The spatial metric representation of body parts: behavioural and neuropsychological evidence

The mental representation of the body is being a subject of intensive research from different perspectives starting from the 20th Century. Indeed, the body is a peculiar object for the brain, being at the same time a physical, space-occupying object and the critical mean for perception and action in the world around us. The present doctoral work focussed on the spatial representation of the body; in particular it was investigated whether the body holds a specific metric representation, which is supposed to be useful for action programming and interaction with the environment, as introduced in Chapter 1. To this aim Experimental Part 1 (Chapter 2 and 3) investigated the stable properties of the body metrics, while Experimental Part 2 (Chapter 4) focussed on its plastic and dynamic features. Chapter 2 discusses the differences between the spatial metric representation of body parts and non bodily three-dimensional objects. In particular, Experiment 1 investigated the possibility that Unilateral Spatial Neglect (USN) may affect to a different extent the spatial analysis of body parts relative to extrapersonal three-dimensional objects. Participants were required to bisect their left forearm and a length-matched cylinder with their right index finger. Both USN patients and neurologically unimpaired participants showed a significantly more accurate estimation of the subjective midpoint of the forearm, relative to the solid object. Besides the main pattern of an advantage in the forearm bisection, a further analysis suggested the possibility of a double dissociation, with two patients exhibiting the opposite advantage in the solid bisection. Experiment 2, asking unimpaired volunteers to perform the same bisection task in three different conditions (Forearm, Fake Forearm, Cylinder), showed a similar kind of spatial analysis for stimuli displaying bodily features, either real or fake, relative to non-corporeal objects. Thus, it can be suggested that the spatial processing of body parts critically depends upon their prototypical visuo-spatial shape and that the spatial metrics of body parts, relatively to noncorporeal objects, is also more resistant to the disruption of spatial processing and representation brought about by USN. Chapter 3, starting from recent evidence showing how the body can be used as an intrinsic metric system for the representation of near space, illustrates how the length of extrapersonal objects can be scaled using the metric representation of body parts, and to what extent a higher-order metric representation of the body relays upon the somatosensory system. Experiment 3 showed, by means of a bisection task, that the spatial encoding of an extracorporeal object (i.e., a cylinder) may be facilitated by the presence of the forearm in that space –i.e. when the forearm was placed inside the cylinder- as if participants can unconsciously rely on its well known metric representation in order to better estimate the length of the cylinder. In Experiment 4 the same task was administered to a group of right-brain damaged patients, with or without somatosensory and proprioceptive defict, and to a matched control group. The results showed that the spatial metric representation of body parts might be distorted, or even not available, when the somatosensory sensitivity is altered by a cerebral lesion. Data about the plasticity of the metric representation of body parts are presented in Chapter 4. In this last group of experiments, blindfolded participants were required to perform a radial proprioceptive bisection of their forearm before and after a training with a tool, which allowed an extension of the action space in the far space. The results of Experiment 5 supported the working hypothesis that the arm metric representation can be changed by tool-use. In this experiment participants performed a radial bisection of their arm and indicated the subjective midpoint of their arm more distally after the training, suggesting that the perceived length of their own arm was increased. Interestingly, no effect was obtained following a training with a shorter tool (i.e., 20 cm long). Experiment 6 further supported this interpretation by showing, through a proprioceptive control task, that the dynamic lengthening induced in the metric representation of the arm was not due to a mere illusory distal drift of the whole arm. Furthermore, it demonstrated that the spatial metric representations of the dominant and the non-dominant arms share similar plastic features, being both equally prone to be modified by tool use. In conclusion this doctoral work showed that body size holds a mental representation that is very stable (even more than that of extrapersonal objects), but also characterized by flexible functional plasticity.