Authors: Grazia Fernanda Spitoni Giorgio Pireddu Rocco Luca Cimmino Gaspare Galati Alberto Priori Michal Lavidor Liron Jacobson Luigi Pizzamiglio
Publish Date: 2013/05/12
Volume: 228, Issue: 1, Pages: 63-72
Abstract
The parietal lobes contribute to bodyspace representation The present work aims at characterizing the functional role of the inferior parietal lobe in bodyspace representation and at studying the different roles of the angular gyrus in the right and left hemisphere We conducted three separate transcranial direct current stimulation tDCS experiments using “tactile distance task” as an implicit measure of body representation Whereas anodal tDCS on the right angular gyrus influences vocal reaction times vRT for stimuli delivered on the ipsilateral body parts without changes of accuracy right tDCS improved both vRT and accuracy for tactile stimuli on the contralateral limbs Sham or left parietal anodal tDCS had no effect These evidences support the view that right parietal areas have a crucial role in the metric component of the body representationWe are constantly requested to process information in the environment regarding the position of all objects surrounding our body and our body itself A theoretical framework that well describe the relations between the sense of touch and the body has been proposed by Serino and Haggard 2010 The authors describe an analytical model in which tactile afferent information can either be influenced by or have influence upon a mental body representations MBRs Following the authors’ model four separate processes are necessary to link the physical body the sense of touch and the mental representation of one’s own body 1 The physical body organizes tactile sensations 2 Tactile sensations contribute to a mental MBR 3 MBRs reciprocally influence primary tactile processing and 4 MBRs mediate the formation of object representation from primary tactile sensationsThis latter process seems particularly important whenever one is asked to consider the intrinsic properties of a given object touching the skin For example it has been shown that altering the visual experience of the body alters perceived tactile distances Cardini et al 2012 Longo et al 2008 TaylorClarke et al 2004 This evidence is interpreted as the brain attempt to preserve tactile size constancy by rescaling the primary distorted bodysurface representation into objectcentered space according to visual experience of the body A further study by de Vignemont et al 2005 developed this issue showing that proprioception as well as vision plays a role in the formation of a MBR mediating tactile distance judgement The authors also showed that the MBR used for body referencing of touch is not only a memorized body template stored in longterm memory but also is updated online to integrate current sensory information From this perspective Serino and Haggard 2010 claimed that tactile perceptions are always referenced to the body even if the content of the perception is an external objectThe right parietal lobe contributes to body representation in several ways Tsakiris et al 2008 used TMS to show that the right temporal parietal junction rTPJ supports an internal model of the body They suggested that the internal model of the body acts as a stored template against which novel stimuli are compared allowing the maintenance of a basic sense of embodied self Various functional magnetic resonance imaging fMRI studies investigated the properties of the parietal lobe in processing body information see CorradiDell’Acqua et al 2008 Pellijeff et al 2006 Parkinson et al 2010 showed that the superior parietal lobule SPL maintains an uptodate estimate of the current postural configuration of the part of the body that is used during the planning and the execution of a reaching movement Dijkerman and de Haan 2007 described a model that specifies separable functional entities for somatosensory processing subserving perception and action The authors propose that actionrelated processing occurs mainly in the posterior parietal cortex PPC whereas recognition and perception involve the insula as well as the PPC Relevant to our issue this model also distinguishes between somatosensory processing about the body where you have been touched and about external stimuli eg surface features of objects On the basis of neurophysiological and human lesions studies Dijkerman and de Haan suggested a key role of PPC in the metric aspects of the body such as its spatial configuration and size whereas the insula is more concerned with higher order somatosensory processing of the body that is either related to a sense of ownership or to emotional experienceThe importance of the parietal lobe in dealing with the localization of somatic stimuli on the skin was previously highlighted by Van Boven et al 2005 Authors described two areas selectively involved in a grating orientation task stimulus orientation and in a simple tactile localization task stimulus location In contrast to grating orientation which specifically activates IPL tactile localization preferentially engages the TPJSpecifically we refer to the concept of metric component of body representation as one of the functions that the somatoperception uses to form a model of body size and shape see Longo et al 2010 Moreover in a previous work Spitoni et al 2010 we argued that the right angular gyrus may house the process of body referencing of tactile inputs that is required to compare the tactile distance across different body part Among this frame of evidences we used the term metric component of body representation to describe a selective component of the body representation used to process tactile distances discriminationDespite a rich description of the functional properties of the parietal areas in dealing with body representations little is known about the neural basis of the specific metric component of body representation In a previous work Spitoni et al 2010 we pointed out the importance of parietal structures in dealing with the perception of the metric component of MBR In that study subjects were stimulated with pairs of wooden cylinders separated by a variable distance and attached to nylon filaments of variable length and had to evaluate either the distance between the two cylinders distance task or the amount of skin deformation generated by the touch contact task The distance but not the contact task implicitly required to access a stored metric representation of the touched body part Results showed that while both tasks bilaterally activated parietal and frontal areas only the evaluation of distances on the body surface activated the angular gyrus and the temporoparietooccipital junction in the right hemisphere Our conclusion was that the right angular gyrus together with other cortical sensory modalities such as proprioception plays a key role in the metric tactile perception by linking touched locations on the skin to an internal representation of body parts
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