The functional architecture of the human body: assessing body representation by sorting body parts and activities
We investigated mental representations of body parts and body-related activities in two subjects with congenitally absent limbs (one with, the other without phantom sensations), a wheelchair sports group of paraplegic participants, and two groups of participants with intact limbs. To analyse mental representation structures, we applied Structure Dimensional Analysis. Verbal labels indicating body parts and related activities were presented in randomized lists that had to be sorted according to a hierarchical splitting paradigm. Participants were required to group the items according to whether or not they were considered related, based on their own body perception. Results of the groups of physically intact and paraplegic participants revealed separate clusters for the lower body, upper body, fingers and head. The participant with congenital phantom limbs also showed a clear separation between upper and lower body (but not between fingers and hands). In the participant without phantom sensations of the absent arms, no such modularity emerged, but the specific practice of his right foot in communication and daily routines was reflected. Sorting verbal labels of body parts and activities appears a useful method to assess body representation in individuals with special body anatomy or function and leads to conclusions largely compatible with other assessment procedures.
Words denoting manipulable objects activate sensorimotor brain areas, likely reflecting action experience with the denoted objects. In particular, these sensorimotor lexical representations have been found to reflect the way in which an object is used. In the current paper we present data from two experiments (one behavioral and one neuroimaging) in which we investigate whether body schema information, putatively necessary for interacting with functional objects, is also recruited during lexical processing. To this end, we presented participants with words denoting objects that are typically brought towards or away from the body (e.g., cup or key, respectively). We hypothesized that objects typically brought to a location on the body (e.g., cup) are relatively more reliant on body schema representations, since the final goal location of the cup (i.e., the mouth) is represented primarily through posture and body co-ordinates. In contrast, objects typically brought to a location away from the body (e.g., key) are relatively more dependent on visuo-spatial representations, since the final goal location of the key (i.e., a keyhole) is perceived visually. The behavioral study showed that prior planning of a movement along an axis towards and away from the body facilitates processing of words with a congruent action semantic feature (i.e., preparation of movement towards the body facilitates processing of cup.). In an fMRI study we showed that words denoting objects brought towards the body engage the resources of brain areas involved in the processing information about human bodies (i.e., the extra-striate body area, middle occipital gyrus and inferior parietal lobe) relatively more than words denoting objects typically brought away from the body. The results provide converging evidence that body schema are implicitly activated in processing lexical information.