The Rubber Hand Illusion: Two's a company, but three's a crowd

On the one hand, it is often assumed that the Rubber Hand Illusion (RHI) is constrained by a structural body model so that one cannot implement supernumerary limbs. On the other hand, several recent studies reported illusory duplication of the right hand in subjects exposed to two adjacent rubber hands. The present study tested whether spatial constraints may affect the possibility of inducing the sense of ownership to two rubber hands located side by side to the left of the subject's hand. We found that only the closest rubber hand appeared both objectively (proprioceptive drift) and subjectively (ownership rating) embodied. Crucially, synchronous touch of a second, but farther, rubber hand disrupted the objective measure of the RHI, but not the subjective one. We concluded that, in order to elicit a genuine RHI for multiple rubber hands, the two rubber hands must be at the same distance from the subject's hand/body

Categorical perception of tactile distance

The tactile surface forms a continuous sheet covering the body. And yet, the perceived distance between two touches varies across stimulation sites. Perceived tactile distance is larger when stimuli cross over the wrist, compared to when both fall on either the hand or the forearm. This effect could reflect a categorical distortion of tactile space across body-part boundaries (in which stimuli crossing the wrist boundary are perceptually elongated) or may simply reflect a localised increased in acuity surrounding anatomical landmarks (in which stimuli near the wrist are perceptually elongated). We tested these two interpretations, by comparing a well-documented bias to perceive mediolateral tactile distances across the forearm/hand as larger than proximodistal ones along the forearm/hand at three different sites (hand, wrist, and forearm). According to the ‘categorical’ interpretation, tactile distances should be elongated selectively in the proximodistal axis thus reducing the anisotropy. According to the ‘localised acuity’ interpretation, distances will be perceptually elongated in the vicinity of the wrist regardless of orientation, leading to increased overall size without affecting anisotropy. Consistent with the categorical account, we found a reduction in the magnitude of anisotropy at the wrist, with no evidence of a corresponding specialized increase in precision. These findings demonstrate that we reference touch to a representation of the body that is categorically segmented into discrete parts, which consequently influences the perception of tactile distance

Seeing the body distorts tactile size perception

Vision of the body modulates somatosensation, even when entirely non-informative about stimulation. For example, seeing the body increases tactile spatial acuity, but reduces acute pain. While previous results demonstrate that vision of the body modulates somatosensory sensitivity, it is unknown whether vision also affects metric properties of touch, and if so how. This study investigated how non-informative vision of the body modulates tactile size perception. We used the mirror box illusion to induce the illusion that participants were directly seeing their stimulated left hand, though they actually saw their reflected right hand. We manipulated whether participants: (a) had the illusion of directly seeing their stimulated left hand, (b) had the illusion of seeing a non-body object at the same location, or (c) looked directly at their non-stimulated right-hand. Participants made verbal estimates of the perceived distance between two tactile stimuli presented simultaneously to the dorsum of the left hand, either 20, 30, or 40 mm apart. Vision of the body significantly reduced the perceived size of touch, compared to vision of the object or of the contralateral hand. In contrast, no apparent changes of perceived hand size were found. These results show that seeing the body distorts tactile size perception

The plasticity of near space: evidence for contraction

The distinction between near space and the space farther away has been well established, as has the relation of this distinction to arm length. Recent studies provide evidence for the plasticity of near space, showing that it is possible to expand its extent ("size") through tool-use. In the present study, we examine the converse effect, whether contraction of near space results from increasing the effort involved on a line bisection task. Adult participants bisected lines at different distances, while, in some cases, wearing weights. In Experiment 1, the arms, specifically, were weighted (wrist weights), and in Experiment 2, more general body weights were used (heavy backpack). As in previous studies, unencumbered participants showed leftward bias when bisecting lines at the closest distances and a rightward shift in bias with increasingly farther distances. With wrist weights, but not a heavy backpack, participants showed more rightward bias at the closest distances, and a more gradual rightward shift with increasing distance, as if the nearest locations were represented as being farther away. These results suggest that increased effort, when specifically related to the arm, can serve to reduce the size of near space, providing support for the generally symmetrical plasticity of near space representations

The impact of music on learning and consolidation of novel words

Music can be a powerful mnemonic device, as shown by a body of literature demonstrating that listening to text sung to a familiar melody results in better memory for the words compared to conditions where they are spoken. Furthermore, patients with a range of memory impairments appear to be able to form new declarative memories when they are encoded in the form of lyrics in a song, while unable to remember similar materials after hearing them in the spoken modality. Whether music facilitates the acquisition of completely new information, such as new vocabulary, remains unknown. Here we report three experiments in which adult participants learned novel words in the spoken or sung modality. While we found no benefit of musical presentation on free recall or recognition memory of novel words, novel words learned in the sung modality were more strongly integrated in the mental lexicon compared to words learned in the spoken modality. This advantage for the sung words was only present when the training melody was familiar. The impact of musical presentation on learning therefore appears to extend beyond episodic memory and can be reflected in the emergence and properties of new lexical representations

Visual enhancement of touch and the bodily self

We experience our own body through both touch and vision. We further see that others’ bodies are similar to our own body, but we have no direct experience of touch on others’ bodies. Therefore, relations between vision and touch are important for the sense of self and for mental representation of one’s own body. For example, seeing the hand improves tactile acuity on the hand, compared to seeing a non-hand object. While several studies have demonstrated this visual enhancement of touch (VET) effect, its relation to the ‘bodily self’, or mental representation of one’s own body remains unclear. We examined whether VET is an effect of seeing a hand, or of seeing my hand, using the rubber hand illusion. In this illusion, a prosthetic hand which is brushed synchronously—but not asynchronously—with one’s own hand is felt to actually be one’s hand. Thus, we manipulated whether or not participants felt like they were looking directly at their hand, while holding the actual stimulus they viewed constant. Tactile acuity was measured by having participants judge the orientation of square-wave gratings. Two characteristic effects of VET were observed: (1) cross-modal enhancement from seeing the hand was inversely related to overall tactile acuity, and (2) participants near sensory threshold showed significant improvement following synchronous stroking, compared to asynchronous stroking or no stroking at all. These results demonstrate a clear functional relation between the bodily self and basic tactile perception

A supramodal representation of the body surface

The ability to accurately localize both tactile and painful sensations on the body is one of the most important functions of the somatosensory system. Most accounts of localization refer to the systematic spatial relation between skin receptors and cortical neurons. The topographic organization of somatosensory neurons in the brain provides a map of the sensory surface. However, systematic distortions in perceptual localization tasks suggest that localizing a somatosensory stimulus involves more than simply identifying specific active neural populations within a somatotopic map. Thus, perceptual localization may depend on both afferent inputs and other unknown factors. In four experiments, we investigated whether localization biases vary according to the specific skin regions and subset of afferent fibers stimulated. We represented localization errors as a ‘perceptual map’ of skin locations. We compared the perceptual maps of stimuli that activate Aβ (innocuous touch), Aδ (pinprick pain), and C fibers (non-painful heat) on both the hairy and glabrous skin of the left hand. Perceptual maps exhibited systematic distortions that strongly depended on the skin region stimulated. We found systematic distal and radial (i.e., towards the thumb) biases in localization of touch, pain, and heat on the hand dorsum. A less consistent proximal bias was found on the palm. These distortions were independent of the population of afferent fibers stimulated, and also independent of the response modality used to report localization. We argue that these biases are likely to have a central origin, and result from a supramodal representation of the body surface

Emotional Correlates of Unirhinal Odor Identification

It seems self-evident that smell profoundly shapes emotion, but less clear is the nature of this interaction. Here we sought to determine whether the ability to identify odors co-varies with self-reported feelings of empathy and emotional expression recognition, as predicted if the two capacities draw on common resource. Thirty six neurotypical volunteers were administered the Alberta Smell Test, The Interpersonal Reactivity Index and an emotional expression recognition task. Statistical analyses indicated that feelings of emotional empathy positively correlated with odor discrimination in right nostril, while the recognition of happy and fearful facial expressions positively correlated with odor discrimination in left nostril. These results uncover new links between olfactory discrimination and emotion which, given the ipsilateral configuration of the olfactory projections, point towards intra- rather than inter-hemispheric interaction. The results also provide novel support for the proposed lateralisation of emotional empathy and the recognition of facial expression, and give reason to further explore the diagnostic sensitivity of smell tests because reduced sensitivity to others’ emotions can mark the onset of certain neurological diseases

What is embodiment? a psychometric approach

What is it like to have a body? The present study takes a psychometric approach to this question. We collected structured introspective reports of the rubber hand illusion, to systematically investigate the structure of bodily self-consciousness. Participants observed a rubber hand that was stroked either synchronously or asynchronously with their own hand and then made proprioceptive judgments of the location of their own hand and used Likert scales to rate their agreement or disagreement with 27 statements relating to their subjective experience of the illusion. Principal components analysis of this data revealed four major components of the experience across conditions, which we interpret as: embodiment of rubber hand, loss of own hand, movement, and affect. In the asynchronous condition, an additional fifth component, deafference, was found. Secondary analysis of the embodiment of runner hand component revealed three subcomponents in both conditions: ownership, location, and agency. The ownership and location components were independent significant predictors of proprioceptive biases induced by the illusion. These results suggest that psychometric tools may provide a rich method for studying the structure of conscious experience, and point the way towards an empirically rigorous phenomenology

Owning an overweight or underweight body: distinguishing the physical, experienced and virtual body

Our bodies are the most intimately familiar objects we encounter in our perceptual environment. Virtual reality provides a unique method to allow us to experience having a very different body from our own, thereby providing a valuable method to explore the plasticity of body representation. In this paper, we show that women can experience ownership over a whole virtual body that is considerably smaller or larger than their physical body. In order to gain a better understanding of the mechanisms underlying body ownership, we use an embodiment questionnaire, and introduce two new behavioral response measures: an affordance estimation task (indirect measure of body size) and a body size estimation task (direct measure of body size). Interestingly, after viewing the virtual body from first person perspective, both the affordance and the body size estimation tasks indicate a change in the perception of the size of the participant’s experienced body. The change is biased by the size of the virtual body (overweight or underweight). Another novel aspect of our study is that we distinguish between the physical, experienced and virtual bodies, by asking participants to provide affordance and body size estimations for each of the three bodies separately. This methodological point is important for virtual reality experiments investigating body ownership of a virtual body, because it offers a better understanding of which cues (e.g. visual, proprioceptive, memory, or a combination thereof) influence body perception, and whether the impact of these cues can vary between different setups