A Psychophysical Approach to Touch: from Multi-sensory Processes to Body Representations

The goal of this PhD dissertation is to investigate the sense of touch in its complexity. The somatosensory system is made up of a number of submodalities, and it is a pivotal building block of our experience of the external world and the body itself. In a natural context, the somatosensory signals are continuously integrated with the signals coming from other sensory modalities, allowing to come up with unambiguous percepts of the world. When facing a somatosensory deficit, the benefits of such an optimal integration could be lost, with dramatic consequences. Due to the importance of these sensory processes, quantitative and rigorous psychophysical methods have to be carefully implemented and adopted to unveil crucial somatosensory processes. Therefore, to investigate multisensory integration in different clinical populations suffering from somatosensory deficits, two studies were conducted. In the first one, the integration of visual and artificial somatosensory information are investigated in an amputee provided with a sensorized bionic hand prosthesis. The results indicate that sensory substitution based on intraneural feedback can be integrated with natural visual feedback. The study opens encouraging scenarios in the development of increasingly sophisticated technology devoted to the restoration of lost limb functioning. The second study involves patients with Anorexia Nervosa (AN) and healthy individuals with and without body concerns. Its findings suggest that patients show visual and tactile objects shape distortion and lower sensory acuity. To a lesser extent, the biases in shape perception (i.e. width overestimation) were observed also in the other two healthy groups, and was more pronounced in women concerned about their bodies. These biases are interpreted as a reflection of proven distortions in the Mental Body Representations of the body (MBRs). Finally, our results do not support the multi-modal integration impairment hypothesis. The study encourages to evaluate the potential of perceptual indexes as a tool to support clinical practice. Another fundamental feature of the somatosensory system is that the inputs can be static or dynamic, implying movement and an active interaction between the skin and the perceived objects. To analyze this important perceptual aspect in depth, I carried out two studies comparing bimanual performances in static and dynamic conditions. The first study investigated sensory integration mechanisms within the somatosensory modality (i.e. the integration of information coming from the two hands). The results showed that the information coming from the two hands are integrated in a statistically optimal fashion. Slightly better performances are observed in the dynamic conditions providing a little evidence of movements’ benefits on the perceptual performances. This work confirms the importance of cognitive effects in bimanual integration processing. The second study targeted the effect of movement on lower-level information processing. A bimanual manipulandum was used to measure size-change detection thresholds of a virtual bar at rest or during continuous movements. The results showed that the humans’ ability to localize the relative positions of their upper-limbs decreases during movement. The existence of different processes underlying size discrimination in static and dynamic objects perception is suggested. Finally, the sense of touch has an essential role in the constitution of our sense of-self. Different MBRs and their interaction provide a reference context to perceive our own body and the external objects. Accordingly, the last experiment presents a paradigm designed to investigate different cognitive maps of the body and their interaction in a complex task involving their synergy. The figure below proposed a graphical abstract of this PhD dissertation, which is part of a research activity carried out in collaboration between different national and international Universities, Research Institutions and Hospitals. Overall, it provides an overview of the different peculiar aspects underlying the somatosensory experience by using a quantitative approach based on the methods of psychophysics.