Masakazu Ide

The effect of "anatomical plausibility" of hand angle on the rubber-hand illusion

In the rubber-hand illusion (RHI), when an actual hand hidden from view and a rubber hand in view are simultaneously stimulated, participants mistakenly perceive tactile sensation as arising from the rubber hand, not from the actual hand. Some studies have revealed that the magnitude of RHI decreases when the actual and rubber hand are incongruent in terms of hand angle. However, the acceptable range of angular deviations between the actual and rubber hand and its determination factor has not been investigated so far. I attempt to examine the angle-congruency effect between the actual and rubber hands on RHI. The left rubber hand simulated by 3-D computer graphics was rotated at 8 angles. Participants perceived higher ownership of the stimulated rubber hand when the angles were at 0°, 45°, 90°, and 315° (these are easy to mimic with the actual hand) than at 180°, 225°, and 270° (these are difficult to mimic with the actual hand). The perceived location of the actual hand became closer to that of the simulated rubber hand with increased perception of ownership of the rubber hand. Moreover, the onset duration of RHI became shorter, especially for the angles of 0°, 45°, and 315°. These results suggest that RHI occurs mainly within the range where people usually rotate their hand and that body representation might include the knowledge of “anatomical plausibility”.

The Rubber Tail Illusion as Evidence of Body Ownership in Mice.

The ownership of ones body parts represents a fundamental aspect of self-consciousness. Accumulating empirical evidence supports the existence of this concept in humans and nonhuman primates, but it is unclear whether nonprimate mammals experience similar feelings. Therefore, the present study used rubber tails to investigate body ownership in rodents. When the real tails and rubber tails were synchronously stroked, the mice responded as if their own tails were touched when the rubber tails were grasped. In contrast, when the stimuli were delivered asynchronously, there was a significantly lower mean response rate when the rubber tail was grasped. These findings suggest that mice may experience body ownership of their tails, suggestive of the rubber hand illusion in humans. SIGNIFICANCE STATEMENT To explore the manner in which the ownership of body parts is experienced, this study specifically used the rubber hand illusion (RHI), in which self-consciousness can be extended out of ones own body. Accumulating empirical evidence supports the existence of this concept in humans and nonhuman primates, but it remains unclear whether nonprimate mammals experience similar feelings. This study demonstrated for the first time that mice may experience body ownership of their tails, which is suggestive of the RHI in humans and provides evidence that may highlight how humans experience the ownership of body parts.

Tactile stimulation can suppress visual perception

An input (e.g., airplane takeoff sound) to a sensory modality can suppress the percept of another input (e.g., talking voices of neighbors) of the same modality. This perceptual suppression effect is evidence that neural responses to different inputs closely interact with each other in the brain. While recent studies suggest that close interactions also occur across sensory modalities, crossmodal perceptual suppression effect has not yet been reported. Here, we demonstrate that tactile stimulation can suppress the percept of visual stimuli: Visual orientation discrimination performance was degraded when a tactile vibration was applied to the observers index finger of hands. We also demonstrated that this tactile suppression effect on visual perception occurred primarily when the tactile and visual information were spatially and temporally consistent. The current findings would indicate that neural signals could closely and directly interact with each other, sufficient to induce the perceptual suppression effect, even across sensory modalities.

Visual presentation of hand image modulates visuo–tactile temporal order judgment

Perceptual systems can distinguish among a variety of inputs in the temporal domain, including even different sensory inputs. This process has been investigated mainly by using a temporal task (temporal order judgment: TOJ). For example, studies have reported estimated critical limits (just noticeable difference: JND) of the TOJ between a visual stimulus and a tactile stimulus (visuo–tactile TOJ, e.g., flashes and vibrations) fell within a certain temporal range. Recent studies have also suggested that the visual presentation of a hand image could modulate visuo–tactile integrations in the temporal domain, but these studies did not thoroughly examine such effects by using temporal tasks. Here, we investigated the effect of visual presentation of a hand image on visuo–tactile TOJ. In our experiments, a visual stimulus was presented on the index finger of a hand image and a tactile stimulus was presented on the index finger of a participant’s hand. We found that the JND of visuo–tactile TOJ became larger when a forward hand image was presented than when inverted hand or arrow images were presented. However, this effect was not observed for the TOJ between an auditory stimulus and a visual stimulus. Thus, the visual presentation of a hand image whose angle corresponds to that of one’s own hand could selectively degrade visuo–tactile TOJ. This finding indicates that visual hand images implicitly enhance the internal proximity between the visual and tactile stimuli and make them difficult to distinguish from each other in the temporal domain.

Sound can suppress visual perception

In a single modality, the percept of an input (e.g., voices of neighbors) is often suppressed by another (e.g., the sound of a car horn nearby) due to close interactions of neural responses to these inputs. Recent studies have also suggested that close interactions of neural responses could occur even across sensory modalities, especially for audio-visual interactions. However, direct behavioral evidence regarding the audio-visual perceptual suppression effect has not been reported in a study with humans. Here, we investigated whether sound could have a suppressive effect on visual perception. We found that white noise bursts presented through headphones degraded visual orientation discrimination performance. This auditory suppression effect on visual perception frequently occurred when these inputs were presented in a spatially and temporally consistent manner. These results indicate that the perceptual suppression effect could occur across auditory and visual modalities based on close and direct neural interactions among those sensory inputs.

Periodic Visuotactile Stimulation Slowly Enhances the Rubber Hand Illusion in Individuals with High Autistic Traits.

In a rubber hand illusion (RHI) task, synchronous brush stroking of a rubber hand and a participants hidden hand induces body ownership of the rubber hand. The effects of spatial distances and temporal lags on the RHI have been extensively examined; however, the effect of periodicity of the stimuli on illusory body ownership has not been examined. Meanwhile, the occurrence of RHI tends to be weak in individuals with autism-spectrum disorders (ASD) and high autistic traits. Preference for stimulus having regularity of tempo is generally observed in individuals with ASD, and thus, periodic stimulation might be more effective to elicit the body ownership illusion in individuals with high autistic traits. Hence, we investigated whether stimulus periodicity influenced RHI as well as its association with participants autistic traits. Brush strokes were applied to a participants own hand and the rubber hand periodically (2 s) or non-periodically (1–3 s), either synchronously or asynchronously. Two blocks were performed in each condition. We found that periodic stimulation enhanced the spatial updating of tactile sensation induced by RHI in the subsequent block in participants with high autistic traits, whereas both periodic and non-periodic stimulation strongly elicited RHI in blocks 1 and 2. These results indicate that the periodicity of stimulation has different effects based on an individuals autistic traits. Since individuals with ASD are known to sustain their focus on interoceptive sensations (heartbeats), a periodic stimulation that is potentially correlated with heartbeats might be effective to enhance the visuotactile integration during RHI in individuals with high autistic traits.

Rubber hand presentation modulates visuotactile interference effect especially in persons with high autistic traits

Persons with high autistic traits showed diverse reactions as to their multisensory integration, whereas neurotypical persons adequately integrate visual and tactile information. Successive visual stimuli sometimes interfere ordering of successive tactile stimuli. Presentation of a hand-shaped object would affect the interference. Besides, its associations with autistic traits have not been reported. Here, we investigated the effect of a rubber hand presentation on interferences to tactile temporal order judgment by successive visual stimuli. We also investigated whether individual differences associated with autistic traits. A rubber hand was placed palm down in front of the participant in one condition, while in other conditions, it was inverted or was not presented. Participants were required to judge the temporal order of tactile stimuli presented to the index finger and ring finger of their unseen right hand, and needed to ignore the visual stimuli placed on the corresponding fingers of the rubber hand. When incongruent visual stimuli were delivered along with presentation of the rubber hand, the participant’s judgment was notably reversed. In contrast, the degree of reversals significantly decreased when the rubber hand was not presented or presented in an inverted direction. Additionally, we found that participants with high autistic traits tended to show large reversal with the rubber hand presentation, while they showed small reversal when the rubber hand was inverted. Our results suggest that rubber hand corresponding to one’s own hand facilitates visuotactile interference. Furthermore, autistic traits may affect the integration of visuotactile inputs when the rubber hand is presented.

Salivary Oxytocin Concentration Associates with the Subjective Feeling of Body Ownership during the Rubber Hand Illusion

Oxytocin is a hormone of the posterior pituitary that promotes lactation, maternal bonding, and birth. Recent studies have shown that oxytocin may modulate social recognition in both sexes, and thus it may be related to empathy. Brain regions that are associated with social recognition and empathy (e.g., the insular cortex) are activated in the rubber hand illusion (RHI), which involves illusory ownership of a rubber hand caused by brush strokes applied synchronously to both a rubber hand and one of the participants hand, which is hidden from view. It is intriguing to examine whether oxytocin modulates plastic changes in body representation, such as the changes occurring in the RHI. In the present study, we investigated the relationship between salivary oxytocin concentration and the feeling of rubber hand ownership. Brush strokes were applied synchronously or asynchronously to the participants hand and a rubber hand on different days. Salivary oxytocin was measured before and after the behavioral tasks. We found that participants who had high concentrations of salivary oxytocin tended to feel strong ownership of the rubber hand. We also found that the participants with a high autism spectrum quotient (AQ) score who particularly felt difficulties in social skills and communications tended to feel weak rubber hand ownership. We observed that illusory body ownership was closely linked to social communications and a related neuroendocrine basis. The results of the present study suggest that an individuals salivary oxytocin concentration can predict the extent to which the individual experiences the RHI; furthermore, oxytocin might modulate the sensation of body ownership.

Higher Tactile Temporal Resolution as a Basis of Hypersensitivity in Individuals with Autism Spectrum Disorder

Many individuals with autism spectrum disorder (ASD) have symptoms of sensory hypersensitivity. Several studies have shown high individual variations in temporal processing of tactile stimuli. We hypothesized that these individual differences are linked to differences in hyper-reactivity among individuals with ASD. Participants performed two tasks as to vibrotactile stimuli: One is a temporal order judgement task, and another is a detection task. We found that individuals with ASD with higher temporal resolution tended to have more severe hypersensitivity symptoms. In contrast, the tactile detection threshold/sensitivity were related to the severities of stereotyped behaviour and restricted interests, rather than to hypersensitivity. Our findings demonstrate that higher temporal resolution to sensory stimuli may contribute to sensory hypersensitivity in individuals with ASD.

Neural mechanisms underlying touch-induced visual perceptual suppression: An fMRI study

Crossmodal studies have demonstrated inhibitory as well as facilitatory neural effects in higher sensory association and primary sensory cortices. A recent human behavioral study reported touch-induced visual perceptual suppression (TIVS). Here, we introduced an experimental setting in which TIVS could occur and investigated brain activities underlying visuo-tactile interactions using a functional magnetic resonance imaging technique. While the suppressive effect of touch on vision was only found for half of the participants who could maintain their baseline performance above chance level (i.e. TIVS was not well replicated here), we focused on individual differences in the effect of touch on vision. This effect could be suppressive or enhancement, and the neuronal basis of these differences was analyzed. We found larger inhibitory responses in the anterior part of the right visual cortex (V1, V2) with higher TIVS magnitude when visuo-tactile stimuli were presented as spatially congruent. Activations in the right anterior superior temporal region, including the secondary somatosensory cortical area, were more strongly related to those in the visual cortex (V1, V2) with higher TIVS magnitude. These results indicate that inhibitory neural modulations from somatosensory to visual cortices and the resulting inhibitory neural responses in the visual cortex could be involved in TIVS.