Somatosensory cortical representation of the body size

Multisensory integration and motor resonance in the primary motor cortex

Humans are endowed with a motor system that resonates to speech sounds, but whether concurrent visual information from lip movements can improve speech perception at a motor level through multisensory integration mechanisms remains unknown. Therefore, the aim of the study was to explore behavioral and neurophysiological correlates of multisensory influences on motor resonance in speech perception. Motor-evoked potentials (MEPs), by single pulse transcranial magnetic stimulation (TMS) applied over the left lip muscle (orbicularis oris) representation in the primary motor cortex, were recorded in healthy participants during the presentation of syllables in unimodal (visual or auditory) or multisensory (audio-visual) congruent or incongruent conditions. At the behavioral level, subjects showed better syllable identification in the congruent audio-visual condition as compared to the unimodal conditions, hence showing a multisensory enhancement effect. Accordingly, at the neurophysiological level, increased MEPs amplitudes were found in the congruent audio-visual condition, as compared to the unimodal ones. Incongruent audio-visual syllables resulting in illusory percepts did not increase corticospinal excitability, which in fact was comparable to that induced by the real perception of the same syllable. In conclusion, seeing and hearing congruent bilabial syllables increases the excitability of the lip representation in the primary motor cortex, hence documenting that multisensory integration can facilitate speech processing by influencing motor resonance. These findings highlight the modulation role of multisensory processing showing that it can boost speech perception and that multisensory interactions occur not only within higher-order regions, but also within primary motor areas, as shown by corticospinal excitability changes.

Unlocking the potential of 'passive' modulation: How sensory stimulation shapes hand and face size

Knowledge of the body size is intricately tied to multisensory integration processes that rely on the dynamic interplay of top-down and bottom-up mechanisms. Recent years have seen the development of passive sensory stimulation protocols aimed at investigating the modulation of various cognitive functions, primarily inducing perceptual learning and behaviour change without the need for extensive training. Given that reductions in sensory input have been associated with alterations in body size perception, it is reasonable to hypothesize that increasing sensory information through passive sensory stimulation could similarly influence the perception of the size of body parts. The primary aim of this study was to investigate the potential modulatory effects of passive sensory stimulation on the perception of hand and face size in a group of young adults. Passive sensory stimulation effectively modulated the size representation of the stimulated hand, supporting the notion that access to somatosensory and proprioceptive information is prioritised for the hands but may not extend to the face. Increased somatosensory input resulted in a reduction of distortion, providing evidence for bottom-up modulation of size representation. Passive sensory stimulation can induce subjective changes in body size perception without the need for extensive training. This paradigm holds promise as a potential alternative for modulating distorted size representation in individuals with body representational deficits.

Increased temporal binding during voluntary motor task under local anesthesia

Temporal binding refers to a systemic bias in the perceived time interval between two related events, most frequently voluntary motor actions and a subsequent sensory effect. An inevitable component of most instrumental motor actions is tactile feedback. Yet, the role of tactile feedback within this phenomenon remains largely unexplored. Here, we used local anesthesia of the index finger to temporarily inhibit incoming sensory input from the finger itself, while participants performed an interval-estimation task in which they estimated the delay between a voluntary motor action (button press) and a second sensory event (click sound). Results were compared to a control condition with intact sensation. While clear binding was present in both conditions, the effect was significantly enhanced when tactile feedback was temporarily removed via local anesthesia. The results are discussed in light of current debates surrounding the underlying mechanisms and function of this temporal bias.

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.

Selective effects of a brain tumor on the metric representation of the hand: a pre- versus post-surgery comparison

Body representation disorders are complex, varied, striking, and very disabling in most cases. Deficits of body representation have been described after lesions to multimodal and sensorimotor cortical areas. A few studies have reported the effects of tumors on the representation of the body, but little is known about the changes after tumor resection. Moreover, the impact of brain lesions on the hand size representation has been investigated in few clinical cases. Hands are of special importance, as no other body part has the ability for movement and interaction with the environment that the hands have, and we use them for a multitude of daily activities. Studies with clinical population can add further knowledge into the way hands are represented. Here, we report a single case study of a patient (AM) who was an expert bodybuilder and underwent a surgery to remove a glioblastoma in the left posterior prefrontal and precentral cortex at the level of the hand's motor region. Pre- (20 days) and post- (4 months) surgery assessment did not show any motor or cognitive impairments. A hand localization task was used, before and after surgery (12 months), to measure possible changes of the metric representation of his right hand. Results showed a post-surgery modulation of the typically distorted hand representation, with an overall accuracy improvement, especially on width dimension. These findings support the direct involvement of sensorimotor areas in the implicit representation of the body size and its relevance on defining specific size representation dimensions.

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.

I am the metre: The representation of one's body size affects the perception of tactile distances on the body

Humans must ground the perception of one's body in a mental representation to move in space and interact with objects. This representation can be temporarily altered artificially. In the full-body illusion (FBI), participants see a virtual (or filmed) body receiving a tactile stimulation. When participants receive touches on their body similarly to the seen one (i.e., homologous location and synchronous timing), they embody the seen alien body. While the subjective embodiment of alien bodies of different sizes has been already manipulated with the FBI, it remains unexplored whether the body-metric perception is impacted too. We first developed a new setup for the FBI using 360° videos to favour the embodiment. The FBI was induced for bodies of three sizes adopting anatomical and non-anatomical viewpoints, and we measured the subjective embodiment. The results suggest that humans can embody normal size or bigger bodies seen from anatomical viewpoints, but not smaller ones. We then investigated if the FBI modulates the body-metric representation. We found that the resized bodies' vision affects the perception of one's body-metric representation, but this was independent of the embodiment, suggesting that the FBI alters the body representation at different levels with a specific impact.

Lewis J. The structural and functional connectivity neural underpinnings of body image

How we perceive our bodies is fundamental to our self-consciousness and our experience in the world. There are two types of interrelated internal body representations-a subjective experience of the position of a limb in space (body schema) and the subjective experience of the shape and size of the limb (body image). Body schema has been extensively studied, but there is no evidence of the brain structure and network dynamics underpinning body image. Here, we provide the first evidence for the extrastriate body area (EBA), a multisensory brain area, as the structural and functional neural substrate for body shape and size. We performed a multisensory finger-stretch illusion that elongated the index finger. EBA volume and functional connectivity to the posterior parietal cortex are both related to the participants' susceptibility to the illusion. Taken together, these data suggest that EBA structure and connectivity encode body representation and body perception disturbances.

Perceptual Representation of Own Hand Size in Early Childhood and Adulthood

Hand size perceptual distortions characterize adult human cognition. Notwithstanding the importance of uncovering how hand size representation develops in humans, studies in this field are still at a preliminary stage. Indeed, it is yet to be understood whether hand size distortions are present and reliable in early childhood and whether they differ from adults’ distortions, offering a more in-depth insight into the emergence and development of such representations. We addressed this issue by comparing 4- to 6- year-old children and adults’ representation of their own hand size, as assessed with a 2-forced choice visual perceptual task. To test participants’ ability to estimate their own hand size, children and adults judged whether pictures of their own hand, resized to appear smaller or bigger than their own hand, matched or not its actual dimension. Results show that children aged 4 to 6 years tend to underestimate their own hand size, while adults underestimate their own hand more weakly. This evidence suggests that body-parts perceptual distortions are already in place in early childhood, and thus represent a characteristic of the human body representation.

Somatosensory cortical representation of the body size

The knowledge of the size of our own body parts is essential for accurately moving in space and efficiently interact with objects. A distorted perceptual representation of the body size often represents a core diagnostic criterion for some psychopathological conditions. The metric representation of the body was shown to depend on somatosensory afferences: local deafferentation indeed causes a perceptual distortion of the size of the anesthetized body part. A specular effect can be induced by altering the cortical map of body parts in the primary somatosensory cortex. Indeed, the present study demonstrates, in healthy adult participants, that repetitive Transcranial Magnetic Stimulation to the somatosensory cortical map of the hand in both hemispheres causes a perceptual distortion (i.e., an overestimation) of the size of the participants' own hand (Experiments 1-3), which does not involve other body parts (i.e., the foot, Experiment 2). Instead, the stimulation of the inferior parietal lobule of both hemispheres does not affect the perception of the own body size (Experiment 4). These results highlight the role of the primary somatosensory cortex in the building up and updating of the metric of body parts: somatosensory cortical activity not only shapes our somatosensation, it also affects how we perceive the dimension of our body.