Everyday use of the computer mouse extends peripersonal space representation
Introduction
The body is the focus of certain spatial representations. Converging evidence from neurophysiology, neuropsychology and psychology suggests that spatial representation is not uniform, but that there are multiple, modular representations of space, with specific characteristics and functions; the body is at the centre of such representations. To simplify, we might distinguish at least three spatial representations originating from the body (see, e.g. Rizzolatti, Fadiga, Fogassi, & Gallese, 1997): the body space (see de Vignemont, this issue), the space far from the body, i.e. not reachable by a simple movement of the arm, named extrapersonal space, and the space immediately surrounding the body, i.e. peripersonal space (PPS), which is the topic of the present paper.
Several lines of evidence support the existence of a specialized brain system that specifically represents the PPS. Neurophysiological studies in monkeys described multisensory neurons in subcortical and cortical fronto-parietal regions which respond both to tactile stimuli delivered on a given body part (namely the head, arm, trunk) and to visual stimuli presented close to the same body part. Importantly, these neurons show null or lower responses when visual stimuli are presented far (at more than about 30 cm) from the body part where the tactile receptive field is located (Colby et al., 1993, Duhamel et al., 1998, Fogassi et al., 1996, Graziano et al., 1994, Rizzolatti et al., 1981). Similar response properties have been described for premotor (Graziano, Reiss, & Gross, 1999) and parietal (Schlack, Sterbing-D’Angelo, Hartung, Hoffmann & Bremmer, 2005) neurons sensitive to tactile stimuli administered on the head and auditory stimuli presented near the head. Thus, these neurons integrate tactile information on the body with visual and auditory information presented close to the body.
Similar integrative properties of PPS in humans have been described in neuropsychological studies conducted on brain damaged patients with cross-modal extinction. In these patients, the perception of contralesional tactile stimuli was affected by concurrent ipsilesional visual or auditory stimuli, and this effect is much stronger when visual or auditory stimuli are presented close to the patient's body rather than far apart, in the extrapersonal space (Di Pellegrino et al., 1997, Farnè and Làdavas, 2000). The near-far modulation of cross-modal extinction has been considered the behavioral hallmark of multisensory integrative systems coding PPS in humans (see Làdavas and Farnè, 2004b, Làdavas and Serino, 2008 for reviews).
An important property of PPS is the possibility of being modified as a function of experience. We can use a tool to reach portions of the extrapersonal space, and, consequently, to make reachable the unreachable space. This activity has been shown to extend the representation of the PPS. For instance, visual peripersonal space (VPPS) around the hand extends after a training that consists in using a rake to reach and collect objects placed far from the body, both in monkeys (Iriki et al., 1996, Ishibashi et al., 2000) and in humans (see also Farnè and Làdavas, 2000, Holmes and Spence, 2005, Ishibashi et al., 2004, Làdavas and Serino, 2008, Làdavas, 2002, Maravita and Iriki, 2004, Maravita et al., 2001 for reviews). In order to extend VPPS actual use of the tool is necessary, because no extension occurs if the tool is passively held in the subjects’ hands (Ishibashi et al., 2004, Làdavas and Farnè, 2006). Interestingly, the extension of peripersonal space after tool-use has been described by previous studies as lasting only briefly, because multisensory VPPS contracts to the pre-tool-use level several minutes after the end of training. However, tool-use is quite a common experience in everyday-life, and indeed there are some subjects who habitually and functionally use a tool to interact with extrapersonal space, such as blind people who use a cane to navigate in their daily environment. A recent study by our group investigated audio–tactile integration in the space around the hand and in extrapersonal space in order to measure the extension of the auditory peripersonal space (APPS) in blind cane users and in a control group of sighted, blindfolded, subjects (Serino, Bassolino, Farnè, & Làdavas, 2007). The results showed that while in sighted subjects the APPS is normally limited around the hand, in blind subjects it is immediately extended as soon as they hold their cane, even without any active momentary use of the tool. These findings suggest that the long-term experience with the cane in blind people produces a special and durably extended representation of APPS, which can be dynamically and functionally engaged depending on contextual demands. As far as we know, results from Serino et al. (2007) are the first demonstration of a durable extension of PPS representation. However, that study was carried out in a particular population of subjects, using a quite special tool.
The aim of the present work is to study whether sighted subjects who regularly use a common everyday-life tool, such as the computer mouse, have a durably extended PPS representation. To this end, we investigated the properties of APPS representation around the hand in subjects using the computer mouse everyday. The computer mouse can be conceived as a common tool linking peripersonal and extrapersonal space: it is used in the space near the hand but has an effect in far space, on the computer screen. Thus, a long-term experience with the computer mouse might durably extend the integrative space surrounding the hand. To test this hypothesis, we conducted two experiments on subjects who use the computer mouse for several hours per day with their right hand. In a first experiment, we measured the extension of APPS around the right hand, to study the effect of long-term experience of mouse-use. In a second experiment, we measured the extension of the APPS around the left hand in order to test whether any change in PPS representation was really induced by long-term mouse-use experience and therefore was selective for the hand used to operate with the mouse.
Section snippets
Experiment 1
We selected sixteen subjects who use a computer mouse everyday for work. In order to measure the extension of the auditory peri-hand space, we used the same task as in Serino et al. (2007): participants sat in front of the computer screen and were requested to verbally respond as fast as they could to a tactile target administered on their right hand, while concurrent task-irrelevant sounds were presented either near the stimulated hand (near sounds) or 70 cm away from the hand (far sounds).
Experiment 2
A new group of 16 subjects was recruited to participate in Experiment 2. APPS representation around the left hand was studied with the same procedure described above. Subjects performed a tactile discrimination task, during the presentation of a near or a far sound, in 3 experimental conditions, as in Experiment 1: the “No-mouse condition”, the “Passive mouse-hold condition” and the “Active mouse-use condition”. In the “No-mouse condition”, a segregation of the APPS is expected, as in
General discussion
The computer mouse is a tool which is used in the near space in order to act upon the far space; in this regard, it is a special tool because the space where it is used and the space where it exerts an effect are not physically connected. In the present study, we asked whether, in subjects using the computer mouse everyday, such “virtual” tool-use experience can extend APPS representation from near to far space. To this aim, the extension of audio–tactile integrative space around the hand was
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Now at: Istituto Italiano di Tecnologia, Genova, Italy.