Frédérique de Vignemont

Bodily Illusions Modulate Tactile Perception

Touch differs from other exteroceptive senses in that the body itself forms part of the tactile percept. Interactions between proprioception and touch provide a powerful way to investigate the implicit body representation underlying touch. Here, we demonstrate that an intrinsic primary quality of a tactile object, for example its size, is directly affected by the perceived size of the body part touching it. We elicited proprioceptive illusions that the left index finger was either elongating or shrinking by vibrating the biceps or triceps tendon of the right arm while subjects grasped the tip of their left index finger. Subjects estimated the distance between two simultaneous tactile contacts on the left finger during tendon vibration. We found that tactile distances feel bigger when the touched body part feels elongated. Control tests showed that the modulation of touch was linked to the perceived index-finger size induced by tendon vibration. Vibrations that did not produce proprioceptive illusion had no effect on touch. Our results show that the perception of tactile objects is referenced to an implicit body representation and that proprioception contributes to this body representation. We also provide, for the first time, a quantitative, implicit measure of distortions of body size.

Embodiment, ownership and disownership ☆

There are two main pathways to investigate the sense of body ownership, (i) through the study of the conditions of embodiment for an object to be experienced as ones own and (ii) through the analysis of the deficits in patients who experience a body part as alien. Here, I propose that E is embodied if some properties of E are processed in the same way as the properties of ones body. However, one must distinguish among different types of embodiment, and only self-specific embodiment can lead to feelings of ownership. I address issues such as the functional role and the dynamics of embodiment, degrees and measures of ownership, and shared body representations between self and others. I then analyse the interaction between ownership and disownership. On the one hand, I show that there is no evidence that in the Rubber Hand Illusion, the rubber hand replaces the biological hand. On the other hand, I argue that the sense of disownership experienced by patients towards their body part cannot be reduced to the mere lack of ownership.There are two main pathways to investigate the sense of body ownership, (i) through the study of the conditions of embodiment for an object to be experienced as ones own and (ii) through the analysis of the deficits in patients who experience a body part as alien. Here, I propose that E is embodied if some properties of E are processed in the same way as the properties of ones body. However, one must distinguish among different types of embodiment, and only self-specific embodiment can lead to feelings of ownership. I address issues such as the functional role and the dynamics of embodiment, degrees and measures of ownership, and shared body representations between self and others. I then analyse the interaction between ownership and disownership. On the one hand, I show that there is no evidence that in the Rubber Hand Illusion, the rubber hand replaces the biological hand. On the other hand, I argue that the sense of disownership experienced by patients towards their body part cannot be reduced to the mere lack of ownership.

Is this hand for real? attenuation of the rubber hand illusion by transcranial magnetic stimulation over the inferior parietal lobule

In the rubber hand illusion (RHI), participants incorporate a rubber hand into a mental representation of ones body. This deceptive feeling of ownership is accompanied by recalibration of the perceived position of the participants real hand toward the rubber hand. Neuroimaging data suggest involvement of the posterior parietal lobule during induction of the RHI, when recalibration of the real hand toward the rubber hand takes place. Here, we used off-line low-frequency repetitive transcranial magnetic stimulation (rTMS) in a double-blind, sham-controlled within-subjects design to investigate the role of the inferior posterior parietal lobule (IPL) in establishing the RHI directly. Results showed that rTMS over the IPL attenuated the strength of the RHI for immediate perceptual body judgments only. In contrast, delayed perceptual responses were unaffected. Furthermore, ballistic action responses as well as subjective self-reports of feeling of ownership over the rubber hand remained unaffected by rTMS over the IPL. These findings are in line with previous research showing that the RHI can be broken down into dissociable bodily sensations. The illusion does not merely affect the embodiment of the rubber hand but also influences the experience and localization of ones own hand in an independent manner. Finally, the present findings concur with a multicomponent model of somatosensory body representations, wherein the IPL plays a pivotal role in subserving perceptual body judgments, but not actions or higher-order affective bodily judgments.

Perception of self-generated action in schizophrenia

Introduction. Self-generated actions involve central processes of sensorimotor integration that continuously monitor sensory inputs to ensure that motor outputs are congruent with our intentions. This mechanism works automatically in normal conditions but becomes conscious whenever a mismatch happens during the execution of action between expected and current sensorimotor reafferences. It is now admitted in the literature that sensorimotor processes as well as the ability to predict the consequences of our own actions imply the existence of a forward model of action, which is based on efference copies. Recently, it has been proposed that positive symptoms expressed by schizophrenic patients, such as delusions of control or thought insertions, arise because of a deficiency in this forward model, and more particularly, because of a lack of awareness of certain aspects of motor control derived from such an internal model. Method. To test further this hypothesis, 19 schizophrenic patients (10 with and 9 without Schneiderian symptoms) and 19 control subjects performed a visuo-motor conflict task and had verbally to report the felt position of their hand at the end of each trial. Results. Under this experimental procedure, schizophrenic patients - whatever their clinical phenotype - failed to switch to a conscious representation of their hand movements, and then consequently to maintain their level of performance for the sensorimotor adjustment in comparison with controls. Conclusion. Our findings suggest two facts. First, that a functional monitoring of action, based on a forward

The weight of representing the body: addressing the potentially indefinite number of body representations in healthy individuals

There is little consensus about the characteristics and number of body representations in the brain. In the present paper, we examine the main problems that are encountered when trying to dissociate multiple body representations in healthy individuals with the use of bodily illusions. Traditionally, task-dependent bodily illusion effects have been taken as evidence for dissociable underlying body representations. Although this reasoning holds well when the dissociation is made between different types of tasks that are closely linked to different body representations, it becomes problematic when found within the same response task (i.e., within the same type of representation). Hence, this experimental approach to investigating body representations runs the risk of identifying as many different body representations as there are significantly different experimental outputs. Here, we discuss and illustrate a different approach to this pluralism by shifting the focus towards investigating task-dependency of illusion outputs in combination with the type of multisensory input. Finally, we present two examples of behavioural bodily illusion experiments and apply Bayesian model selection to illustrate how this different approach of dissociating and classifying multiple body representations can be applied.

Cooling the Thermal Grill Illusion through Self-Touch

Acute peripheral pain is reduced by multisensory interactions at the spinal level [1]. Central pain is reduced by reorganization of cortical body representations [2, 3]. We show here that acute pain can also be reduced by multisensory integration through self-touch, which provides proprioceptive, thermal, and tactile input forming a coherent body representation [4, 5]. We combined self-touch with the thermal grill illusion (TGI) [6]. In the traditional TGI, participants press their fingers on two warm objects surrounding one cool object. The warm surround unmasks pain pathways, which paradoxically causes the cool object to feel painfully hot. Here, we warmed the index and ring fingers of each hand while cooling the middle fingers. Immediately after, these three fingers of the right hand were touched against the same three fingers on the left hand. This self-touch caused a dramatic 64% reduction in perceived heat. We show that this paradoxical release from paradoxical heat cannot be explained by low-level touch-temperature interactions alone. To reduce pain, we often clutch a painful hand with the other hand. We show here that self-touch not only gates pain signals reaching the brain [7-9] but also, via multisensory integration, increases coherence of cognitive body representations to which pain afferents project [10].

The co-consciousness hypothesis

Self-knowledge seems to be radically different from the knowledge of other people. However, rather than focusing on the gap between self and others, we should emphasize their commonality. Indeed, different “mirror matching mechanisms” have been found in monkeys as well as in humans showing that one uses the same representations for oneself and for the others. But do these shared representations allow one to report the mental states of others as if they were ones own? I intend in this essay to address the epistemic problem of other minds by developing Ayers notion of co-consciousness.

How many representations of the body

Based on functional differences, Dijkerman and de Haan emphasize the duality of somatosensory processing and therefore of body representations. But how many body representations do we really have? And what kind of criterion can we use to distinguish them? I review here the empirical and conceptual difficulties in drawing such distinctions and the way to progress.

The Marginal Body

According to Gurwitsch, the body is at least at the margin of consciousness. If all components of the field of consciousness were experienced as equally salient, we would indeed not be able to think and behave appropriately. Though the body may become the focus of our conscious field when we are introspectively aware of it, it remains most of the time only at the background of consciousness. However, we may wonder if bodily states do really need to be conscious, even at the margin, or cannot be simply non-conscious. Action control requires permanent proprioceptive and visual feedback about the state and the position of our body parts. Experimental data show that action monitoring operates at a nonconscious level and we may similarly suggest that we have a continuous unconscious access to bodily information. In this chapter, I thus intend to describe the various levels of body representations with the help of Gurwitschs distinction. I will investigate the properties and the function of each of these levels.

How do the body schema and the body image interact

Despite their differences, body schema and the body image representations are not only consistent in everyday life, but also sometimes consistent in pathological disorders, such as in Alice in Wonderland syndrome and anorexia nervosa. The challenge is to understand how they achieve such consistency. Recently, we suggested that these two representations were co-constructed (Pitron & Vignemont, 2017). In his reply, Gadsby (2018) invited us to clarify how this co-construction works and to what extent the body schema and the body image can reshape each other. Here we motivate conceptual grounds for a model on which these two forms of representation modify one another and explore theoretical options for the way(s) in which they might do so. In particular, we highlight the virtues of a serial model in which the body schema has some primacy over the body image, while also acknowledging the special role played by the body image.