Hand dominance influences the processing of observed bodies

Abstract

In motor tasks, subgroups of lefthanders have been shown to differ in the distribution of attention about their own bodies. The present experiment examined whether similar attentional biases also apply when processing observed bodies. Sixteen right handers (RHs), 22 consistent left handers (CLHs) and 11 relatively ambidextrous inconsistent left handers (ILHs) performed an own body transformation task in which they were instructed to make speeded left–right judgements about a schematic human figure. Attentional biases associated with handedness were found to extend to observed bodies: CLHs’ judgements were faster to the figure’s left side, while ILHs, like RHs, showed facilitated performance to the figure’s right side. These results demonstrate a novel embodiment effect whereby the processing of a static schematic human figure is modulated by an individual’s personal motor capabilities. This finding suggests that motor simulation may contribute to whole body perception in the absence of actual or implied actions.

Introduction

There is widespread evidence for the recruitment of motor resources for perceptual tasks (Jeannerod, 2001; see Wilson & Knoblich (2005), for a review), and emerging evidence that this may be modulated by the idiosyncratic motor capabilities of the participant. For example, activation of the mirror neuron system (e.g., Decety and Grezes, 1999, di Pellegrino et al., 1992, Rizzolatti et al., 2001) has been shown to be dependent upon whether the observed actions are included within an individual’s personal motor repetoire (Calvo-Merino et al., 2005, Calvo-Merino et al., 2006). This suggests that one’s own motor capabilities modulate the extent to which motor simulation may enable the understanding of actions. The extent to which we spontaneously adopt the spatial perspective of an actor has also been shown to be modulated by motor experience: when answering questions about the spatial layout of scenes depicting another person in action, participants have been shown to be more likely to respond from the actor’s point of view than their own, particularly for actors with the same handedness as themselves (Lozano, Martin Hard, & Tversky, 2007). In the present study, we aim to evaluate further the role of motor competencies during imagined perspective taking by examining the effect of hand dominance on performance in an “own body transformation” (OBT) task (Arzy et al., 2006, Blanke et al., 2005, Zacks et al., 1999). The OBT task requires participants to take the perspective of a schematic human figure in order to perform simple spatial judgements.

Left and right handers have been found to differ in the allocation of attention to one side or the other in motor tasks involving bimanual coordination (Amazeen et al., 1997, Amazeen et al., 2005, Peters and Servos, 1989), with left handers favouring their left side and right handers favouring their right. Where the two hands perform different but complementary functions, participants tend to use the dominant hand for the more demanding task and the non-dominant hand in a supporting role (Peters, 1994). Where they have to perform identical movements, the dominant hand tends to lead: for example, when drawing circles (Summers, Semjen, Carson, & Thomas, 1995); pendulum swinging (Amazeen et al., 1997; Riley et al., 1997, Treffner and Turvey, 1995, Treffner and Turvey, 1996) or tapping in a Fitts’ law task (Amazeen et al., 2005). It has been suggested that these handedness effects arise from an inherent attentional bias towards the dominant limb which can be moderated, but not reversed, by the voluntary direction of attention to the non-dominant limb (de Poel, Peper, & Beek, 2008). By this account, attention is not distributed equally about the midline but is biased to one side or the other, leading to superior performance on the attended side (Kinsbourne, 1970).

Similar biases in processing information to the right or left of the participants’ own bodies have been demonstrated in spatial choice-reaction tasks. Responses to visual stimuli are faster to stimuli in the visual field corresponding to the dominant hand (Pierce, Harris, & Henderson, 1996) and the Simon effect is greater in the right visual field for right handers and the left visual field for left handers (Rubichi and Nicoletti, 2006, Simon and Rudell, 1967). These findings have been explained (Rubichi & Nicoletti, 2006) in terms of a premotor theory of attention, which posits a perception-for-action system in which attention and response processes are closely linked because they are controlled by shared sensorimotor mechanisms. According to this account, spatial attention involves a readiness to act in the space toward which a motor programme is prepared, and it also facilitates the processing of stimuli coming from that same space sector (Craighero, Fadiga, Rizzolatti, & Umiltà, 1999). Rubichi and Nicoletti (2006) propose that the handedness effect arises from the larger cortical representation of the dominant hand (Triggs, Subramanium, & Rossi, 1999) leading to spatial attention processes related to the dominant hand being more efficient than those related to the non-dominant hand. In support of this idea, a crossed hands manipulation of the Simon task brought about a reversal of the asymmetry associated with handedness (Rubichi & Nicoletti, 2006), in that the Simon effect was now larger in the left visual field for right handers and the right visual field for left handers, suggesting that the attentional bias associated with handedness is toward the side of space in which the dominant hand is currently operating, rather than where it habitually operates.

The own body transformation (OBT) task provides an opportunity to investigate the modulatory role of motor competence for the processes involved in taking the perspective of an observed body. Several studies (Arzy et al., 2007, Arzy et al., 2006, Blanke et al., 2005, Mohr et al., 2006, Zacks et al., 1999) have employed such a task to investigate the processes involved in imagining the rotation of one’s whole body to match a target body. In a typical OBT task participants are required to make left–right judgements about a schematic human figure, which may be facing away from or toward the observer. Behavioural and neurophysiological data from these studies have been taken as evidence that the left–right judgement is accomplished by performing such a mental self-rotation, with consequently longer response times to front-facing figures than to back-facing figures, consistent with the longer time it would take to rotate oneself physically through 180°. The role of motor competence in imagined transformations of perspective may therefore be investigated by examining interactions between handedness and OBT task performance.

Although this question has yet to be examined systematically, there is suggestive evidence that handedness may influence OBT task performance (Richardson, 1994). In a pen and paper predecessor to this task, participants were required to judge which of a manikin’s hands was marked by a black disc. Performance was more accurate when the black disc marked the right hand than when it marked the left, regardless of whether the manikin was facing toward or away from the viewer. Given that the sample was likely to be predominantly right-handed, these results are suggestive of a facilitation of perspective transformation performance when processing the body side corresponding to the dominant hand. The current study aims to address this specific question by investigating the performance of both right-handers and left-handers in the chronometric OBT task.

While many studies have treated left handers as a homogenous group, there is evidence that subgroups of left handers show different patterns of performance on a variety of tasks (Peters & Servos, 1989). As might be expected in a predominantly right-handed world, some individuals who write with the left hand show a preference for using the right hand for certain other activities, while others are consistently left handed and use the left hand for all activities where a right hander would use the right hand. Failure to subdivide lefthanders into consistent (CLH) and inconsistent left handers (ILH) may lead to performance differences being obscured. For example, while right handers show a stronger right hand than left, this asymmetry is not found with left handers when they are treated as a single group (Borod, Caron, & Koff, 1984; Peters, 1983). When they are divided into subgroups, however, CLHs show a stronger left hand than right and ILHs show a stronger right hand than left, at least for males (Peters & Servos, 1989). Similar differences were found in a bimanual tapping task, in which participants tapped twice with one hand for each single tap with the other, with ILHs performing like right handers in that they were faster with the R2/L1 combination, and CLHs showing an asymmetry favouring the L2/R1 combination (Peters & Servos, 1989). When left handers were considered as a single group, no asymmetry in performance was observed, again offering support for making this distinction. The current study therefore follows Peters and Servos (1989) by subdividing left handers into consistent (CLH) and inconsistent (ILH) groups.

The aim of the current study was to investigate the modulatory role of motor competencies for visual cognition by evaluating whether the attentional bias towards the dominant hand, found in bimanual coordination tasks and spatial choice-reaction time tasks, also affects judgements made when performing imagined perspective transformations. We examined this using the OBT task. In this task, a bias toward processing the side of an observed body that corresponds to the participant’s dominant hand would support this hypothesis. We also used a control task requiring participants to make left–right judgements from their own spatial perspective, in order to control for artifacts relating to any response preference in favour of the dominant hand. If the attentional bias for the dominant hand affects imagined perspective transformations, then right handers should show faster response times in the OBT task when the ball is in the manikin’s right hand, regardless of whether the manikin is front- or back-facing. There should be no such difference in the control task. Consistent left handers, by contrast, would show faster response times when the ball is in the manikin’s left hand, again selectively for the OBT task. Inconsistent left handers, if they show any bias, are likely to behave more like right handers.