Abstract
The representation of body parts holds a special status in the brain, due to their prototypical shape and the contribution of multisensory (visual and somatosensory-proprioceptive) information. In a previous study (Sposito et al., 2010), we showed that patients with left unilateral spatial neglect exhibit a rightward bias in setting the midpoint of their left forearm, which becomes larger when bisecting a cylindrical object comparable in size. This body part advantage, found also in control participants, suggests partly different processes for computing the extent of body parts and objects. In this study we tested 16 right-brain-damaged patients, and 10 unimpaired participants, on a manual bisection task of their own (real) left forearm, or a size-matched fake forearm. We then explored the effects of adaptation to rightward displacing prism exposure, which brings about leftward aftereffects. We found that all participants showed prism adaptation (PA) and aftereffects, with right-brain-damaged patients exhibiting a reduction of the rightward bias for both real and fake forearm, with no overall differences between them. Second, correlation analyses highlighted the role of visual and proprioceptive information for the metrics of body parts. Third, single-patient analyses showed dissociations between real and fake forearm bisections, and the effects of PA, as well as a more frequent impairment with fake body parts. In sum, the rightward bias shown by right-brain-damaged patients in bisecting body parts is reduced by prism exposure, as other components of the neglect syndrome; discrete spatial representations for real and fake body parts, for which visual and proprioceptive codes play different roles, are likely to exist. Multisensory information seems to render self bodily segments more resistant to the disruption brought about by right-hemisphere injury.
Highlights
After an unilateral brain damage patients may show an altered representation of the space contralateral to the damaged hemisphere, that produces the neuropsychological syndrome of unilateral spatial neglect (USN)
We found that all participants showed prism adaptation (PA) and aftereffects, with right-brain-damaged patients exhibiting a reduction of the rightward bias for both real and fake forearm, with no overall differences between them
Other main effects and interactions were not significant: Group [F (1,24) = 3.12; P = 0.07, pη2 = 0.2], and Group by Target location [F (2,48) = 0.07; P = 0.9, pη2 = 0.01]. These findings suggest that both controls and RHD patients were accurate in pointing to visual targets in a baseline condition
Summary
After an unilateral brain damage patients may show an altered representation of the space contralateral to the damaged hemisphere, that produces the neuropsychological syndrome of unilateral spatial neglect (USN). This task has been typically employed to explore the spatial representation, the lateral extent, of extra-personal objects, most frequently segments (Schenkenberg et al, 1980; Bisiach et al, 1983; Halligan and Marshall, 1994; Vallar et al, 2000) We used this task to explore the spatial metrics of body parts in rightbrain-damaged patients with left USN, and in neurologically unimpaired participants (Sposito et al, 2010). Single-patient analyses reveal that in USN patients the rightward bias can be significantly more severe either in cylinder than in forearm bisection (the prevailing pattern, as indicated by the above mentioned group analysis), or, vice versa, in forearm bisection This double dissociation suggests the existence of independent representations for extra-personal objects and body parts, likely supported by discrete spatial processes (Sposito et al, 2010). We predicted that if real and fake body parts share a similar visuo-spatial representation, PA is expected to modulate their bisection bias in a similar way
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