Feeling of ownership over an embodied avatar's hand brings about fast changes of fronto-parietal cortical dynamics

When we look at our body parts, we are immediately aware that they belong to us and we rarely doubt about the integrity, continuity, and sense of ownership of our body. Despite this certainty, immersive virtual reality (IVR) may lead to a strong feeling of embodiment over an artificial body part seen from a first-person perspective (1PP). Although such feeling of ownership (FO) has been described in different situations, it is not yet understood how this phenomenon is generated at neural level. To track the real-time brain dynamics associated with FO, we delivered transcranial magnetic stimuli over the hand region in the primary motor cortex (M1) and simultaneously recorded electroencephalography (EEG) in 19 healthy volunteers (11 male/8 female) watching IVR renderings of anatomically plausible (full-limb) versus implausible (hand disconnected from the forearm) virtual limbs. Our data show that embodying a virtual hand is temporally associated with a rapid drop of cortical activity of the onlookers' hand region in the M1 contralateral to the observed hand. Spatiotemporal analysis shows that embodying the avatar's hand is also associated with fast changes of activity within an interconnected fronto-parietal circuit ipsilateral to the brain stimulation. Specifically, an immediate reduction of connectivity with the premotor area is paralleled by an enhancement in the connectivity with the posterior parietal cortex (PPC) which is related to the strength of ownership illusion ratings and thus likely reflects conscious feelings of embodiment. Our results suggest that changes of bodily representations are underpinned by a dynamic cross talk within a highly-plastic, fronto-parietal network

The sense of embodiment in Virtual Reality and its assessment methods

The sense of embodiment refers to the sensations of being inside, having, and controlling a body. In virtual reality, it is possible to substitute a person’s body with a virtual body, referred to as an avatar. Modulations of the sense of embodiment through modifications of this avatar have perceptual and behavioural consequences on users that can influence the way users interact with the virtual environment. Therefore, it is essential to define metrics that enable a reliable assessment of the sense of embodiment in virtual reality to better understand its dimensions, the way they interact, and their influence on the quality of interaction in the virtual environment. In this review, we first introduce the current knowledge on the sense of embodiment, its dimensions (senses of agency, body ownership, and self-location), and how they relate the ones with the others. Then, we dive into the different methods currently used to assess the sense of embodiment, ranging from questionnaires to neurophysiological measures. We provide a critical analysis of the existing metrics, discussing their advantages and drawbacks in the context of virtual reality. Notably, we argue that real-time measures of embodiment, which are also specific and do not require double tasking, are the most relevant in the context of virtual reality. Electroencephalography seems a good candidate for the future if its drawbacks (such as its sensitivity to movement and practicality) are improved. While the perfect metric has yet to be identified if it exists, this work provides clues on which metric to choose depending on the context, which should hopefully contribute to better assessing and understanding the sense of embodiment in virtual reality

A Ilusão da Mão de Borracha: Evidências de Integração Multissensorial da Propriocepção

The present review aims to describe the consolidation of a multisensory integration hypothesis for proprioception through the description of different Rubber Hand Illusion (RHI) experimental settings. RHI is a paradigm created in 1998 to explore the relation between visual and tactile sensory systems. The task involves a synchronous stroking, using a paintbrush, of one of the subjects hand, occluded from his vision, and a prosthetic rubber hand located in front of the subject. Instructed to look at the rubber hand, after approximately half a minute, the subject starts to feel as if the rubber hand is his own hand. The illusion produces a feeling of ownership of the rubber hand. Scientific research has widely investigated the experiment in the last 15 years, indicating the dynamic functions of the brain and body sensory systems, as well as shedding light on amputee rehabilitation and different types of paresthesia The review is structured around three topics: 1) the definition of the RHI, its limits and scope, 2) the physiological and neurocognitive evidence backing the RHI, and 3) the use of RHI in action experimental settings. The paper concludes that RHI is a convenient example of a neuroscientific trend towards an integrated account of body, brain and perceptual space. The illusion has also established an alternative way for studying proprioception and related brain dynamics in normal subjects.    La presente revisión tiene como objetivo describir una hipótesis de integración multisensorial para la propiocepción a través de la descripción de distintos configuraciones experimentales con la Ilusión de la Mano de Goma (img). img es un paradigma creado en 1998 para explorar la relación entre los sistemas sensoriales visual y táctil. La tarea implica una estimulación síncrona de una mano de los sujetos, oculto de su visión, y una mano de goma en frente del sujeto. Instruidos para mirar la mano de goma, después de aproximadamente medio minuto, el sujeto comienza a sentir la mano de goma como su propia mano. La ilusión produce un sentimiento de propiedad de la mano de goma. La literatura ha investigado ampliamente el experimento en los últimos 15 años, demostrando las funciones dinámicas de los sistemas sensoriales del cerebro y del cuerpo, así como clarificando aspectos de la rehabilitación de sujetos amputados y diferentes tipos de parestesia. La revisión se estructura en torno de tres temas: (1) definición de la img, sus límites y alcances, (2) evidencias fisiológicas y neurocognitivas que dan apoyo a la img, y (3) la img en configuraciones experimentales implicando acción. La revisión concluye que la img es un ejemplo práctico de una tendencia neurocientífica innovadora para el estudio integrado del cuerpo, el cerebro y el espacio perceptual. La ilusión también ha establecido una forma alternativa para el estudio de la propiocepción y la dinámica del cerebro en sujetos normalesA presente revisão tem como objetivo descrever uma hipótese de integração multissensorial para a propriocepção através da descrição de diferentes configurações experimentais com a Ilusão da Mão de Borracha (img). img é um paradigma criado em 1998 para explorar a relação entre os sistemas sensoriais visual e táctil. A tarefa implica uma estimulação síncrona de uma mão dos sujeitos, oculto de sua visão, e uma mão de borracha em frente ao sujeito. Instruídos para olhar a mão de borracha, depois de aproximadamente meio minuto, o sujeito começa a sentir a mão de borracha como a sua própria mão. A ilusão produz um sentimento de propriedade da mão de borracha. A literatura tem investigado amplamente o experimento nos últimos 15 anos, demostrando as funções demonstrando as funções dinâmicas dos sistemas sensoriais do cérebro e do corpo, assim como clarificando aspetos da reabilitação de sujeitos amputados e diferentes tipos de parestesia. A revisão estrutura-se em torno a três temas: (1) definição da img, os seus limites e alcances, (2) evidências fisiológicas e neurocognitivas que dão apoio à img e (3) a img em configurações experimentais implicando ação. A revisão conclui que a img é um exemplo prático de uma tendência neurocientífica inovadora para o estudo integrado do corpo, o cérebro e o espaço percetual. A ilusão também tem estabelecido uma alternativa para o estudo da propriocepção e a dinâmica do cérebro em sujeitos normais