Bodily Illusions and Motor Imagery in Fibromyalgia

Eliciting the rubber hand illusion by the activation of nociceptive C and Aδ fibers

ABSTRACT

The coherent perceptual experience of one's own body depends on the processing and integration of signals from multiple sensory modalities, including vision, touch, and proprioception. Although nociception provides critical information about damage to the tissues of one's body, little is known about how nociception contributes to own-body perception. A classic experimental approach to investigate the perceptual and neural mechanisms involved in the multisensory experience of one's own body is the rubber hand illusion (RHI). During the RHI, people experience a rubber hand as part of their own body (sense of body ownership) caused by synchronized stroking of the rubber hand in the participant's view and the hidden participant's real hand. We examined whether the RHI can be elicited by visual and "pure" nociceptive stimulation, ie, without tactile costimulation, and if so, whether it follows the basic perceptual rules of the illusion. In 6 separate experiments involving a total of 180 healthy participants, we used a Nd:YAP laser stimulator to specifically target C and Aδ fibers in the skin and compared the illusion condition (congruent visuonociceptive stimulation) to control conditions of incongruent visuonociceptive, incongruent visuoproprioceptive, and no nociceptive stimulation. The illusion was quantified through direct (questionnaire) and indirect (proprioceptive drift) behavioral measures. We found that a nociceptive rubber hand illusion (N-RHI) could be elicited and that depended on the spatiotemporal congruence of visuonociceptive signals, consistent with basic principles of multisensory integration. Our results suggest that nociceptive information shapes multisensory bodily awareness and contributes to the sense of body ownership.

How pain and body representations transform each other: A narrative review

Pain, as a multidimensional and subjective experience, intertwines with various aspects of body representation, involving sensory, affective and motivational components. This review explores the bidirectional relationship between pain and body representations, emphasizing the impact of the sense of ownership on pain perception, the transformative impact of pain on motor imagery, the effects associated with vicarious pain perception on body representations and the role of pain in the maintenance of body representations in specific clinical conditions. Literature indicates complex interactions between pain and body representations, with the sense of ownership inducing analgesic effects in some cases and hyperalgesia in others, contingent upon factors such as the appearance of the affected limb. Pain sensations inform the body on which actions might be executed without harm, and which are potentially dangerous. This information impacts on motor imagery too, showing reduced motor imagery and increased reaction times in tasks where motor imagery involves the painful body parts. Finally, contrary to the conventional view, according to which pain impairs body representation, evidence suggests that pain can serve as an informative somatosensory index, preserving or even enhancing the representation of the absent or affected body parts. This bidirectional relationship highlights the dynamic and multifaceted nature of the interplay between pain and body representations, offering insights into the adaptive nature of the central nervous system in response to perceived bodily states.

Preserved tactile distance estimation despite body representation distortions in individuals with fibromyalgia

Our mental representation of our body depends on integrating various sensory modalities, such as tactile information. In tactile distance estimation (TDE) tasks, participants must estimate the distance between two tactile tips applied to their skin. This measure of tactile perception has been linked to body representation assessments. Studies in individuals with fibromyalgia (FM), a chronic widespread pain syndrome, suggest the presence of body representation distortions and tactile alterations, but TDE has never been examined in this population. Twenty participants with FM and 24 pain-free controls performed a TDE task on three Body regions (upper limb, trunk, lower limb), in which they manually estimated the interstimuli distance on a tablet. TDE error, the absolute difference between the estimation and the interstimuli distance, was not different between the Groups, on any Body region. Drawings of their body as they felt it revealed clear and frequent distortions of body representation in the group with FM, compared to negligible perturbations in controls. This contrast between distorted body drawings and unaltered TDE suggests a preserved integration of tactile information but an altered integration of this information with other sensory modalities to generate a precise and accurate body representation. Future research should investigate the relative contribution of each sensory information and prior knowledge about the body in body representation in individuals with FM to shed light on the observed distortions.

Visuotactile integration in individuals with fibromyalgia

Our brain constantly integrates afferent information, such as visual and tactile information, to perceive the world around us. According to the maximum-likelihood estimation (MLE) model, imprecise information will be weighted less than precise, making the multisensory percept as precise as possible. Individuals with fibromyalgia (FM), a chronic pain syndrome, show alterations in the integration of tactile information. This could lead to a decrease in their weight in a multisensory percept or a general disruption of multisensory integration, making it less beneficial. To assess multisensory integration, 15 participants with FM and 18 pain-free controls performed a temporal-order judgment task in which they received pairs of sequential visual, tactile (unisensory conditions), or visuotactile (multisensory condition) stimulations on the index and the thumb of the non-dominant hand and had to determine which finger was stimulated first. The task enabled us to measure the precision and accuracy of the percept in each condition. Results indicate an increase in precision in the visuotactile condition compared to the unimodal conditions in controls only, although we found no intergroup differences. The observed visuotactile precision was correlated to the precision predicted by the MLE model in both groups, suggesting an optimal integration. Finally, the weights of the sensory information were not different between the groups; however, in the group with FM, higher pain intensity was associated with smaller tactile weight. This study shows no alterations of the visuotactile integration in individuals with FM, though pain may influence tactile weight in these participants.

Motor imagery vividness and symptom severity in Parkinson's disease

Motor imagery (MI), the mental simulation of movement in the absence of overt motor output, has demonstrated potential as a technique to support rehabilitation of movement in neurological conditions such as Parkinson's disease (PD). Existing evidence suggests that MI is largely preserved in PD, but previous studies have typically examined global measures of MI and have not considered the potential impact of individual differences in symptom presentation on MI. The present study investigated the influence of severity of overall motor symptoms, bradykinesia and tremor on MI vividness scores in 44 individuals with mild to moderate idiopathic PD. Linear mixed effects modelling revealed that imagery modality and the severity of left side bradykinesia significantly influenced MI vividness ratings. Consistent with previous findings, participants rated visual motor imagery (VMI) to be more vivid than kinesthetic motor imagery (KMI). Greater severity of left side bradykinesia (but not right side bradykinesia) predicted increased vividness of KMI, while tremor severity and overall motor symptom severity did not predict vividness of MI. The specificity of the effect of bradykinesia to the left side may reflect greater premorbid vividness for the dominant (right) side or increased attention to more effortful movements on the left side of the body resulting in more vivid motor imagery.