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

Changing ideas about others' intentions: updating prior expectations tunes activity in the human motor system

Predicting intentions from observing another agent’s behaviours is often thought to depend on motor resonance – i.e., the motor system’s response to a perceived movement by the activation of its stored motor counterpart, but observers might also rely on prior expectations, especially when actions take place in perceptually uncertain situations. Here we assessed motor resonance during an action prediction task using transcranial magnetic stimulation to probe corticospinal excitability (CSE) and report that experimentally-induced updates in observers’ prior expectations modulate CSE when predictions are made under situations of perceptual uncertainty. We show that prior expectations are updated on the basis of both biomechanical and probabilistic prior information and that the magnitude of the CSE modulation observed across participants is explained by the magnitude of change in their prior expectations. These findings provide the first evidence that when observers predict others’ intentions, motor resonance mechanisms adapt to changes in their prior expectations. We propose that this adaptive adjustment might reflect a regulatory control mechanism that shares some similarities with that observed during action selection. Such a mechanism could help arbitrate the competition between biomechanical and probabilistic prior information when appropriate for prediction

The Agent is Right: When Motor Embodied Cognition is Space-Dependent

The role of embodied mechanisms in processing sentences endowed with a first person perspective is now widely accepted. However, whether embodied sentence processing within a third person perspective would also have motor behavioral significance remains unknown. Here, we developed a novel version of the Action-sentence Compatibility Effect (ACE) in which participants were asked to perform a movement compatible or not with the direction embedded in a sentence having a first person (Experiment 1: You gave a pizza to Louis) or third person perspective (Experiment 2: Lea gave a pizza to Louis). Results indicate that shifting perspective from first to third person was sufficient to prevent motor embodied mechanisms, abolishing the ACE. Critically, ACE was restored in Experiment 3 by adding a virtual “body” that allowed participants to know “where” to put themselves in space when taking the third person perspective, thus demonstrating that motor embodied processes are space-dependent. A fourth, control experiment, by dissociating motor response from the transfer verb's direction, supported the conclusion that perspective-taking may induce significant ACE only when coupled with the adequate sentence-response mapping

Tactile masking within and between hands: Insights for spatial coding of touch at the fingers

A tactile stimulus at the fingers can be encoded according to multiple reference frames (hand-, body- or space-specific). We examined the relative importance of these reference frames by adapting a tactile masking paradigm for stimuli at the index or middle fingers of either hand (unseen). In Exp.1, participants performed a go-no-go task to detect a vibrotactile target at a pre-specified finger (e.g., right index), when this was presented alone or with a concurrent distractor either on the same hand (right middle finger), or on the opposite hand (at homologous or non-homologous locations with respect to the target finger; e.g., left index or left middle finger, respectively). Tactile masking emerged under double stimulation, both for a distractor within the target-hand and for distractors at the non-homologous location on the opposite hand. This suggests use of hand-specific (than body- or space-specific) reference frames when solving this task. In Exp.2, one hand rotated by 180° around the wrist in half of the trials. Masking effects changed only between-hands. Intriguingly, masking from the non-homologous finger reduced only when the hand that changed posture contained the target. This suggests that spatial coding for touches at the fingers depend upon the behavioural relevance of tactile stimuli

Losing One's Hand: Visual-Proprioceptive Conflict Affects Touch Perception

BACKGROUND: While the sense of bodily ownership has now been widely investigated through the rubber hand illusion (RHI), very little is known about the sense of disownership. It has been hypothesized that the RHI also affects the ownership feelings towards the participant's own hand, as if the rubber hand replaced the participant's actual hand. Somatosensory changes observed in the participants' hand while experiencing the RHI have been taken as evidence for disownership of their real hand. Here we propose a theoretical framework to disambiguate whether such somatosensory changes are to be ascribed to the disownership of the real hand or rather to the anomalous visuo-proprioceptive conflict experienced by the participant during the RHI. METHODOLOGY/PRINCIPAL FINDINGS: In experiment 1, reaction times (RTs) to tactile stimuli delivered to the participants' hand slowed down following the establishment of the RHI. In experiment 2, the misalignment of visual and proprioceptive inputs was obtained via prismatic displacement, a situation in which ownership of the seen hand was doubtless. This condition slowed down the participants' tactile RTs. Thus, similar effects on touch perception emerged following RHI and prismatic displacement. Both manipulations also induced a proprioceptive drift, toward the fake hand in the first experiment and toward the visual position of the participants' hand in the second experiment. CONCLUSIONS/SIGNIFICANCE: These findings reveal that somatosensory alterations in the experimental hand resulting from the RHI result from cross-modal mismatch between the seen and felt position of the hand. As such, they are not necessarily a signature of disownership

Tactile stimuli at the fingers coding is modulated by hand posture

Introduzione. In due esperimenti abbiamo esaminato l’importanza relativa dei sistemi di riferimento (mano-, corpo- o spazio-specifico) per la codifica di stimoli tattili alle mani usando un paradigma di mascheramento tattile. Metodo. 12 soggetti hanno preso parte all’Esp. 1 (età media 29 anni, 3 femmine) e 12 soggetti hanno preso parte all’Esp. 2 (età media 27 anni, 5 femmine). In entrambi gli esperimenti, il soggetto eseguiva un compito di detezione di un bersaglio vibrotattile, presentato per 5 ms al dito indice o medio di una delle due mani. Il dito bersaglio era specificato dallo sperimentatore all’inizio di ciascun blocco (es., indice destro). Il bersaglio poteva comparire da solo o contemporaneamente ad un distrattore sulla stessa mano (es., medio destro) o sulla mano opposta (al dito omologo o non-omologo rispetto al dito bersaglio; es., indice sinistro o medio sinistro). Inoltre erano previsti catch trial nei quali comparivano i soli distrattori. Nell’Esp.1 entrambe le mani erano in posizione prona; nell’Esp.2 una delle due mani veniva ruotata in posizione supina in metà delle prove. Le variabili dipendenti erano i tempi di reazione e la sensitività (d’). Risultati. Nelle prove in cui il bersaglio era presentato assieme ad un distrattore è emerso un generale effetto di mascheramento. Tuttavia, nell’Esp.1, l’effetto di mascheramento era presente in misura simile per distrattori alla mano contenente il bersaglio (mascheramento intra-mano), o distrattori al dito non-omologo dell’altra mano (mascheramento inter-mano). L’Esp.2 replicava questo risultato quando le mani erano entrambe in posizione prona. Tuttavia, quando una delle due mani era in posizione supina, l’unica condizione interferente rimaneva quella intra-mano. Conclusioni. Questo risultato dimostra che la codifica spaziale di uno stimolo tattile alle dita varia in funzione di quanto le informazioni posturali richiedono un aggiornamento della posizione spaziale dello stimolo tattile

Visual perceptual learning is effective in the illusory far but not in the near space

Visual shape discrimination is faster for objects close to the body, in the peripersonal space (PPS), compared to objects far from the body. Visual processing enhancement in PPS occurs also when perceived depth is based on 2D pictorial cues. This advantage has been observed from relatively low-level (detection, size, orientation) to high-level visual features (face processing). While multisensory association also displays proximal advantages, whether PPS influences visual perceptual learning remains unclear. Here, we investigated whether perceptual learning effects vary according to the distance of visual stimuli (near or far) from the observer, illusorily induced by leveraging the Ponzo illusion. Participants performed a visual search task in which they reported whether a specific target object orientation (e.g., triangle pointing downward) was present among distractors. Performance was assessed before and after practising the visual search task (30 minutes/day for 5 days) at either the close (near group) or far (far group) distance. Results showed that participants that performed the training in the near space did not improve. By contrast, participants that performed the training in the far space showed an improvement in the visual search task in both the far and near spaces. We suggest that such improvement following the far training is due to a greater deployment of attention in the far space, which could make the learning more effective and generalize across spaces

Prismatic Adaptation Induces Plastic Changes onto Spatial and Temporal Domains in Near and Far Space

A large literature has documented interactions between space and time suggesting that the two experiential domains may share a common format in a generalized magnitude system (ATOM theory). To further explore this hypothesis, here we measured the extent to which time and space are sensitive to the same sensorimotor plasticity processes, as induced by classical prismatic adaptation procedures (PA). We also exanimated whether spatial-attention shifts on time and space processing, produced through PA, extend to stimuli presented beyond the immediate near space. Results indicated that PA affected both temporal and spatial representations not only in the near space (i.e., the region within which the adaptation occurred), but also in the far space. In addition, both rightward and leftward PA directions caused opposite and symmetrical modulations on time processing, whereas only leftward PA biased space processing rightward. We discuss these findings within the ATOM framework and models that account for PA effects on space and time processing. We propose that the differential and asymmetrical effects following PA may suggest that temporal and spatial representations are not perfectly aligned

The toolish hand illusion: embodiment of a tool based on similarity with the hand

A tool can function as a body part yet not feel like one: Putting down a fork after dinner does not feel like losing a hand. However, studies show fake body-parts are embodied and experienced as parts of oneself. Typically, embodiment illusions have only been reported when the fake body-part visually resembles the real one. Here we reveal that participants can experience an illusion that a mechanical grabber, which looks scarcely like a hand, is part of their body. We found changes in three signatures of embodiment: the real hand’s perceived location, the feeling that the grabber belonged to the body, and autonomic responses to visible threats to the grabber. These findings show that artificial objects can become embodied even though they bear little visual resemblance to the hand