Bisecting real and fake body parts: effects of prism adaptation after right brain damage

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.

Keywords: body representation; multisensory; prismatic adaptation; space coding; unilateral spatial neglect.

Figure 1

Lesion mapping. Lesions were mapped onto a standard template (Montreal Neurological Institute) using the software MRIcro (Rorden and Brett, 2000). White areas represent the extension of the lesion of each patient.

Figure 2

(A) Schematic bird’s eye view of the experimental setting for the real (left, light gray) and fake (right, pattern) forearm. Participants wore a black mantle, and they could see only their real forearm, which lay outside of two side holes of the mantle, or the fake forearm; black dots limited the extent of the real/fake forearm considered for the bisection task. (B) On the left, the box used for prism adaptation by repeated pointing trials, closed by the removable plexiglas, seen from the examiner’s side (Fortis et al., 2010). Marks for the recording of the patients’ pointing errors are shown. On the right, the prismatic goggles inducing a 10° shift of the visual field to the right.

Figure 3

(A) Aftereffects: Invisible condition. Mean pointing error (in degrees of visual angle, ±SE) in the pre-exposure and post-exposure sessions, made by neurologically unimpaired control participants, and RHD patients. (B) Mean percent deviation error (±SE) made by control participants and RHD patients, by stimulus condition (real and fake forearm), before and after PA.

Figure 4

(A) Pre-exposure condition: mean percent deviation error by stimulus condition in each of the 16 RHD patients. (B) Effects of PA on forearm bisection, i.e., difference in the deviation error between post-exposure and pre-exposure to prisms (post minus pre PA), for the real and the fake forearm conditions in each of the 16 patients. A negative score indicates a leftward, PA-induced, net effect, a positive score a rightward effect. Asterisk: significant difference between each patient’s score and the mean score of the control group for each stimulus condition. Circles and brackets: significant difference between fake and real forearm bisection in the individual patient, as compared to the same average difference in the control group. N−, patients without USN; N+, patients with USN.