More than skin deep: body representation beyond primary somatosensory cortex

The neural circuits underlying initial sensory processing of somatic information are relatively well understood. In contrast, the processes that go beyond primary somatosensation to create more abstract representations related to the body are less clear. In this review, we focus on two classes of higher-order processing beyond somatosensation. Somatoperception refers to the process of perceiving the body itself, and particularly of ensuring somatic perceptual constancy. We review three key elements of somatoperception: (a) remapping information from the body surface into an egocentric reference frame (b) exteroceptive perception of objects in the external world through their contact with the body and (c) interoceptive percepts about the nature and state of the body itself. Somatorepresentation, in contrast, refers to the essentially cognitive process of constructing semantic knowledge and attitudes about the body, including: (d) lexical-semantic knowledge about bodies generally and one’s own body specifically, (e) configural knowledge about the structure of bodies, (f) emotions and attitudes directed towards one’s own body, and (g) the link between physical body and psychological self. We review a wide range of neuropsychological, neuroimaging and neurophysiological data to explore the dissociation between these different aspects of higher somatosensory function

Anisotropies of tactile distance perception on the face

The distances between pairs of tactile stimuli oriented across the width of the hand dorsum are perceived as about 40% larger than equivalent distances oriented along the hand length. Clear anisotropies of varying magnitudes have been found on different sites on the limbs and less consistently on other parts of the body, with anisotropies on the center of the forehead, but not on the belly. Reported anisotropies on the center of the forehead, however, might reflect an artefact of categorical perception from the face midline, which might be comparable to the expansion of tactile distance perception observed for stimuli presented across joint boundaries. To test whether tactile anisotropy is indeed a general characteristic of the tactile representation of the face, we assessed the perceived distance between pairs of touches on the cheeks and three locations on the forehead: left, right, and center. Consistent with previous results, a clear anisotropy was apparent on the center of the forehead. Importantly, similar anisotropies were also evident on the left and right sides of the forehead and both cheeks. These results provide evidence that anisotropy of perceived tactile distance is not a specific feature of tactile organization at the limbs but it also exists for the face, and further suggest that the spatial distortions found for tactile distances that extend across multiple body parts are not present for stimuli that extend across the body midline

The standard posture of the hand

Perceived limb position is known to rely on sensory signals and motor commands. Another potential source of input is a standard representation of body posture, which may bias perceived limb position towards more stereotyped positions. Recent results show that tactile stimuli are processed more efficiently when delivered to a thumb in a relatively low position or an index finger in a relatively high position. This observation suggests that we may have a standard posture of the body that promotes a more efficient interaction with the environment. In this study, we mapped the standard posture of the entire hand by characterizing the spatial associations of all five digits. Moreover, we show that the effect is not an artefact of intermanual integration. Results showed that the thumb is associated with low positions, while the other fingers are associated with upper locations

Rubber Hands Feel Touch, but Not in Blind Individuals

Psychology and neuroscience have a long-standing tradition of studying blind individuals to investigate how visual experience shapes perception of the external world. Here, we study how blind people experience their own body by exposing them to a multisensory body illusion: the somatic rubber hand illusion. In this illusion, healthy blindfolded participants experience that they are touching their own right hand with their left index finger, when in fact they are touching a rubber hand with their left index finger while the experimenter touches their right hand in a synchronized manner (Ehrsson et al. 2005). We compared the strength of this illusion in a group of blind individuals (n = 10), all of whom had experienced severe visual impairment or complete blindness from birth, and a group of age-matched blindfolded sighted participants (n = 12). The illusion was quantified subjectively using questionnaires and behaviorally by asking participants to point to the felt location of the right hand. The results showed that the sighted participants experienced a strong illusion, whereas the blind participants experienced no illusion at all, a difference that was evident in both tests employed. A further experiment testing the participants' basic ability to localize the right hand in space without vision (proprioception) revealed no difference between the two groups. Taken together, these results suggest that blind individuals with impaired visual development have a more veridical percept of self-touch and a less flexible and dynamic representation of their own body in space compared to sighted individuals. We speculate that the multisensory brain systems that re-map somatosensory signals onto external reference frames are less developed in blind individuals and therefore do not allow efficient fusion of tactile and proprioceptive signals from the two upper limbs into a single illusory experience of self-touch as in sighted individuals

Spatiotemporal processing of somatosensory stimuli in schizotypy

Unusual interaction behaviors and perceptual aberrations, like those occurring in schizotypy and schizophrenia, may in part originate from impaired remapping of environmental stimuli in the body space. Such remapping is contributed by the integration of tactile and proprioceptive information about current body posture with other exteroceptive spatial information. Surprisingly, no study has investigated whether alterations in such remapping occur in psychosis-prone individuals. Four hundred eleven students were screened with respect to schizotypal traits using the Schizotypal Personality Questionnaire. A subgroup of them, classified as low, moderate, and high schizotypes were to perform a temporal order judgment task of tactile stimuli delivered on their hands, with both uncrossed and crossed arms. Results revealed marked differences in touch remapping in the high schizotypes as compared to low and moderate schizotypes. For the first time here we reveal that the remapping of environmental stimuli in the body space, an essential function to demarcate the boundaries between self and external world, is altered in schizotypy. Results are discussed in relation to recent models of 'self-disorders' as due to perceptual incoherence

Anisotropies of tactile distance perception on the face

The distances between pairs of tactile stimuli oriented across the width of the hand dorsum are perceived as about 40% larger than equivalent distances oriented along the hand length. Clear anisotropies of varying magnitudes have been found on different sites on the limbs and less consistently on other parts of the body, with anisotropies on the centre of the forehead, but not on the belly. Reported anisotropies on the centre of forehead, however, might reflect an artefact of categorical perception from the face midline, which might be comparable to the expansion of tactile distance perception observed for stimuli presented across joint boundaries. To test whether tactile anisotropy is indeed a general characteristic of the tactile representation of the face, we assessed the perceived distance between pairs of touches on the cheeks and three locations on the forehead: left, right and centre. Consistent with previous results, a clear anisotropy was apparent on the centre of the forehead. Importantly, similar anisotropies were also evident on the left and right sides of the forehead and both cheeks. These results provide evidence that anisotropy of perceived tactile distance is not a specific feature of tactile organization at the limbs but it also exists at the face, and further suggest that the spatial distortions found for tactile distances that extend across multiple body parts are not present for stimuli that extend across the body midline

Combining proprioception and touch to compute spatial information

Localising a tactile stimulus in egocentric space involves integrating information from skin receptors with proprioceptive inputs about body posture. We investigated whether body posture automatically influences tactile spatial judgements, even when it is irrelevant to the task. In Experiment 1, participants received two successive tactile stimuli on the forearm and were asked to indicate whether the first or second touch of the pair was closer to an anatomical body landmark, either the wrist or the elbow. The task was administered in three experimental conditions involving different body postures: canonical body posture with extended forearm and hand pointing distally; a non-canonical body posture with forearm and hand pointing vertically up at 90° and a ‘reversed’ body posture with the elbow fully flexed at 180°, so that the hand pointed proximally. Thus, our task required localising touch on the skin and then relating skin locations to anatomical body landmarks. Critically, both functions are independent of the posture of the body in space. We nevertheless found reliable effects of body posture: judgement errors increased when the canonical forearm posture was rotated through 180°. These results were further confirmed in Experiment 2, in which stimuli were delivered to the finger. However, additionally reversing the canonical posture of the finger, as well as that of the forearm, so that the finger was restored to its canonical orientation in egocentric space, restored performance to normal levels. Our results confirm an automatic process of localising the body in external space underlying the process of tactile perception. This process appears to involve a combination of proprioceptive and tactile information