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On the nature of near space: Effects of tool use and the transition to far space

https://doi.org/10.1016/j.neuropsychologia.2005.09.003Get rights and content

Abstract

Many researchers have proposed that the near space immediately surrounding the body is represented differently than more distant space. Indeed, it has often been suggested that near space encompasses that within arm's reach. The present study used a line bisection task in healthy adults to investigate the effects of tool use on space perception, and the nature of the transition between near and far space. Subjects bisected lines at four distances controlled for both veridical and angular size using a laser pointer and a set of sticks. When the laser pointer was used, a left to right shift in bias was observed as stimuli were moved from near to far space. When a tool was used, however, a leftward bias was observed at all distances, similar to that observed with the laser pointer in near space. These results suggest that the tool expanded the range of near space. Additionally, the transition from near to far space was gradual, with no abrupt shift at arm's length (or at any other distance). In contrast to theories describing near space as that within arm's reach, these findings suggest that the representation of near space is less rigid, extending with tool use and gradually transitioning into far space.

Introduction

Perceiving the space around us is crucial for successful action. Given that we cannot act at indefinite distance, it is of primary importance to represent the space closest to us. Accordingly, many researchers in diverse fields have differentiated the near space immediately surrounding the body from that farther away (e.g., Brain, 1941, Hall, 1966, Sommer, 1969). Brain, for example, described two patients with opposite localization deficits, one unable to locate “objects within arm's reach” (p. 255), the other unable to localize objects farther than a yard from his body. Subsequent research has replicated this double-dissociation (Cowey, Small, & Ellis, 1994; Halligan & Marshall, 1991). Brain distinguished the grasping distance within arm's reach from the walking distance beyond. Following Brain, others have described near space as that within arm's reach (e.g., Berti et al., 2002, Cowey et al., 1994; Halligan, Fink, Marshall, & Vallar, 2003; Rizzolatti, Matelli, & Pavesi, 1983; Weiss, Marshall, Zilles, & Fink, 2003). The distinction between near and far space, however, may be less rigid (cf. Berti & Rizzolatti, 2002). For example, near space may be extended through the use of tools, and the transition from near to far space may be more gradual than abrupt. The present study examined these issues in neurologically healthy individuals.

Recent neurophysiological and neuropsychological findings have demonstrated that tool use affects space perception. Rizzolatti, Scandolara, Matelli, and Gentilucci (1981) described neurons in the monkey with visual RFs consisting of the space either immediately adjacent to tactile RFs (pericutaneous), or within arm's reach (distant peripersonal). Iriki, Tanaka, and Iwamura (1996) found that these RFs extend to include the space around a rake wielded by a monkey. Similarly, visual-tactile interference around the hand in human patients with cross-modal extinction extended to incorporate the space surrounding a wielded tool (Farnè & Làdavas, 2000).

Case studies of patients with neglect have also investigated effects of tool use on perception of near and far space. Berti and Frassinetti (2000) described a patient who showed left neglect (i.e., rightward bias) when bisecting lines with a laser pointer in near, but not far, space. However, when responding with a stick, neglect appeared in both near and far space. Berti and Frassinetti suggested that tool use, by extending the range of effective action, remapped near space, projecting neglect into far space. In contrast, Pegna et al.'s (2001) patient showed a rightward bias using a tool, but not a laser pointer, regardless of distance. These authors suggested that differential motor requirements, rather than extension of near space, might account for effects of tool use. Humphreys, Riddoch, Forti, and Ackroyd (2004) proposed, alternatively, that tools affect performance by drawing attention to neglected regions of space. Thus, while it is clear that tool use affects performance, it is less clear whether this represents extension of near space.

Another sense in which near space may differ from Brain's (1941) conception is that the transition to far space may be more gradual than abrupt. The extent of peripersonal RFs from the skin varies widely. Fogassi et al. (1996), for example, found visual RFs ranging in extent from 5 to 35 cm from tactile RFs. Nevertheless, these visual RFs always remain within the monkey's reaching distance (Fogassi & Gallese, 2004). Thus, while there appears to be a gradient of neuronal responses coding (at a population level) the location of objects within reach, near space itself may terminate rather abruptly at the periphery of arm's reach.

Most studies of near and far space in human patients have presented stimuli at only two distances, making inferences about the transition between near and far space impossible. Two exceptions are studies by Berti et al. (2002) and Cowey, Small, and Ellis (1999). Berti et al. used three distances, finding more severe neglect beyond (1.5 and 3 m) than within arm's reach (.5 m). There was no difference in performance at 1.5 and 3 m, consistent with an abrupt shift between near and far space. In contrast, Cowey et al. (1999) studied five patients with greater neglect in near than far space at six distances, failing to find any abrupt shift at the distance of arm's reach. These results are difficult to interpret, however, since only one patient displayed what could be characterized as a continuous shift (Berti & Rizzolatti, 2002). The others displayed patterns consistent with abrupt shifts, but at different distances. When averaged, these data appear to indicate a gradual shift, potentially masking individual differences. Another interpretive problem with studies such as Cowey et al. (1999) is that, in controlling visual angle, progressively longer lines were presented at farther distances. Consequently, any effects of distance may actually be effects of line length. Indeed, neglect patients often bisect longer lines farther to the right than shorter lines (Marshall & Halligan, 1989; Tegnér, Caneman, & Levander, 1990).

The distinction between near and far space has also been examined in healthy adults using line bisection tasks. On standard versions, presented in near space, subjects demonstrate a slight leftward bias, known as pseudoneglect (Bowers & Heilman, 1980; see Jewell & McCourt, 2000 for review). While several studies have either failed to find effects of distance on pseudoneglect (e.g., Cowey et al., 1999, Weiss et al., 2000) or found only inconsistent effects (e.g., Cowey et al., 1994; Wilkinson & Halligan, 2003), three recent studies reported consistent rightward shifts in bias from near to far space (Bjoertomt, Cowey, & Walsh, 2002; McCourt & Garlinghouse, 2000; Varnava, McCarthy, & Beaumont, 2002). Varnava et al., unlike the other studies, presented lines at more than two distances, finding an apparent gradual shift. As with Cowey et al.'s (1999) study, however, these studies controlled visual angle, presenting longer lines at farther distances. Given that line length has been found to modulate pseudoneglect (McCourt & Jewell, 1999; Reuter-Lorenz, Kinsbourne, & Moscovitch, 1990), these effects of distance are ambiguous. Indeed, Wilkinson and Halligan found less consistent distance effects when veridical size was controlled.

As described above, existing studies are ambiguous about two important issues: whether effects of tool use actually represent extension of near space and about the nature of the transition between near and far space (i.e., abrupt versus gradual). To examine these issues, healthy subjects in the present study bisected lines at four distances (controlled for both veridical and angular size) using either a laser pointer or a set of sticks. If modulation of bisection performance observed in previous studies reflects effects of near versus far space, a rightward shift in bias with distance was predicted on laser pointer trials. In contrast, if tool use extends the range of near space, no such shift was expected with the sticks; rather, a constant leftward bias was anticipated, comparable to that obtained with the laser pointer in near space. Alternate interpretations of the effects of tool use (e.g., Humphreys et al., 2004, Pegna et al., 2001) would not predict such an interaction. Furthermore, since more than two distances were used, abrupt versus gradual models of the transition from near to far space can be distinguished controlling both veridical and angular size.

Section snippets

Participants

Sixty students (35 female, 25 male), between 18 and 30 years, participated. All but one were right-handed as determined by the Edinburgh Handedness Inventory (Oldfield, 1971), M: 68.93; range: −26.3 to 100.

Stimuli

Lines of 2, 4, 8, 16, and 32 cm (1 mm in height) were used, centered on legal-sized paper attached horizontally to a wall, 156 cm above the floor. A subset of lengths was selected from three of the four distances – at 30 cm (2, 4 and 8 cm), 60 cm (4, 8 and 16 cm), and 120 cm (8, 16 and 32 cm) – such

Results

Analysis of variance revealed effects of distance, F(3, 117) = 4.40, p < .01, and device-type, F(1, 39) = 12.91, p < .001, and an interaction between distance and device-type, F(3, 117) = 5.83, p < .001. When the laser pointer was used – but not the sticks – a clear left to right shift in bias was observed with increasing distance (see Fig. 1, top panel). Planned contrasts revealed bias to be significantly farther to the right on laser pointer than stick trials at 60 cm, t(39) = 1.90, p < .05, 90 cm, t(39) = 2.59,

Discussion

The present study examined two important issues concerning the nature of near space: whether wielding a tool expands the range of near space, and whether the transition to far space is abrupt or gradual. When subjects bisected lines with a laser pointer, a left to right shift in bias with increasing distance was observed, consistent with previous findings. Importantly, this shift occurred whether controlling for visual angle or veridical size, suggesting a modulating effect of distance per se.

Acknowledgements

This research was supported by grants from the Psychology Graduate Student Travel and Research Committee to both authors, and an NSF graduate fellowship to MRL. The authors thank Bennett Bertenthal, Janellen Huttenlocher, Susan Levine, and Linda Suriyakham for helpful discussion.

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