Peripersonal and Extrapersonal Space: Line Bisection Experiments in Real and Virtual Environments

The human interest on understanding how the surrounding space is perceived and processed has stimulated the scientific and experimental research in this field for several decades. Moreover, from the earliest human interaction with computer-generated virtual environments, interest in this subject has received increasing attention, in order to verify whether the observed behavior in virtual environments is similar to that seen in real environments. Virtual reality technologies are currently used in different fields, from the implementation in medicine to the rehabilitation of neuropsychological and psychological disorders, from the use in the military to the exploitation in industry. The positive results obtained through the use of virtual reality continue to support this new technology and its evolution. Research on the human brain mechanisms involved during interactions with these synthetic environments plays an important role from a neuropsychological point of view, and in terms of reliability and credibility of the instrument. Of particular importance in the study of spatial perception is the question of its representation that distinguishes the surrounding environment in peripersonal space, or near space, the space within arm reach, and extrapersonal space, or far space, defined the space beyond the arm reach. Neuropsychological evidences confirm the existence of different neural mechanisms involved in peripersonal and extrapersonal space representation. Moreover, it is known that the use of standard tools results in a modulation of the perceptual space, expanding the representation of peripersonal space to include the part of extrapersonal space occupied by the end of the tools manipulated. The aim of the present study is to analyze and understand some unclear aspects of this area of research, and to add information to the basic theory. To this end, several experiments were conducted to investigate the following aspects: • peripersonal and extrapersonal space limits during tool-use; • neural circuits involved in representation of peripersonal and extrapersonal space; • body and arm position influence in the perception of peripersonal and extrapersonal space; • the mechanism involved in extrapersonal space perception. The first chapter is an overview of peripersonal and extrapersonal space theories, and of the brain areas involved in their representation, through the analysis of researches and experiments with animal subjects; next, further studies are examined with human participants, in different situations and methods, in order to define the characteristics of this perceptual phenomenon. The second chapter presents the literature concerning the applications in the virtual reality field in the study of artificial environments spatial perception. The first part of the chapter introduces the basic concepts of the technology. Then, empirical evidences that support the utility and potential of this new technology are exposed Finally, an analysis of the researches concerning the phenomenon of peripersonal space perception in virtual environments is provided. The third chapter presents the research, and particularly explains the, methodology, experimental procedures and materials used for the experiments. The experimental paradigm used in the present work was the line bisection task. It is a widely used experimental paradigm since relatively easy for participants to perform, but provides valid results in terms of perception and visuospatial attention. The first study aimed to understand how far up tool-use can expand peripersonal space. The results showed an expansion of perceived peripersonal space when handling a tool up to a distance of 240 cm. The second study explored which brain areas are involved in a visuospatial attention task performed in a virtual environment, by using the neuroimaging technique functional Near Infrared Spectroscopy (i.e., fNIRS). The experiment represents one of the first attempts to investigate the neural correlates by using the fNIRS during an immersive virtual reality experience. Body position can influence the perception of the surrounding space and the space beyond arm reaching distance. Based on previous results, the third experiment investigated if the feeling of having the body blocked or free to move during a visuospatial attention task has implications in the attentional shift that modulates peripersonal and extrapersonal space perception. The results showed that both for the first and the second case, an abrupt attentional shift, not gradual, during the transition from peripersonal to extrapersonal space was observed. Arm position can influence the perception of the surrounding space and the space beyond arm reaching distance. Based on previous results, the fourth experiment investigated whether having the arm stretched or bent during a visuospatial attention task, has implications in the modulation of peripersonal and extrapersonal space perception. The results confirmed that the position of the arm affects visuospatial attention. The last experiment has specifically investigated the underlying causes of peripersonal space expansion when using a tool. It is believed that is the ability to actively manipulate the space the essential feature to induce the peripersonal space expansion. But, as noted in previous studies, it is possible that the visual continuity from the hand toward the manipulated region of space represents the key feature in modulating peripersonal space expansion. The results confirm the hypothesis that the essential feature in order to induce peripersonal space expansion is represented by the active manipulation of the observed region of space. The studies reported in the present study explored several issues relating to the understanding of the perception of the surrounding space and its implications on the attentional processes related. In the fourth chapter the results are discussed and evaluated.