Visual and somatosensory information contribute to distortions of the body model

The characteristics of the implicit body model of the trunk

Knowing where the body is in space requires reference to a stored model of the size and shape of body parts, termed the body model. This study sought to investigate the characteristics of the implicit body model of the trunk by assessing the position sense of midline and lateral body landmarks. Sixty-nine healthy participants localised midline and lateral body landmarks on their thorax, waist and hips, with perceived positions of these landmarks compared to actual positions. This study demonstrates evidence of a significant distortion of the implicit body model of the trunk, presenting as a squatter trunk, wider at the waist and hips. A significant difference was found between perceived and actual location in the horizontal (x) and vertical (y) directions for the majority of trunk landmarks. Evidence of a rightward bias was noted in the perception of six of the nine body landmarks in the horizontal (x) direction, including all midline levels. In the vertical (y) direction, a substantial inferior bias was evident at the thorax and waist. The implicit body model of the trunk is shown to be distorted, with the lumbar spine (waist-to-hip region) held to be shorter and wider than reality.

Proprioceptive acuity for landmarks on the hand and digits

Previous work using visually guided reaches to localize landmarks on a hidden hand has suggested that proprioceptive acuity for hand targets is low and representation of hand dimensions is highly distorted (e.g., hand width estimated to be 60% wider than actual hand width). We re-examined these issues using a pure proprioceptive task in which 20 blindfolded subjects reached in a single movement without terminal corrections to touch the right index-tip to landmarks of the left hand placed in various locations in 3D space. Subjects were also tested with vision allowed to estimate minimal errors. Based on previous reports of high proprioceptive acuity for some hand landmarks, we hypothesized that the proprioceptive representation of the hand was much less distorted than described previously and that errors were not correlated with target hand location. Mean distance errors in proprioceptively guided reaches to the landmarks averaged less than 3 cm and were only 0.5-1.3 cm larger than when vision was allowed. Errors were not correlated with hand location in most subjects. Distortions of hand width averaged less than 20% wider than actual width and were not correlated with hand location in most subjects. We conclude that relatively accurate proprioceptive awareness of locations of hand/digit structures and dimensions is available for use in control of hand movements, which are executed largely subconsciously. Studying acuity of proprioception using conscious perceptual tasks and involving vision may not provide accurate measures of proprioceptive acuity as used by the motor system.

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.

Persistence of metric biases in body representation during the body ownership illusion

Our perception of the body’s metric is influenced by bias according to the axis, called the systematic metric bias in body representation. Systematic metric bias was first reported as Weber’s illusion and observed in several parts of the body in various patterns. However, the systematic metric bias was not observed with a fake hand under the influence of the body ownership illusion during the line length judgment task. The lack of metric bias observed during the line length judgment task with a fake hand implies that the tactile modality occupies a relatively less dominant position than perception occurring through the real body. The change in weight between visual and tactile modalities during the body ownership illusion has not been adequately investigated yet, despite being a factor that influences the perception through body ownership illusion. Therefore, this study aimed to investigate whether the dominance of vision over tactile modality is prominent, regardless of the task type. To investigate whether visual dominance persists during the process of inducing body ownership illusion regardless of task type, we introduced spatial visuotactile incongruence (2 cm, 3 cm) in the longitudinal and transverse axes during the visuotactile localization tasks and measured the intensity of the body ownership illusion using a questionnaire. The results indicated that participants perceived smaller visuotactile incongruence when the discrepancy occurred in the transverse axis rather than in the longitudinal axis. The anisotropy in the tolerance of visuotactile incongruence implies the persistence of metric biases in body representation. The results suggest the need for further research regarding the factors influencing the weight of visual and tactile modalities.

Distortion of mental body representations

Our body is central to our sense of self, and distorted body representations are found in several serious medical conditions. This paper reviews evidence that distortions of body representations are also common in healthy individuals, and occur in domains including tactile spatial perception, proprioception, and the conscious body image. Across domains, there is a general tendency for body width to be overestimated compared to body length. Intriguingly, distortions in both eating disorders and chronic pain appear to be exaggerations of this baseline pattern of distortions, suggesting that these conditions may relate to dysfunction of mechanisms for body perception. Distortions of body representations provide a revealing window into basic aspects of self-perception.

Exploring the Interaction Between Handedness and Body Parts Ownership by Means of the Implicit Association Test

The experience of owning a body is built upon the integration of exteroceptive, interoceptive, and proprioceptive signals. Recently, it has been suggested that motor signals could be particularly important in producing the feeling of body part ownership. One thus may hypothesize that the strength of this feeling may not be spatially uniform; rather, it could vary as a function of the degree by which different body parts are involved in motor behavior. Given that our dominant hand plays a leading role in our motor behavior, we hypothesized that it could be more strongly associated with one’s self compared to its non-dominant counterpart. To explore whether this possible asymmetry manifests as a stronger implicit association of the right hand (vs left hand) with the self, we administered the Implicit Association Test to a group of 70 healthy individuals. To control whether this asymmetric association is human-body specific, we further tested whether a similar asymmetry characterizes the association between a right (vs left) animal body part with the concept of self, in an independent sample of subjects (N = 70, 140 subjects total). Our results revealed a linear relationship between the magnitude of the implicit association between the right hand with the self and the subject’s handedness. In detail, the strength of this association increased as a function of hand preference. Critically, the handedness score did not predict the association of the right-animal body part with the self. These findings suggest that, in healthy individuals, the dominant and non-dominant hands are differently perceived at an implicit level as belonging to the self. We argue that such asymmetry may stem from the different roles that the two hands play in our adaptive motor behavior.

Hand size representation in healthy children and young adults

Whereas we experience our body as a coherent volumetric object, the brain appears to maintain highly fragmented representations of individual body parts. Little is known about how body representations of hand size and shape are built and evolve during infancy and young adulthood. This study aimed to investigate the effect of hand side, handedness, and age on the development of central hand size representation. The observational study with comparison groups was conducted with 90 typically developing Belgian school children and young adults (48 male and 42 female; age range = 5.0-23.0 years; 49 left-handed and 41 right-handed). Participants estimated their hand size and shape using two different tasks. In the localization task, participants were verbally cued to judge the locations of 10 anatomical landmarks of an occluded hand. An implicit hand size map was constructed and compared with actual hand dimensions. In the template selection task, the explicit hand shape was measured with a depictive method. Hand shape indexes were calculated and compared for the actual, implicit, and explicit conditions. Participants were divided into four age groups (5-8 years, 9-10 years, 11-16 years, and 17-23 years). Implicit hand maps featured underestimation of finger length and overestimation of hand width, which is already present in the youngest children. Linear mixed modeling revealed no influence of hand side on finger length underestimation; nonetheless, a significant main effect of age (p = .001) was exposed. Sinistrals aged 11 to 16 years showed significantly less underestimation (p = .03) than dextrals of the same age. As for the hand shape, the implicit condition differed significantly with the actual and explicit conditions (p < .001). Again, the implicit shape index was subjected to handedness and age effects, with significant differences being found between sinistrals and dextrals in the age groups of 9 and 10 years (p = .029) and 11 to 16 years (p < .001). In conclusion, the implicit metric component of the hand representation in children and young adults is misperceived, featuring shortened fingers and broadened hands since a very young age. Crucially, the finger length underestimation increases with age and shows a different developmental trajectory for sinistrals and dextrals. In contrast, the explicit hand shape is approximately veridical and seems immune from age and handedness effects. This study confirms the dual character of somatoperception and establishes a point of reference for children and young adults.

Metric biases in body representation extend to objects

We typically misestimate the dimensions of our body e.g., we perceive our fingers as shorter, and our torso as more elongated, than they actually are. It stands to reason that those metric biases may also extend to objects that we interact with, to facilitate attunement with the environment. To explore this hypothesis, we compared the metric representations of seven objects and the subjects' own hand using the Line Length Judgment task, in six experiments involving 152 healthy subjects. We evaluated the size estimation errors made for each target (hand or previously observed objects) by asking subjects to compare the vertical or horizontal dimension of a specific target against the length of a vertical or horizontal line. As expected, we showed that the hand is misperceived in its dimensions. Interestingly, we found that metric biases are also present for daily-life objects, such as a mobile phone and a coffee mug, and are not affected by familiarity with the objects. In contrast, objects that are less likely to be manipulated, either because they are potentially harmful or disgusting, were differently represented. Furthermore, the propensity to interact with an object, rated by an independent sample of subjects, best predicted the pattern of metric biases associated with that object. Taken together, these findings support the hypothesis that biases affecting the hand representation extend to objects that elicit action-oriented behavior, highlighting the importance of studying the body as integrated and active in the environment.

Proprioceptive errors in the localization of hand landmarks: What can be learnt about the hand metric representation?

Proprioception acquires a crucial role in estimating the configuration of our body segments in space when visual information is not available. Proprioceptive accuracy is assessed by asking participants to match the perceived position of an unseen body landmark through reaching movements. This task was also adopted to study the perceived hand structure by computing the relative distances between averaged proprioceptive judgments (hand Localization Task). However, the pattern of proprioceptive errors leading to the misperceived hand structure is unexplored. Here, we aimed to characterize this pattern across different hand landmarks, having different anatomo-physiological properties and cortical representations. Furthermore, we sought to describe the error consistency and its stability over time. To this purpose, we analyzed the proprioceptive errors of 43 healthy participants during the hand Localization Task. We found larger but more consistent errors for the fingertips compared to the knuckles, possibly due to poorer proprioceptive signal, compensated by other sources of spatial information. Furthermore, we found a shift (overlap effect) and a temporal drift of the hand perceived position towards the shoulder of origin, which was consistent within and between subjects. The overlap effect had a greater influence on lateral compared to medial landmarks, leading to the hand width overestimation. Our results are compatible with domain-general and body-specific spatial biases affecting the proprioceptive localization of the hand landmarks, thus the apparent hand structure misperception.

The motor system (partially) deceives body representation biases in absence of visual correcting cues

The internal models of our body dimensions are prone to bias, but little evidence exists to explain how the motor system achieves fine-grained control despite these distortions. Previous work showed that the hand representation, assessed in a dynamic task (Proprioceptive Matching Task), was less distorted compared to that measured through a static body representation task (Localization Task), suggesting that either the hand representation was updated or the motor trajectory was adjusted during movement. The present study set out to shed light on this phenomenon by administering the Localization Task before and after either the Proprioceptive Matching Task or a control condition in a within-subjects design. Our results showed that hand map biases decreased during the Proprioceptive Matching Task, but that this increase in accuracy did not carry over to the Localization Task. In other words, more accurate performance in the dynamic body representation task does not reflect a change in how the hand is represented. Rather, it likely reflects a refinement of the motor trajectory, due to the integration of multisensory information, providing interesting insights into how the motor system partially overcomes biases in body representations.