Evidence from patients has shown that primary somatosensory representations are plastic, dynamically changing in response to central or peripheral alterations, as well as experience. Furthermore, recent research has also demonstrated that altering body posture results in changes in the perceived sensation and localization of tactile stimuli. Using evidence from behavioral studies with brain damaged and healthy subjects, as well as functional imaging, we propose that the traditional concept of the body schema should be divided into three components. First are primary somatosensory representations, which are representations of the skin surface that are typically somatotopically organized, and have been shown to change dynamically due to peripheral (usage, amputation, deafferentation) or central (lesion) modifications. Second, we argue for a mapping from a primary somatosensory representation to a secondary representation of body size and shape (body form representation). Finally, we review evidence for a third set of representations that encodes limb position and is used to represent the location of tactile stimuli relative to the subject using external, non-somatotopic reference frames (postural representations). From Maps to Skin to Space -Touch and Body RepresentationsInformation regarding body position in space comes from tactile, proprioceptive, visual, vestibular, auditory and enteroceptive sources. These inputs are integrated to generate representations of the body that are crucial for perception and action. Head and Holmes (1911) introduced the concept of multiple integrated body representations, dividing them into three categories -a postural schema that represents the position of the body in space before and after movement, a superficial schema used to localize the position of sensation on the body surface, (both which form an unconscious body schema), and a conscious representation known as the body image. Later characterizations of body representations focused primarily on the conscious/unconscious distinction in the body schema and body image (see Gallagher, 1986;Gallagher, 2005; Paillard, 1999). However, this conscious/unconscious dichotomy is likely to be overly simplistic in characterizing body representations (for a discussion, see de Vignemont, this issue; Gallese & Sinigaglia, this issue). We propose to use evidence from studies of tactile perception to provide a theoretical framework for understanding body representations. We argue that representations of the body used in sensory and motor processing (i.e. the body schema as described in Schwoebel & Coslett, 2005) can be divided into three distinct representations used to localize tactile stimuli and interact with the environment 1 .

There is evidence for different levels of visuospatial processing with their own frames of reference: viewer-centered, stimulus-centered, and object-centered. The neural locus of these levels can be explored by examining lesion location in subjects with unilateral spatial neglect (USN) manifest in these reference frames. Most studies regarding the neural locus of USN have treated it as a homogenous syndrome, resulting in conflicting results. In order to further explore the neural locus of visuospatial processes differentiated by frame of reference, we presented a battery of tests to 171 subjects within 48 hr after right supratentorial ischemic stroke before possible structural and/or functional reorganization. The battery included MR perfusion weighted imaging (which shows hypoperfused regions that may be dysfunctional), diffusion weighted imaging (which reveals areas of infarct or dense ischemia shortly after stroke onset), and tests designed to disambiguate between various types of neglect. Results were consistent with a dorsal/ventral stream distinction in egocentric/allocentric processing. We provide evidence that portions of the dorsal stream of visual processing, including the right supramarginal gyrus, are involved in spatial encoding in egocentric coordinates, whereas parts of the ventral stream (including the posterior inferior temporal gyrus) are involved in allocentric encoding.

No abstract