Cerebral processes in mental transformations of body parts: recognition prior to rotation

There is growing evidence that the visual processing of human body stimuli is particular and distinct from that of other objects. This is due to implicit knowledge of anatomical and biomechanical constraints of the human body. The question arises whether body stimuli in which biomechanical constraints are violated are processed in the same way as realistic bodies. This study investigated the neural mechanisms of anatomically plausible and implausible body stimuli. Event-related potentials (ERP) were recorded in healthy participants during mental rotation of body parts. Subjects were shown pictures of body parts or whole bodies in which one element (finger, arm) could be anatomically accurate or inaccurate (e.g., left forearm attached to right upper arm). Furthermore, the body parts were rotated in 7 different orientations, from 0 degrees to 180 degrees in 30 degrees increments, resulting in some possible and some impossible positions of the body parts. Analysis of the 123-channel ERPs was carried out by determining the successive segments of stable map topographies and comparing them between conditions. A particular segment appeared in the case of anatomically impossible postures at 190-230 ms followed by a segment reflecting mental rotation at 310-380 ms. Anatomically implausible positions are thus detected at a very early stage, before mental rotation occurs. Source estimations derived from the topographic data indicated that left occipital, bilateral frontal and two medial areas were activated in the case of impossible postures, whereas left parietal regions were strongly activated during mental rotation. This result contrasts with mental rotation of objects, which is considered to be a right parietal process.