothesised that AMEDA scores would positively correlate with SOT conditions 4-6 (sway-referenced platform for all) and the somatosensory (SOM) sensory score. 54 participants (28 females, 26 males; mean age 40 ± 14 years) completed the study. Positive correlations were found between the AMEDA score and SOT conditions 5 (eyes closed, sway-reference platform) and 6 scores (sway-referenced visual surround and platform) (p = .041 and p = .006) but not with SOT condition 4 (eyes open, sway-referenced platform) or the SOM sensory score (p > .05). There were positive correlations between the AMEDA score, and SOT composite score and vestibular (VEST) sensory score (p < .001 and p = .007). Somatosensation and postural stability scores were related during the most challenging balance tasks, highlighting the role of somatosensory acuity in postural control. However, AMEDA score did not relate to the SOM scores in the SOT, suggesting different factors influence these measures of somatosensation. This highlights the unique contributions of the AMEDA and SOT in assessing sensory function and its impact on balance.

Reactivated thalamocortical plasticity alters neural activity in sensory-motor cortex during post-critical period

Neuroplasticity in sensory brain areas supports adaptation after nerve injury and fundamentally impacts sensation and movement. However, limited neuroplasticity in somatosensory areas due to the early critical period makes determining the role of thalamocortical (TC) inputs in sensorimotor signal processing challenging. Here, we demonstrated that reactivation of TC neuroplasticity was associated with an increase in the number of neurons in layer IV (L4) of the whisker primary somatosensory cortex (wS1) with a stable excitation-inhibition ratio. Highly synchronized neural activity in L4 propagated throughout the wS1 column and to the downstream areas, including whisker secondary somatosensory, primary motor cortices, and contralateral wS1. These results provide crucial evidence that TC inputs can alter the neural activity of sensory-motor pathways even after the critical period. Altogether, these enormous changes in sensorimotor circuit activity are important for adaptation following an injury such as limb loss, stroke, or other forms of neural injury.

Integrative intrinsic brain activity and molecular analyses of the interaction between first-episode depression and age

BACKGROUND

Numerous studies have underscored the presence of abnormal intrinsic neural activity (INA) in individuals with depression. However, recognizing that the age stage may influence the pathophysiology of depression, our study sought to delve into the interplay of depression and age on INA and molecular architecture.

METHODS

One hundred and thirty-eight first-episode depression patients and 120 healthy controls (HC) were recruited and underwent resting-state functional magnetic resonance imaging. The participants were stratified into four groups based on age. Utilizing amplitude of low-frequency fluctuation (ALFF) analyses, we employed an ANCOVA to compare INA patterns in four groups. Additionally, we conducted correlation analyses between ALFF and neurotransmitter maps to elucidate molecular underpinnings of INA abnormalities.

RESULTS

In comparison to adolescents with early-onset depression and adult HC, adult-onset depression exhibited increased ALFF in the right paracentral lobule. Conversely, early-onset depression, when contrasted with adolescent HC, displayed reduced ALFF in the right paracentral lobule. The interactive brain regions affected by ALFF alterations were associated with serotonergic, GABAergic, and opioid neurotransmitter systems.

LIMITATIONS

The present study was limited to its cross-sectional design.

CONCLUSIONS

This study illuminates an antagonistic effect of depression and age on brain activity in paracentral lobule and provides molecular underpinnings of the corresponding INA abnormalities related to key neurotransmitter systems. These insights may prove valuable in the development of neuromarkers for clinical intervention and treatment of depression.

Hand and foot overestimation in visually impaired human adults

Previous research has shown that visual impairment results in reduced audio, tactile and proprioceptive ability. One hypothesis is that these issues arise from inaccurate body representations. Few studies have investigated metric body representations in a visually impaired population. We designed an ecologically valid behavioural task in which visually impaired adults haptically explored various sized gloves or shoes. They were asked to indicate if they perceived each clothing item as bigger than the size of their hand or foot. In the post-hoc analyses we fit psychometric curves to the data to extract the point of subjective equality. We then compared the results to age/sex matched controls. We hypothesized the blind participants body representations should be more distorted. Because previous research has shown that females are more likely to overestimate body size, we predicted sex differences in the sighted participants. However, because blind adults have no exposure to visual ideals of body size, we predicted that there would be no sex differences. Our results showed thatblind participants overestimated their hands and feetto a similar degree. Sighted controls overestimated their hands significantly more than their feet. Taken together, our results partially support our hypothesis and suggest that visual deprivation, even for short periods result in hand size overestimation.

Neural adaptations to congenital deafness: enhanced tactile discrimination through cross-modal neural plasticity - an fMRI study

BACKGROUND

This study explores the compensatory neural mechanisms associated with congenital deafness through an examination of tactile discrimination abilities using high-resolution functional magnetic resonance imaging (fMRI).

OBJECTIVE

To analyze the neural substrates underlying tactile processing in congenitally deaf individuals and compare them with hearing controls.

METHODS

Our participant pool included thirty-five congenitally deaf individuals and thirty-five hearing controls. All participants engaged in tactile discrimination tasks involving the identification of common objects by touch. We utilized an analytical suite comprising voxel-based statistics, functional connectivity multivariate/voxel pattern analysis (fc-MVPA), and seed-based connectivity analysis to examine neural activity.

RESULTS

Our findings revealed pronounced neural activity in congenitally deaf participants within regions typically associated with auditory processing, including the bilateral superior temporal gyrus, right middle temporal gyrus, and right rolandic operculum. Additionally, unique activation and connectivity patterns were observed in the right insula and bilateral supramarginal gyrus, indicating a strategic reorganization of neural pathways for tactile information processing. Behaviorally, both groups demonstrated high accuracy in the tactile tasks, exceeding 90%. However, the deaf participants outperformed their hearing counterparts in reaction times, showcasing significantly enhanced efficiency in tactile information processing.

CONCLUSION

These insights into the brain's adaptability to sensory loss through compensatory neural reorganization highlight the intricate mechanisms by which tactile discrimination is enhanced in the absence of auditory input. Understanding these adaptations can help develop strategies to harness the brain's plasticity to improve sensory processing in individuals with sensory impairments, ultimately enhancing their quality of life through improved tactile perception and sensory integration.

Touch to text: Spatiotemporal evolution of braille letter representations in blind readers

Visual deprivation does not silence the visual cortex, which is responsive to auditory, tactile, and other nonvisual tasks in blind persons. However, the underlying functional dynamics of the neural networks mediating such crossmodal responses remain unclear. Here, using braille reading as a model framework to investigate these networks, we presented sighted (N=13) and blind (N=12) readers with individual visual print and tactile braille alphabetic letters, respectively, during MEG recording. Using time-resolved multivariate pattern analysis and representational similarity analysis, we traced the alphabetic letter processing cascade in both groups of participants. We found that letter representations unfolded more slowly in blind than in sighted brains, with decoding peak latencies ~200 ms later in braille readers. Focusing on the blind group, we found that the format of neural letter representations transformed within the first 500 ms after stimulus onset from a low-level structure consistent with peripheral nerve afferent coding to high-level format reflecting pairwise letter embeddings in a text corpus. The spatiotemporal dynamics of the transformation suggest that the processing cascade proceeds from a starting point in somatosensory cortex to early visual cortex and then to inferotemporal cortex. Together our results give insight into the neural mechanisms underlying braille reading in blind persons and the dynamics of functional reorganization in sensory deprivation.

Multilayer network analysis reveals instability of brain dynamics in untreated first-episode schizophrenia

Although aberrant static functional brain network activity has been reported in schizophrenia, little is known about how the dynamics of neural function are altered in first-episode schizophrenia and are modulated by antipsychotic treatment. The baseline resting-state functional magnetic resonance imaging data were acquired from 122 first-episode drug-naïve schizophrenia patients and 128 healthy controls (HCs), and 44 patients were rescanned after 1-year of antipsychotic treatment. Multilayer network analysis was applied to calculate the network switching rates between brain states. Compared to HCs, schizophrenia patients at baseline showed significantly increased network switching rates. This effect was observed mainly in the sensorimotor (SMN) and dorsal attention networks (DAN), and in temporal and parietal regions at the nodal level. Switching rates were reduced after 1-year of antipsychotic treatment at the global level and in DAN. Switching rates at baseline at the global level and in the inferior parietal lobule were correlated with the treatment-related reduction of negative symptoms. These findings suggest that instability of functional network activity plays an important role in the pathophysiology of acute psychosis in early-stage schizophrenia. The normalization of network stability after antipsychotic medication suggests that this effect may represent a systems-level mechanism for their therapeutic efficacy.

Ketamine reduces the neural distinction between self- and other-produced affective touch: a randomized double-blind placebo-controlled study

A coherent sense of self is crucial for social functioning and mental health. The N-methyl-D-aspartate antagonist ketamine induces short-term dissociative experiences and has therefore been used to model an altered state of self-perception. This randomized double-blind placebo-controlled cross-over study investigated the mechanisms for ketamine's effects on the bodily sense of self in the context of affective touch. Thirty healthy participants (15 females/15 males, age 19-39) received intravenous ketamine or placebo while performing self-touch and receiving touch by someone else during functional MRI - a previously established neural measure of tactile self-other-differentiation. Afterwards, tactile detection thresholds during self- and other-touch were assessed, as well as dissociative states, interoceptive awareness, and social touch attitudes. Compared to placebo, ketamine administration elicited dissociation and reduced neural activity associated with self-other-differentiation in the right temporoparietal cortex, which was most pronounced during other-touch. This reduction correlated with ketamine-induced reductions in interoceptive awareness. The temporoparietal cortex showed higher connectivity to somatosensory cortex and insula during other- compared to self-touch. This difference was augmented by ketamine, and correlated with dissociation strength for somatosensory cortex. These results demonstrate that disrupting the self-experience through ketamine administration affects neural activity associated with self-other-differentiation in a region involved in touch perception and social cognition, especially with regard to social touch by someone else. This process may be driven by ketamine-induced effects on top-down signaling, rendering the processing of predictable self-generated and unpredictable other-generated touch more similar. These findings provide further evidence for the intricate relationship of the bodily self with the tactile sense.

Have I Been Touched? Subjective and Objective Aspects of Tactile Awareness

Somatosensory tactile experience is a key aspect of our interaction with the environment. It is involved in object manipulation, in the planning and control of actions and, in its affective components, in the relationships with other individuals. It is also a foundational component of body awareness. An intriguing aspect of sensory perception in general and tactile perception in particular is the way in which stimulation comes to consciousness. Indeed, although being aware of something seems a rather self-evident and monolithic aspect of our mental states, sensory awareness may be in fact modulated by many different processes that impact on the mere stimulation of the skin, including the way in which we perceive our bodies as belonging to us. In this review, we first took into consideration the pathological conditions of absence of phenomenal experience of touch, in the presence of implicit processing, as initial models for understanding the neural bases of conscious tactile experience. Subsequently, we discussed cases of tactile illusions both in normal subjects and in brain-damaged patients which help to understand which high-order processes impact tactile awareness. Finally, we discussed the observations reported in the review in light of some influential models of touch and body representation.

Does Body Memory Exist? A Review of Models, Approaches and Recent Findings Useful for Neurorehabilitation

Over the past twenty years, scientific research on body representations has grown significantly, with Body Memory (BM) emerging as a prominent area of interest in neurorehabilitation. Compared to other body representations, BM stands out as one of the most obscure due to the multifaceted nature of the concept of "memory" itself, which includes various aspects (such as implicit vs. explicit, conscious vs. unconscious). The concept of body memory originates from the field of phenomenology and has been developed by research groups studying embodied cognition. In this narrative review, we aim to present compelling evidence from recent studies that explore various definitions and explanatory models of BM. Additionally, we will provide a comprehensive overview of the empirical settings used to examine BM. The results can be categorized into two main areas: (i) how the body influences our memories, and (ii) how memories, in their broadest sense, could generate and/or influence metarepresentations-the ability to reflect on or make inferences about one's own cognitive representations or those of others. We present studies that emphasize the significance of BM in experimental settings involving patients with neurological and psychiatric disorders, ultimately analyzing these findings from an ontogenic perspective.

The efficacy of low frequency repetitive transcial magnetic stimulation for treating auditory verbal hallucinations in schizophrenia: Insights from functional gradient analyses

Background

Auditory Verbal Hallucinations (AVH) constitute a prominent feature of schizophrenia. Although low-frequency repetitive transcranial magnetic stimulation (rTMS) has demonstrated therapeutic benefits in ameliorating AVH, the underlying mechanisms of its efficacy necessitate further elucidation.

Objective

This study investigated the cortical gradient characteristics and their associations with clinical responses in schizophrenia patients with AVH, mediated through 1 Hz rTMS targeting the left temporoparietal junction.

Method

Functional gradient metrics were employed to examine the hierarchy patterns of cortical organization, capturing whole-brain functional connectivity profiles in patients and controls.

Results

The 1 Hz rTMS treatment effectively ameliorated the positive symptoms in patients, specifically targeting AVH. Initial evaluations revealed expanded global gradient distribution patterns and specific principal gradient variations in certain brain regions in patients at baseline compared to a control cohort. Following treatment, these divergent global and local patterns showed signs of normalizing. Furthermore, there was observed a closer alignment in between-network dispersion among various networks after treatment, including the somatomotor, attention, and limbic networks, indicating a potential harmonization of brain functionality.

Conclusion

Low-frequency rTMS induces alternations in principal functional gradient patterns, may serve as imaging markers to elucidate the mechanisms underpinning the therapeutic efficacy of rTMS on AVH in schizophrenia.

Perception, action, and the body model

In 1992, Goodale and Milner proposed to study the visual system based on function, thus dissociating vision for perception (ventral stream) and vision for action (dorsal stream). This became known as the Perception and Action model (PAM). Following the PAM in the visual system, a somatosensory PAM was proposed including a body representation for perception and a separate for action. This review explores the body model of the hand and how it relates to the PAM. The body model refers to the internal representation of the body that is responsible for position sense. Previous research has shown that the representation of the hand features systematic distortions: an overestimation of hand width and an underestimation of finger length. These distortions have been reported using different paradigms, different body parts, and in various settings. Thus, body model distortions appear to be a characteristic of human body representation. If the body model of the hand is distorted, how can actions like reaching and grasping be accurate? We review evidence that body model distortions may in fact provide a functional benefit to our actions, that cortical maps in the somatosensory and motor cortices reflect these distortions, and that actions rely on a distorted body model. We argue that the body model is a product of both the ventral and dorsal somatosensory streams. Further, we suggest that the body model is an example of the inextricable link between the two streams.

Are tactile function and body awareness of the foot related to motor outcomes in children with upper motor neuron lesions?

Introduction

Somatosensory function can be reduced in children with Upper Motor Neuron (UMN) lesions. Therefore, we investigated relationships between somatosensory functions of the foot and motor outcomes in children with UMN lesions.

Method

In this cross-sectional study, we assessed the Tactile Threshold (TT) with monofilaments and body awareness with Tactile Localisation Tasks for spatial-related action (TLTaction) and structural-related perception (TLTperception) body representation at the foot sole. Furthermore, we assessed four motor outcomes: the Selective Control Assessment of the Lower Extremity (SCALE), the modified Timed Up and Go test (mTUG), the Gillette Functional Assessment Questionnaire (FAQ), and the Functional Mobility Scale (FMS). Spearman's correlations (ρ) were applied to assess relationships between the somatosensory function of the foot sole and the applied motor outcomes.

Results

Thirty-five children with UMN lesions, on average 11.7 ± 3.4 years old, participated. TLTperception correlated significantly with all lower limb motor outcomes (|ρ|=0.36-0.57; p < 0.05), but TLTaction (|ρ|=0.00-0.27; p = 0.15-0.97, and TT did not (|ρ|=0.01-0.83; p = 0.73-0.94). TLTperception correlated strongly with the Gross Motor Function Classification System (|ρ|=0.62; p = 0.001) in children with cerebral palsy (n = 24).

Discussion

Assessing structural body representation of the foot sole should be considered when addressing lower limb motor impairments, including gait, in children with upper motor neuron lesions. Our results suggest that the assessment of tactile function and spatial body representation may be less related to lower limb motor function.

Clinical neurophysiology of functional motor disorders: IFCN Handbook Chapter

Functional Motor Disorders are common and disabling. Clinical diagnosis has moved from one of exclusion of other causes for symptoms to one where positive clinical features on history and examination are used to make a "rule in" diagnosis wherever possible. Clinical neurophysiological assessments have developed increasing importance in assisting with this positive diagnosis, not being used simply to demonstrate normal sensory-motor pathways, but instead to demonstrate specific abnormalities that help to positively diagnose these disorders. Here we provide a practical review of these techniques, their application, interpretation and pitfalls. We also highlight particular areas where such tests are currently lacking in sensitivity and specificity, for example in people with functional dystonia and functional tic-like movements.

A conceptual framework on body representations and their relevance for mental disorders

Many mental disorders are accompanied by distortions in the way the own body is perceived and represented (e.g., eating disorders, body dysmorphic disorder including muscle dysmorphia, or body integrity dysphoria). We are interested in the way these distortions develop and aim at better understanding their role in mental health across the lifespan. For this purpose, we first propose a conceptual framework of body representation that defines this construct and integrates different perspectives (e.g., cognitive neuroscience, clinical psychology) on body representations. The framework consists of a structural and a process model of body representation emphasizing different goals: the structural model aims to support researchers from different disciplines to structure results from studies and help collectively accumulate knowledge about body representations and their role in mental disorders. The process model is reflecting the dynamics during the information processing of body-related stimuli. It aims to serve as a motor for (experimental) study development on how distorted body representations emerge and might be changed. Second, we use this framework to review the normative development of body representations as well as the development of mental disorders that relate to body representations with the aim to further clarify the potential transdiagnostic role of body representations.

A robot-based interception task to quantify upper limb impairments in proprioceptive and visual feedback after stroke

BACKGROUND

A key motor skill is the ability to rapidly interact with our dynamic environment. Humans can generate goal-directed motor actions in response to sensory stimulus within ~ 60-200ms. This ability can be impaired after stroke, but most clinical tools lack any measures of rapid feedback processing. Reaching tasks have been used as a framework to quantify impairments in generating motor corrections for individuals with stroke. However, reaching may be inadequate as an assessment tool as repeated reaching can be fatiguing for individuals with stroke. Further, reaching requires many trials to be completed including trials with and without disturbances, and thus, exacerbate fatigue. Here, we describe a novel robotic task to quantify rapid feedback processing in healthy controls and compare this performance with individuals with stroke to (more) efficiently identify impairments in rapid feedback processing.

METHODS

We assessed a cohort of healthy controls (n = 135) and individuals with stroke (n = 40; Mean 41 days from stroke) in the Fast Feedback Interception Task (FFIT) using the Kinarm Exoskeleton robot. Participants were instructed to intercept a circular white target moving towards them with their hand represented as a virtual paddle. On some trials, the arm could be physically perturbed, the target or paddle could abruptly change location, or the target could change colour requiring the individual to now avoid the target.

RESULTS

Most participants with stroke were impaired in reaction time (85%) and end-point accuracy (83%) in at least one of the task conditions, most commonly with target or paddle shifts. Of note, this impairment was also evident in most individuals with stroke when performing the task using their unaffected arm (75%). Comparison with upper limb clinical measures identified moderate correlations with the FFIT.

CONCLUSION

The FFIT was able to identify a high proportion of individuals with stroke as impaired in rapid feedback processing using either the affected or unaffected arms. The task allows many different types of feedback responses to be efficiently assessed in a short amount of time.

Feasibility, Validity, and Reliability of Lower Limb Tactile and Body Awareness Assessments in Children With Upper Motor Neuron Lesions

OBJECTIVE

To investigate the feasibility, discriminative and convergent validity, and inter-rater reliability of a lower limb tactile function and 2 body awareness assessments in children with upper motor neuron (UMN) lesions.

DESIGN

Cross-sectional psychometric study.

SETTING

Pediatric rehabilitation center.

PARTICIPANTS

Forty individuals with UMN lesions (mean age 11.7 years, SD 3.4 years; 27 girls) and 40 neurotypically developing children of the same age participated (N=80).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

We assessed the tactile threshold (TT) with monofilaments and body awareness with tactile localization tasks (TLTs) for structural (TLTaction) and spatial (TLTperception) body representation at the foot sole. We compared the test outcomes between children with UMN lesions and neurotypically developing children with the Wilcoxon signed-rank test. Furthermore, we quantified the relations between the 3 tests with Spearman correlations (rs) and the interrater reliability with quadratic weighted kappa (κQW).

RESULTS

About 80% of the children with UMN lesions perceived the tests easy to perform. The children with UMN lesions had significantly reduced somatosensory function compared with the neurotypically developing children. For the more affected leg, we found good relations between the TT and the TLTaction (rs=0.71; P<.001) and between the 2 TLTs (rs=0.66; P<.001), and a fair relation between the TT and the TLTperception (rs=0.31; P=.06). The inter-rater reliability analyses for the sum scores showed almost perfect agreement for the TT (κQW more affected leg 0.86; less affected leg 0.81), substantial agreement for TLTaction (κQW more affected leg 0.76; less affected leg 0.63), and almost perfect agreement for TLTperception (κQW more affected leg 0.88; less affected leg 0.74).

CONCLUSION

The 3 tests are feasible to assess lower limb somatosensory function in children with UMN lesions. Discriminative and convergent validity and reliability of the 3 tests were confirmed. Further studies should investigate responsiveness and association with motor function of these outcome measures.

Profiling Somatosensory Impairment after Stroke: Characterizing Common "Fingerprints" of Impairment Using Unsupervised Machine Learning-Based Cluster Analysis of Quantitative Measures of the Upper Limb

Altered somatosensory function is common among stroke survivors, yet is often poorly characterized. Methods of profiling somatosensation that illustrate the variability in impairment within and across different modalities remain limited. We aimed to characterize post-stroke somatosensation profiles ("fingerprints") of the upper limb using an unsupervised machine learning cluster analysis to capture hidden relationships between measures of touch, proprioception, and haptic object recognition. Raw data were pooled from six studies where multiple quantitative measures of upper limb somatosensation were collected from stroke survivors (n = 207) using the Tactile Discrimination Test (TDT), Wrist Position Sense Test (WPST) and functional Tactile Object Recognition Test (fTORT) on the contralesional and ipsilesional upper limbs. The Growing Self Organizing Map (GSOM) unsupervised machine learning algorithm was used to generate a topology-preserving two-dimensional mapping of the pooled data and then separate it into clusters. Signature profiles of somatosensory impairment across two modalities (TDT and WPST; n = 203) and three modalities (TDT, WPST, and fTORT; n = 141) were characterized for both hands. Distinct impairment subgroups were identified. The influence of background and clinical variables was also modelled. The study provided evidence of the utility of unsupervised cluster analysis that can profile stroke survivor signatures of somatosensory impairment, which may inform improved diagnosis and characterization of impairment patterns.

Calibration of Impairment Severity to Enable Comparison across Somatosensory Domains

Comparison across somatosensory domains, important for clinical and scientific goals, requires prior calibration of impairment severity. Provided test score distributions are comparable across domains, valid comparisons of impairment can be made by reference to score locations in the corresponding distributions (percentile rank or standardized scores). However, this is often not the case. Test score distributions for tactile texture discrimination (n = 174), wrist joint proprioception (n = 112), and haptic object identification (n = 98) obtained from pooled samples of stroke survivors in rehabilitation settings were investigated. The distributions showed substantially different forms, undermining comparative calibration via percentile rank or standardized scores. An alternative approach is to establish comparable locations in the psychophysical score ranges spanning performance from just noticeably impaired to maximally impaired. Several simulation studies and a theoretical analysis were conducted to establish the score distributions expected from completely insensate responders for each domain. Estimates of extreme impairment values suggested by theory, simulation and observed samples were consistent. Using these estimates and previously discovered values for impairment thresholds in each test domain, comparable ranges of impairment from just noticeable to extreme impairment were found. These ranges enable the normalization of the three test scales for comparison in clinical and research settings.

Role of Immersive Virtual Reality in Motor Behaviour Decision-Making in Chronic Pain Patients

Primary chronic pain is a major contributor to disability worldwide, with an estimated prevalence of 20-33% of the world's population. The high socio-economic impact of musculoskeletal pain justifies seeking an appropriate therapeutic strategy. Immersive virtual reality (VR) has been proposed as a first-line intervention for chronic musculoskeletal pain. However, the growing literature has not been accompanied by substantial progress in understanding how VR exerts its impact on the pain experience and what neurophysiological mechanisms might be involved in the clinical effectiveness of virtual reality interventions in chronic pain patients. The aim of this review is: (i) to establish the state of the art on the effects of VR on patients with chronic pain; (ii) to identify neuroplastic changes associated with chronic pain that may be targeted by VR intervention; and (iii) to propose a hypothesis on how immersive virtual reality could modify motor behavioral decision-making through an interactive experience in patients with chronic pain.

Relating different Dimensions of Bodily Experiences: Review and proposition of an integrative model relying on phenomenology, predictive brain and neuroscience of the self

How we mentally experience our body has been studied in a variety research domains. Each of these domains focuses in its own ways on different aspects of the body, namely the neurophysiological, perceptual, affective or social components, and proposes different conceptual taxonomies. It is therefore difficult to find one's way through this vast literature and to grasp the relationships between the different dimensions of bodily experiences. In this narrative review, we summarize the existing research directions and present their limits. We propose an integrative framework, grounded in studies on phenomenal consciousness, self-consciousness and bodily self-consciousness, that can provide a common basis for evaluating findings on different dimensions of bodily experiences. We review the putative mechanisms, relying on predictive processes, and neural substrates that support this model. We discuss how this model enables a conceptual assessment of the interrelationships between multiple dimensions of bodily experiences and potentiate interdisciplinary approaches.

Analogous cognitive strategies for tactile learning in the rodent and human brain

Evolution has molded individual species' sensory capacities and abilities. In rodents, who mostly inhabit dark tunnels and burrows, the whisker-based somatosensory system has developed as the dominant sensory modality, essential for environmental exploration and spatial navigation. In contrast, humans rely more on visual and auditory inputs when collecting information from their surrounding sensory space in everyday life. As a result of such species-specific differences in sensory dominance, cognitive relevance and capacities, the evidence for analogous sensory-cognitive mechanisms across species remains sparse. However, recent research in rodents and humans yielded surprisingly comparable processing rules for detecting tactile stimuli, integrating touch information into percepts, and goal-directed rule learning. Here, we review how the brain, across species, harnesses such processing rules to establish decision-making during tactile learning, following canonical circuits from the thalamus and the primary somatosensory cortex up to the frontal cortex. We discuss concordances between empirical and computational evidence from micro- and mesoscopic circuit studies in rodents to findings from macroscopic imaging in humans. Furthermore, we discuss the relevance and challenges for future cross-species research in addressing mutual context-dependent evaluation processes underpinning perceptual learning.

The distorted hand: systematic but 'independent' distortions in both explicit and implicit hand representations in young female adults

It has long been assumed that an accurate representation of the size and shape of one's body is necessary to successfully interact with the environment. Previous research has shown accurate representations when healthy participants make overt judgments (i.e. explicit) about the size of their bodies. However, when body size is judged implicitly, studies have shown systematic distortions. One suggestion for these differences, is that explicit and implicit representations are informed by different sensory modalities. Explicit representations rely on vision whereas implicit representations are informed by haptics. We designed an experiment to investigate if explicit representations that are informed by haptics are more like implicit representation featuring systematic distortions. We asked female participants to estimate the size of their fingers and hands in three different tasks: an explicit-haptic, an implicit, and an explicit-vision task. The results showed that all three representations were distorted and furthermore, the distortions for each representation were different from one another. These results suggest that inaccurate finger and hand length are a stereotypical feature of body representation that is present in both visual and haptic domains. We discuss the results in relation to theories of body representation.

Age-related brain atrophy in cats without apparent neurological and behavioral signs using voxel-based morphometry

Introduction

Brain atrophy is observed with aging and may cause cognitive decline or dysfunction. Aging cats may demonstrate behavioral changes related to cognitive dysfunction. In the clinical veterinary field, although the conventional region of interest method by manual or semiauto tracing on magnetic resonance imaging is used to detect atrophy of regional structures, such as the hippocampus, it is difficult to assess atrophy globally. Voxel-based morphometry (VBM) has been developed to detect global and regional abnormalities in humans. The purpose of the present study investigates whether the feline brain volume decreases with aging using VBM analysis.

Materials

A total of 65 cats, aged 17-200 months, without apparent neurological and behavioral signs were included in the statistical analysis.

Results

We observed that the gray matter in the bilateral parietal lobes was decreased significantly with aging. The regions that showed decreased volume included the right postcruciate, cingulate gyrus, rostral suprasylvian/ectosylvian gyri, and the left postcruciate gyrus. No significant reduction in white matter was observed. Together, our results show that age-related brain atrophy can be detected using VBM analysis.

Discussion

The age-related atrophy of the parietal cortex may not cause neurological and behavioral signs in cats. Therefore, veterinarians should consider age when assessing the relation between morphometric and functional abnormalities of the parietal cortex in cats.

Relations between short-term memory and the within-subject variability of experimental pain intensity reports: Results from healthy and Fibromyalgia patients

While factors contributing to between-subjects differences in pain have been studied extensively, factors contributing to the within-subjects variability of pain reports are yet unexplored. The aim of this investigation was to assess possible associations between short-term memory and the within-subjects variability of pain reports in healthy and chronic pain patients. Healthy participants were recruited at the University of Haifa, Israel, and Fibromyalgia patients were recruited at a rheumatology department in a central hospital in Lisbon, Portugal. Following consent, both cohorts underwent the same procedures, including the digit-span test, assessing short-term memory, and the FAST procedure, assessing within-subject variability of pain intensity reports in response to experimental pain. One-hundred twenty-one healthy volunteers and 29 Fibromyalgia patients completed the study. While a significant correlation was found between the within-subjects variability and the total score of the short-term memory task (Spearman's r = 0.394, P = 0.046) in the Fibromyalgia group, a marginal correlation emerged in the healthy cohort (r = 0.174, P = 0.056). A possible interpretation of these results is that in the patients' group, at least some of the within-subjects variability of pain intensity reports might be due to error measurement derived by poorer short-term memory, rather than true fluctuations in perception.

Grip force as a functional window to somatosensory cognition

Analysis of grip force signals tailored to hand and finger movement evolution and changes in grip force control during task execution provide unprecedented functional insight into somatosensory cognition. Somatosensory cognition is the basis of our ability to act upon and to transform the physical world around us, to recognize objects on the basis of touch alone, and to grasp them with the right amount of force for lifting and manipulating them. Recent technology has permitted the wireless monitoring of grip force signals recorded from biosensors in the palm of the human hand to track and trace human grip forces deployed in cognitive tasks executed under conditions of variable sensory (visual, auditory) input. Non-invasive multi-finger grip force sensor technology can be exploited to explore functional interactions between somatosensory brain mechanisms and motor control, in particular during learning a cognitive task where the planning and strategic execution of hand movements is essential. Sensorial and cognitive processes underlying manual skills and/or hand-specific (dominant versus non-dominant hand) behaviors can be studied in a variety of contexts by probing selected measurement loci in the fingers and palm of the human hand. Thousands of sensor data recorded from multiple spatial locations can be approached statistically to breathe functional sense into the forces measured under specific task constraints. Grip force patterns in individual performance profiling may reveal the evolution of grip force control as a direct result of cognitive changes during task learning. Grip forces can be functionally mapped to from-global-to-local coding principles in brain networks governing somatosensory processes for motor control in cognitive tasks leading to a specific task expertise or skill. Under the light of a comprehensive overview of recent discoveries into the functional significance of human grip force variations, perspectives for future studies in cognition, in particular the cognitive control of strategic and task relevant hand movements in complex real-world precision task, are pointed out.

Prefrontal and somatosensory-motor cortex effective connectivity in humans

Effective connectivity, functional connectivity, and tractography were measured between 57 cortical frontal and somatosensory regions and the 360 cortical regions in the Human Connectome Project (HCP) multimodal parcellation atlas for 171 HCP participants. A ventral somatosensory stream connects from 3b and 3a via 1 and 2 and then via opercular and frontal opercular regions to the insula, which then connects to inferior parietal PF regions. This stream is implicated in "what"-related somatosensory processing of objects and of the body and in combining with visual inputs in PF. A dorsal "action" somatosensory stream connects from 3b and 3a via 1 and 2 to parietal area 5 and then 7. Inferior prefrontal regions have connectivity with the inferior temporal visual cortex and orbitofrontal cortex, are implicated in working memory for "what" processing streams, and provide connectivity to language systems, including 44, 45, 47l, TPOJ1, and superior temporal visual area. The dorsolateral prefrontal cortex regions that include area 46 have connectivity with parietal area 7 and somatosensory inferior parietal regions and are implicated in working memory for actions and planning. The dorsal prefrontal regions, including 8Ad and 8Av, have connectivity with visual regions of the inferior parietal cortex, including PGs and PGi, and are implicated in visual and auditory top-down attention.

An integrative perspective on the role of touch in the development of intersubjectivity

Touch concerns a fundamental component of sociality. In this review, we examine the hypothesis that somatomotor development constitutes a crucial psychophysiological element in the ontogeny of intersubjectivity. An interdisciplinary perspective is provided on how the communication channel of touch contributes to the sense of self and extends to the social self. During gestation, the transformation of random movements into organized sequences of actions with sensory consequences parallels the development of the brain's functional architecture. Brain subsystems shaped by the coordinated activity of somatomotor circuits to support these first body-environment interactions are the first brain functional arrangements to develop. We propose that tactile self-referring behaviour during gestation constitutes a prototypic mode of interpersonal exchange that supports the subsequent development of intersubjective exchange. The reviewed research suggests that touch constitutes a pivotal bodily experience that in early stages builds and later filters self-other interactions. This view is corroborated by the fact that aberrant social-affective touch experiences appear fundamentally associated with attachment anomalies, interpersonal trauma, and personality disorders. Given the centrality of touch for the development of intersubjectivity and for psychopathological conditions in the social domain, dedicated research is urged to elucidate the role of touch in the evolution of subjective self-other coding.

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Hand constraint reduces brain activity and affects the speed of verbal responses on semantic tasks

According to the theory of embodied cognition, semantic processing is closely coupled with body movements. For example, constraining hand movements inhibits memory for objects that can be manipulated with the hands. However, it has not been confirmed whether body constraint reduces brain activity related to semantics. We measured the effect of hand constraint on semantic processing in the parietal lobe using functional near-infrared spectroscopy. A pair of words representing the names of hand-manipulable (e.g., cup or pencil) or nonmanipulable (e.g., windmill or fountain) objects were presented, and participants were asked to identify which object was larger. The reaction time (RT) in the judgment task and the activation of the left intraparietal sulcus (LIPS) and left inferior parietal lobule (LIPL), including the supramarginal gyrus and angular gyrus, were analyzed. We found that constraint of hand movement suppressed brain activity in the LIPS toward hand-manipulable objects and affected RT in the size judgment task. These results indicate that body constraint reduces the activity of brain regions involved in semantics. Hand constraint might inhibit motor simulation, which, in turn, would inhibit body-related semantic processing.

What are the relevant categories, modalities, and outcome measures for assessing lower limb somatosensory function in children with upper motor neuron lesions? A Delphi study

PURPOSE

Somatosensory function of the lower limbs is rarely assessed in children with upper motor neuron lesions despite its potential relevance for motor function. We explored consensus regarding somatosensory categories (exteroception, proprioception, interoception, and body awareness), modalities, and outcome measures relevant to lower limb motor function.

METHODS

Fifteen international experts with experience of somatosensory function assessment participated in this Delphi study. Surveys of four rounds, conducted online, included questions on the relevance of somatosensory categories and modalities for motor function and on the use of potential outcome measures in clinical practice.

RESULTS

The experts reached consensus on the relevance of six modalities of the categories exteroception, proprioception, and body awareness. Based on their feedback, we formulated three core criteria for somatosensory outcome measures, namely suitability for clinical practice, child-friendliness, and relevance for motor function. None of the nine available outcome measures fulfilled each criterion. The experts also highlighted the importance of using and interpreting the tests in relation to the child's activity and participation.

CONCLUSION

There was expert consensus on three categories and six modalities of somatosensory function relevant for lower limb motor function. However, existing outcome measures will need to be adapted for use in paediatric clinical practice. IMPLICATION FOR REHABILITATIONConsensus was established for the categories and modalities of somatosensory function relevant for lower limb motor function of children with UMN lesion.Outcome measures should cover tactile function, joint movement and joint position and dynamic position sense, and spatial and structural body representation.None of the nine existing outcome measures fulfilled the core criteria: feasibility for clinical practice, child-friendliness, and relevance to motor function.

Influence of the Somatic Rubber Hand Illusion on Maximum Grip Aperture

The classic rubber hand illusion (RHI), based on visual, proprioceptive, and tactile feedback, can affect actions. However, it is not known whether these effects still occur if the paradigm is administered without visual feedback. In this study, we used the somatic RHI to test in thirty-two healthy individuals whether the incorporation of the rubber hand based on proprioceptive and tactile information only is sufficient to generate changes in actions. We measured maximum grip aperture (GA) changes towards a target and associated brain activations within the dorsal stream before and after the somatic RHI. Behavioural and neuroimaging data do not support an effect on maximum GA when the RHI is based on proprioceptive and tactile information only.

Psychometric Properties of Lower Limb Somatosensory Function and Body Awareness Outcome Measures in Children with Upper Motor Neuron Lesions: A Systematic Review

PURPOSE

A systematic review of the psychometric properties and feasibility of outcome measures assessing lower limb somatosensory function and body awareness in children with upper motor neuron lesion.

METHODS

We followed the COnsensus-based Standards for the selection of health Measurement INstruments guidelines. Two raters independently judged the quality and risk of bias of each study. Data synthesis was performed, and aspects of feasibility were extracted.

RESULTS

Twelve studies investigated eleven somatosensory function measures quantifying four modalities and eight body awareness measures quantifying two modalities. The best evidence synthesis was very low to low for somatosensory function modalities and low for body awareness modalities. Few feasibility aspects were reported (e.g., the percentage or minimum age of participants able to perform the tests).

CONCLUSION

Current evidence on the psychometric characteristics of somatosensory function and body awareness outcome measures are relatively sparse. Further research on psychometric properties and practical application is needed.

Skin and Mechanoreceptor Contribution to Tactile Input for Perception: A Review of Simulation Models

We review four current computational models that simulate the response of mechanoreceptors in the glabrous skin to tactile stimulation. The aim is to inform researchers in psychology, sensorimotor science and robotics who may want to implement this type of quantitative model in their research. This approach proves relevant to understanding of the interaction between skin response and neural activity as it avoids some of the limitations of traditional measurement methods of tribology, for the skin, and neurophysiology, for tactile neurons. The main advantage is to afford new ways of looking at the combined effects of skin properties on the activity of a population of tactile neurons, and to examine different forms of coding by tactile neurons. Here, we provide an overview of selected models from stimulus application to neuronal spiking response, including their evaluation in terms of existing data, and their applicability in relation to human tactile perception.

Do motor plans affect sensorimotor state estimates during temporal decision-making with crossed vs. uncrossed hands? Failure to replicate the dynamic crossed-hand effect

Hermosillo et al. (J Neurosci 31: 10019-10022, 2011) have suggested that action planning of hand movements impacts decisions about the temporal order judgments regarding vibrotactile stimulation of the hands. Specifically, these authors reported that the crossed-hand effect, a confusion about which hand is which when held in a crossed posture, gradually reverses some 320 ms before the arms begin to move from an uncrossed to a crossed posture or vice versa, such that the crossed-hand is reversed at the time of movement onset in anticipation of the movement's end position. However, to date, no other study has attempted to replicate this dynamic crossed-hand effect. Therefore, in the present study, we conducted four experiments to revisit the question whether preparing uncrossed-to-crossed or crossed-to-uncrossed movements affects the temporo-spatial perception of tactile stimulation of the hands. We used a temporal order judgement (TOJ) task at different time stages during action planning to test whether TOJs are more difficult with crossed than uncrossed hands ("static crossed-hand effect") and, crucially, whether planning to cross or uncross the hands shows the opposite pattern of difficulties ("dynamic crossed-hand effect"). As expected, our results confirmed the static crossed-hand effect. However, the dynamic crossed-hand effect could not be replicated. In addition, we observed that participants delayed their movements with late somatosensory stimulation from the TOJ task, even when the stimulations were meaningless, suggesting that the TOJ task resulted in cross-modal distractions. Whereas the current findings are not inconsistent with a contribution of motor signals to posture perception, they cast doubt on observations that motor signals impact state estimates well before movement onset.

Tonic somatosensory responses and deficits of tactile awareness converge in the parietal operculum

Although clinical neuroscience and the neuroscience of consciousness have long sought mechanistic explanations of tactile-awareness disorders, mechanistic insights are rare, mainly because of the difficulty of depicting the fine-grained neural dynamics underlying somatosensory processes.

Here, we combined the stereo-EEG responses to somatosensory stimulation with the lesion mapping of patients with a tactile-awareness disorder, namely tactile extinction.

Whereas stereo-EEG responses present different temporal patterns, including early/phasic and long-lasting/tonic activities, tactile-extinction lesion mapping co-localizes only with the latter. Overlaps are limited to the posterior part of the perisylvian regions, suggesting that tonic activities may play a role in sustaining tactile awareness. To assess this hypothesis further, we correlated the prevalence of tonic responses with the tactile-extinction lesion mapping, showing that they follow the same topographical gradient. Finally, in parallel with the notion that visuotactile stimulation improves detection in tactile-extinction patients, we demonstrated an enhancement of tonic responses to visuotactile stimuli, with a strong voxel-wise correlation with the lesion mapping.

The combination of these results establishes tonic responses in the parietal operculum as the ideal neural correlate of tactile awareness.

Mapping the neural systems driving breathing at the transition to unconsciousness

After falling asleep, the brain needs to detach from waking activity and reorganize into a functionally distinct state. A functional MRI (fMRI) study has recently revealed that the transition to unconsciousness induced by propofol involves a global decline of brain activity followed by a transient reduction in cortico-subcortical coupling. We have analyzed the relationships between transitional brain activity and breathing changes as one example of a vital function that needs the brain to readapt. Thirty healthy participants were originally examined. The analysis involved the correlation between breathing and fMRI signal upon loss of consciousness. We proposed that a decrease in ventilation would be coupled to the initial decline in fMRI signal in brain areas relevant for modulating breathing in the awake state, and that the subsequent recovery would be coupled to fMRI signal in structures relevant for controlling breathing during the unconscious state. Results showed that a slight reduction in breathing from wakefulness to unconsciousness was distinctively associated with decreased activity in brain systems underlying different aspects of consciousness including the prefrontal cortex, the default mode network and somatosensory areas. Breathing recovery was distinctively coupled to activity in deep brain structures controlling basic behaviors such as the hypothalamus and amygdala. Activity in the brainstem, cerebellum and hippocampus was associated with breathing variations in both states. Therefore, our brain maps illustrate potential drives to breathe, unique to wakefulness, in the form of brain systems underlying cognitive awareness, self-awareness and sensory awareness, and to unconsciousness involving structures controlling instinctive and homeostatic behaviors.

Precision control for a flexible body representation

Adaptive body representation requires the continuous integration of multisensory inputs within a flexible 'body model' in the brain. The present review evaluates the idea that this flexibility is augmented by the contextual modulation of sensory processing 'top-down'; which can be described as precision control within predictive coding formulations of Bayesian inference. Specifically, I focus on the proposal that an attenuation of proprioception may facilitate the integration of conflicting visual and proprioceptive bodily cues. Firstly, I review empirical work suggesting that the processing of visual vs proprioceptive body position information can be contextualised 'top-down'; for instance, by adopting specific attentional task sets. Building up on this, I review research showing a similar contextualisation of visual vs proprioceptive information processing in the rubber hand illusion and in visuomotor adaptation. Together, the reviewed literature suggests that proprioception, despite its indisputable importance for body perception and action control, can be attenuated top-down (through precision control) to facilitate the contextual adaptation of the brain's body model to novel visual feedback.

Body size estimation in obesity: a systematic review and meta-analysis

Previous research about body size estimation in obesity reported heterogeneous results. This might be related to the fact that the tasks adopted explored different body representations. Classifying the previous studies according to the specific body representation probed (i.e. implicit, explicit or both) might clarify discordant findings. A systematic review and meta-analysis of research articles assessing body size estimation in individuals affected by obesity compared with healthy weight individuals were performed in PubMed and Web of Science. Additional records were identified by reference lists inspection. The last search was run in May 2021. Two independent authors performed data extraction according to predefined criteria. Both groups either overestimated or underestimated the implicit body representation depending on the task used and the body part considered. Conversely, the explicit representation of the body was mainly overestimated by both individuals with obesity and healthy weight. In tasks relying on both these representations, overestimation and underestimation were reported in both groups, possibly depending on the degree of which each procedure relied on the explicit and/or implicit representation of the body. According to the meta-analysis, individuals with obesity tended to be significantly less accurate in body size estimations than participants with healthy weight. We confirmed that heterogeneous findings in body size estimation in obesity are related to the adoption of different tasks, which likely involve different body representations. We discussed the role of body dissatisfaction and altered somatosensation in the lower accuracy observed in obesity.

Neural Basis of Somatosensory Spatial and Temporal Discrimination in Humans: The Role of Sensory Detection

While detecting somatic stimuli from the external environment, an accurate determination of their spatial and temporal properties is essential for human behavior. Whether and how detection relates to human capacity for somatosensory spatial discrimination (SD) and temporal discrimination (TD) remains unclear. Here, participants underwent functional magnetic resonance imaging scanning when simply detecting vibrotactile stimuli of the leg, judging their location (SD), or deciding their number in time (TD). By conceptualizing tactile discrimination as consisting of detection and determination processes, we found that tactile detection elicited activation specifically involved in SD within the right inferior and superior parietal lobules, 2 regions previously implicated in the control of spatial attention. These 2 regions remained activated in the determination process, during which functional connectivity between these 2 regions predicted individual SD ability. In contrast, tactile detection produced little activation specifically related to TD. Participants' TD ability was implemented in brain regions implicated in coding temporal structures of somatic stimuli (primary somatosensory cortex) and time estimation (anterior cingulate, pre-supplementary motor area, and putamen). Together, our findings indicate a close link between somatosensory detection and SD (but not TD) at the neural level, which aids in explaining why we can promptly respond toward detected somatic stimuli.

Scanned optogenetic control of mammalian somatosensory input to map input-specific behavioral outputs

Somatosensory stimuli guide and shape behavior, from immediate protective reflexes to longer-term learning and higher-order processes related to pain and touch. However, somatosensory inputs are challenging to control in awake mammals due to the diversity and nature of contact stimuli. Application of cutaneous stimuli is currently limited to relatively imprecise methods as well as subjective behavioral measures. The strategy we present here overcomes these difficulties, achieving 'remote touch' with spatiotemporally precise and dynamic optogenetic stimulation by projecting light to a small defined area of skin. We mapped behavioral responses in freely behaving mice with specific nociceptor and low-threshold mechanoreceptor inputs. In nociceptors, sparse recruitment of single-action potentials shapes rapid protective pain-related behaviors, including coordinated head orientation and body repositioning that depend on the initial body pose. In contrast, activation of low-threshold mechanoreceptors elicited slow-onset behaviors and more subtle whole-body behaviors. The strategy can be used to define specific behavioral repertoires, examine the timing and nature of reflexes, and dissect sensory, motor, cognitive, and motivational processes guiding behavior.

Abnormal effective connectivity in the sensory network in writer's cramp

BACKGROUND

Writer's cramp (WC), a task specific form of dystonia, is considered to be a motor network disorder, but abnormal sensory tactile processing has also been acknowledged. The sensory spatial discrimination threshold (SDT) can be determined with a spatial acuity test (JVP domes). In addition to increased SDT, patients with WC exhibited dysfunctional sensory processing in the sensory cortex, insula, basal ganglia and cerebellum in a functional magnetic resonance imaging (fMRI) study while performing the spatial acuity test.

OBJECTIVES

To assess whether effective connectivity (EC) in the sensory network including cortical, basal ganglia, thalamic and cerebellar regions of interest in WC patients is abnormal.

METHODS

We used fMRI and applied a block design, while 19 WC patients and 13 age-matched healthy controls performed a spatial discrimination task. Before we assessed EC using dynamic causal modelling, we compared three model structures based on the current literature. We enclosed regions of interest that are established for sensory processing during right hand stimulation: Left thalamus, somatosensory, parietal and insular cortex, posterior putamen, and right cerebellum.

RESULTS

The EC analysis revealed task-dependent decreased unidirectional connectivity between the insula and the posterior putamen. The connectivity involving the primary sensory cortex, parietal cortex and cerebellum were not abnormal in WC. The two groups showed no differences in their behavioural data.

CONCLUSIONS

Perception and integration of sensory information requires the exchange of information between the insula cortex and the putamen, a sensory process that was disturbed in WC patients.

Human Somatosensory Processing and Artificial Somatosensation

In the past few years, we have gained a better understanding of the information processing mechanism in the human brain, which has led to advances in artificial intelligence and humanoid robots. However, among the various sensory systems, studying the somatosensory system presents the greatest challenge. Here, we provide a comprehensive review of the human somatosensory system and its corresponding applications in artificial systems. Due to the uniqueness of the human hand in integrating receptor and actuator functions, we focused on the role of the somatosensory system in object recognition and action guidance. First, the low-threshold mechanoreceptors in the human skin and somatotopic organization principles along the ascending pathway, which are fundamental to artificial skin, were summarized. Second, we discuss high-level brain areas, which interacted with each other in the haptic object recognition. Based on this close-loop route, we used prosthetic upper limbs as an example to highlight the importance of somatosensory information. Finally, we present prospective research directions for human haptic perception, which could guide the development of artificial somatosensory systems.

Reduced frontal white matter microstructure in healthy older adults with low tactile recognition performance

The aging of the nervous system is a heterogeneous process. It remains a significant challenge to identify relevant markers of pathological and healthy brain aging. A central aspect of aging are decreased sensory acuities, especially because they correlate with the decline in higher cognitive functioning. Sensory and higher cognitive processing relies on information flow between distant brain areas. Aging leads to disintegration of the underlying white matter tracts. While this disintegration is assumed to contribute to higher cognitive decline, data linking structural integrity and sensory function are sparse. The investigation of their interrelation may provide valuable insight into the mechanisms of brain aging. We used a combined behavioral and neuroimaging approach and investigated to what extent changes in microstructural white matter integrity reflect performance declines in tactile pattern recognition with aging. Poor performance in older participants was related to decreased integrity in the anterior corpus callosum. Probabilistic tractography showed that this structure is connected to the prefrontal cortices. Our data point to decreased integrity in the anterior corpus callosum as a marker for advanced brain aging. The correlation between impaired tactile recognition and disintegration in frontal brain networks could provide an explanation why the decrease of sensory function predicts cognitive decline.

Somatosensory deafferentation reveals lateralized roles of proprioception in feedback and adaptive feedforward control of movement and posture

Proprioception provides crucial information necessary for determining limb position and movement, and plausibly also for updating internal models that might underlie the control of movement and posture. Seminal studies of upper-limb movements in individuals living with chronic, large fiber deafferentation have provided evidence for the role of proprioceptive information in the hypothetical formation and maintenance of internal models to produce accurate motor commands. Vision also contributes to sensorimotor functions but cannot fully compensate for proprioceptive deficits. More recent work has shown that posture and movement control processes are lateralized in the brain, and that proprioception plays a fundamental role in coordinating the contributions of these processes to the control of goal-directed actions. In fact, the behavior of each limb in a deafferented individual resembles the action of a controller in isolation. Proprioception, thus, provides state estimates necessary for the nervous system to efficiently coordinate multiple motor control processes.

Neural encoding of actual and imagined touch within human posterior parietal cortex

In the human posterior parietal cortex (PPC), single units encode high-dimensional information with partially mixed representations that enable small populations of neurons to encode many variables relevant to movement planning, execution, cognition, and perception. Here, we test whether a PPC neuronal population previously demonstrated to encode visual and motor information is similarly engaged in the somatosensory domain. We recorded neurons within the PPC of a human clinical trial participant during actual touch presentation and during a tactile imagery task. Neurons encoded actual touch at short latency with bilateral receptive fields, organized by body part, and covered all tested regions. The tactile imagery task evoked body part-specific responses that shared a neural substrate with actual touch. Our results are the first neuron-level evidence of touch encoding in human PPC and its cognitive engagement during a tactile imagery task, which may reflect semantic processing, attention, sensory anticipation, or imagined touch.

Distinguishing left from right: A large-scale investigation of left-right confusion in healthy individuals

The ability to distinguish left from right has been shown to vary substantially within healthy individuals, yet its characteristics and mechanisms are poorly understood. In three experiments, we focused on a detailed description of the ability to distinguish left from right and the role of individual differences, and further explored the potential underlying mechanisms. In Experiment 1, a questionnaire concerning self-reported left-right identification (LRI) and strategy use was administered. Objective assessment was used in Experiment 2 by means of vocal responses to line drawings of a figure, with the participants' hands in a spatially neutral position. In Experiment 3, the arm positions and visibility of the hands were manipulated to assess whether bodily posture influences left-right decisions. Results indicate that 14.6% of the general population reported insufficient LRI and that 42.9% of individuals use a hand-related strategy. Furthermore, we found that spatial alignment of the participants' arms with the stimuli increased performance, in particular with a hand-related strategy and females. Performance was affected only by the layout of the stimuli, not by the position of the participant during the experiment. Taken together, confusion about left and right occurs within healthy population to a limited extent, and a hand-related strategy affects LRI. Moreover, the process involved appears to make use of a stored body representation and not bottom-up sensory input. Therefore, we suggest a top-down body representation is the key mechanism in determining left and right, even when this is not explicitly part of the task.