In anorexia nervosa (AN), body distortions have been associated with parietal cortex (PC) dysfunction. The PC is also the anatomical substrate of a supramodal reference framework involved in spatial orientation constancy. Given the impaired spatial orientation constancy found in hemineglect, we sought to determine whether similar disturbances could be observed in anorexic patients. We investigated the effect of passive lateral body inclination on the tactile subjective vertical (SV). Fifty participants (25 AN patients and 25 healthy controls) were asked to manually set a rod into the vertical position under three postural conditions. For tilted conditions, we observed a significant deviation of the tactile SV towards the body. This effect was abnormally accentuated in AN patients and might be caused by higher weighting with respect to the egocentric frame of reference. Our findings reinforce the role of the PC in AN and suggest that this dysfunction affects spatial orientation constancy as well as body boundaries.
Patients with anorexia nervosa (AN) very frequently report that they feel larger and fatter than they really are. This alteration in body representation is thus a major, troublesome, clinical symptom of AN (American Psychiatric Association, 1994) and can counteract the benefits of therapy by increasing the obsessive will to lose weight and thus maintaining restrictive eating behaviors (Heilbrun and Friedberg, 1990). However, the notion of body representation is not unique and at least two types have been proposed: the body schema and the body image (De Vignemont, 2010). Even though most research to date has tended to emphasize the aesthetical/emotional components of body representation (i.e. the body image), some researchers (Grunwald et al., 2002, Guardia et al., 2010 and Nico et al., 2010) have also suggested that the sensorimotor representation of the body which initiates and guides actions (i.e. the body schema) could be disturbed in AN. It has been suggested that the body schema distortion in AN is related to dysfunction of the parietal cortex (PC), since this cortical lobe has a role in the establishment of a coherent body schema (Daprati et al., 2010).
For instance, the AN patients in Grunwald et al.'s (2002) experiment had to manually adjust a bar (without visual feedback) into a parallel position relative to a reference bar sensed by the other hand. The difference between the results for the right and left hands was interpreted by the authors as evidencing dysfunction of the right parietal cortex. In the experiment by Nico et al. (2010), anorexics underestimated their left body-boundary in the same way as hemineglect patients with right parietal lobe lesions did. The task involved anticipating when an approaching light beam would hit the body.
Even though the PC is viewed as the locus of the body schema, it is also considered to be the anatomical substrate of a supramodal frame of reference involved in spatial orientation constancy (Kerkhoff, 1999 and Funk et al., 2010). Spatial orientation constancy is defined as the central nervous system's capability to maintain the sense of gravitational, vertical orientation (i.e. the sense of verticality) despite inclination of the body and/or the visual context (Howard, 1982). Funk et al. (2010) recently tested spatial orientation constancy in hemineglect patients suffering from right parietal lesions. Participants had to adjust a bar into the vertical or horizontal position with visual or tactile modalities and with the head vertical, left roll-tilted or right roll-tilted. It is well known that in darkness, head and/or body tilts cause the subjective vertical (SV) to deviate in healthy controls (Howard, 1982, Luyat et al., 2001 and Luyat and Gentaz, 2002). Whereas A-effects (deviations of the SV towards the axis of the head) are observed in vision and large tilts, E-effects (deviations of the SV away from the axis of the head) are usually found with tactile adjustments (Bauermeister et al., 1964, Luyat et al., 2001, Bortolami et al., 2006 and Gentaz et al., 2008). In Funk et al.'s (2010) experiment, the hemineglect patients showed strong A-effects after left roll-tilting of the head. The A-effect was also more pronounced under tactile modality and was interpreted by the authors as an abnormal weighting towards an egocentric frame of reference (such as the idiotropic vector; Mittelstaedt, 1983), due to impaired processing of gravitational information in AN.
Given the PC's involvement in spatial orientation constancy, the objective of the present research was to determine whether spatial orientation constancy could also be affected in AN. We expected to find similar disturbances in AN to those observed in Funk et al.'s experiment with hemineglect patients. Such a finding would emphasize the role of the PC in AN and suggest that PC dysfunction affects not only body-boundary representation but also the importance given to the body as a frame of reference.
