Thursday, April 02, 2009

Heterotopagnosia: When I point at parts of your body

Heterotopagnosia is an unusual neurological syndrome, as described below by Laurent Cleret de Langavant and colleagues:
Heterotopagnosia is the acquired inability of brain-lesioned patients to point at someone else's body parts when prompted. The cognitive basis of this disorder is unclear. It might result from a biological function deficit critical for communication in human beings; alternatively, it could result from the disruption of a body representation. Here, we report three patients with heterotopagnosia following a recent left parieto-occipital stroke and a previous insular lesion. The patients were tested on their ability to name, point out and grasp several targets including body parts (own, real others’ and figurative others’). Language, visuo-spatial deficits or any confounding neuropsychological disorders were controlled for. We found that the patients erroneously pointed to their own body parts when asked to point at someone else's. Strikingly, their ability to grasp someone else's body parts was largely unimpaired. The dissociation between their grasping and communicative pointing abilities supports the hypothesis that heterotopagnosia is a disorder of communicative function conveyed by pointing but not by grasping. In addition, pointing performance in our patients varied according to the target: the more similar the target was to a real person, the worse the patients’ pointing performance. We suggest that communicative pointing might require a specific representation of the addressee's body and point of view, a heterocentric representation. In the patients described here this phenomenon resulted from a combined insulo-parietal lesion, which may explain why, in contrast to other patients described previously, the heterotopagnosia was long-lasting.
The area of maximum lesion overlap was a region in the left parieto-occipital cortex, as shown in this drawing depicting the area of damage shared by all three patients.

Fig. 3. (Cleret de Langavant et al., 2009). The overlap of the left parieto-occipital junction lesion (Brodmann area 19), common to all three patients. Note that there is no overlap within the insular region for the three patients since COG has a left insular lesion and ROM and BEG a right one.

What can explain this unusual disorder? First, the authors ruled out several possibilities:
The features of heterotopagnosia set this pointing disorder apart from any other known neurological disorder. The patients’ preserved performance in grasping and touching body parts rules out any causal visual or spatial impairment based on the size, type, complexity or componential analysis of the target. Likewise, a language deficit cannot account for the patients’ performance in pointing. The spared ability of patients in naming body parts they cannot point to, their use of possessive and demonstrative grammatical indices and their perfect understanding of pointing exclude a category-specific lexical impairment.
Instead, they suggested the patients exhibited a deficit selective for another person's body, problems with self-referencing behavior, and a dissociation between impaired pointing versus intact grasping/touching. The authors concluded by asking...
...why is pointing interesting if its impairment has no consequences in daily life? We suggest that pointing is a residual function that is fundamental in infant development. It marks the construction of the three-way relationship of communication (self-addressee-object or I-you-he/she/it), as a keystone for the development of language and theory of mind. In adults, its impairment is compensated by abilities that have integrated and supplanted it, leaving intact relationships with others (e.g. verbal communication and knowledge concerning others).
Cleret de Langavant, L., Trinkler, I., Cesaro, P., & Bachoud-Lévi, A. (2009). Heterotopagnosia: When I point at parts of your body. Neuropsychologia DOI: 10.1016/j.neuropsychologia.2009.02.016

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