Brief articleLeft to right: Representational biases for numbers and the effect of visuomotor adaptation
Introduction
Patients with right parietal damage and unilateral neglect exhibit a perceptual deficit for the left (contralesional) side of physical space (Heilman, Watson, & Valenstein, 1993). As a result, for line bisection tasks, neglect patients bisect the line far to the right of its true centre. Unilateral neglect is not restricted to stimuli that are physically present and also occurs for mental imagery (Bartolomeo, Bachoud-Lévi, Azouvi, & Chokron, 2005) and mental representations of numbers. The mental number line is thought to have a left-to-right organization whereby low and high numbers are represented in the left and right sides of space, respectively (Dehaene, Bossini, & Giraux, 1993). As a result, when judging the distance between two numbers, left neglect patients misplace the midpoint to the right (i.e. toward the higher number) – analogous to their rightward misbisection of physical lines (Vuilleumier et al., 2004, Zorzi et al., 2006, Zorzi et al., 2002; but cf. Dorrichi, Guariglia, Gasparini, & Tomaiuolo, 2005).
While left neglect patients misbisect mental and physical lines to the right, normal participants demonstrate a leftward bias (Nicholls et al., 1999, Nicholls and Loftus, 2007). This leftward bias reflects pseudoneglect, a phenomenon that causes the leftward stimulus properties to be overestimated relative to those on the right (Bowers & Heilman, 1980). Pseudoneglect manifests itself on physical line bisection tasks, where the perceived midpoint of a line is shifted left of the true midpoint (Jewell and McCourt, 2000, McCourt, 2001), but is also observed for judgments of luminance (Mattingley et al., 1994, Nicholls et al., 1999, Nicholls et al., 2004), size and numerosity (Nicholls et al., 1999). Leftward biases have also been observed for the mental representation of stimuli, such as the recall of familiar scenes (McGeorge, Beschin, Colnaghi, Rusconi, & Della Sala, 2007), mental alphabet lines (Nicholls & Loftus, 2007) and mental number lines (Longo & Lourenco, 2007).
The clinical symptoms of neglect can be ameliorated through adaptation to right-shifting prisms, improving performance on a wide range of visuospatial tasks (Frassinetti et al., 2002, Pisella et al., 2006) including explicitly spatial tasks such as physical line bisection (Rossetti et al., 1998) and non-explicitly spatial tasks such as temporal order judgments (Berberovic, Pisella, Morris, & Mattingley, 2004) and mental imagery (Rode, Rossetti, & Boisson, 2001). Rossetti et al. (2004) found that left neglect for the mental number line was improved by adaptation to right-shifting prisms, leading them to suggest that adaptation alters higher-level representations of space.
Wearing right-shifting prisms causes objects to appear to the right of where they actually are, so that when the wearer first points to an object, they miss to the right. Subsequent movements must be adapted if they are to be accurate, a complex process known as ‘prism adaptation’ (PA), which involves two key components – ‘strategic control’ and ‘spatial realignment’ (Redding, Rossetti, & Wallace, 2005). Strategic control is a short-term process whereby initial reaching errors are rapidly detected and reduced. Spatial realignment involves a shift of sensory–motor reference frames so that the felt and seen positions of the moving limb are congruous (Redding & Wallace, 1997). Negative aftereffects (i.e. pointing errors in the direction opposite to the prismatic shift following removal of the prisms) are thought to reflect the amount of spatial realignment that has occurred (Rossetti et al., 1998, Redding and Wallace, 2006), and can be observed within a 5-min PA session (Pisella et al., 2006).
Although the precise mechanism by which PA moderates left neglect remains unclear, it is thought that PA promotes the realignment of dysfunctional spatial maps. Adaptation to right-shifting prisms shifts biased egocentric spatial reference frames to the neglected side, correcting the dysfunctional calibration of the task-work space (Redding et al., 2005, Redding and Wallace, 2006, Rossetti et al., 1998). Left-shifting prisms do not affect bisection judgments (Rossetti et al., 2004, Rossetti et al., 1998), suggesting that there is an inherent asymmetry in the mechanisms that link PA and spatial cognition, which causes the two hemispheres to be differentially engaged during adaptation (Colent et al., 2000, Vallar et al., 1999).
The dramatic effect of PA on neglect patients with disordered spatial maps raises the intriguing possibility that PA also affects normal spatial maps in the intact brain. This issue has been investigated to some extent in studies investigating the effect of prisms on line bisection. Adaptation to left-shifting prisms elicits a rightward shift of midpoint in normal participants (Berberovic and Mattingley, 2003, Colent et al., 2000), whereas right-shifting prisms have no effect (Colent et al., 2000, Michel et al., 2003; but cf. Rossetti et al., 1998). While these studies show that prisms affect the perception of a stimulus, they do not demonstrate that this effect goes beyond relatively low-level congruencies in the mapping of the stimulus with reference to body coordinates and/or the response. In the present study, we sought to demonstrate that PA affects high-order mental representations of space where there are no physical sensory/motor congruencies. The mental number line was explored using low and high numbers, which are known to induce pseudoneglect (Longo & Lourenco, 2007). Given that neglect and pseudoneglect are thought to reflect the operation of a common set of cognitive and neural mechanisms (McCourt and Jewell, 1999, Nicholls et al., 2004) we expected our pseudoneglect study to yield a similar (but mirror-reversed) pattern of results relative to the neglect literature (e.g. Rossetti et al., 2004). Participants completed a mental number line task before and after PA that shifted the visual scene 15° to the left or to the right. In light of the asymmetrical effects of PA reported by Berberovic and Mattingley (2003), we expected left-shifting prisms to reduce pseudoneglect for mental number lines whereas right-shifting prisms should have no effect.
Section snippets
Participants
Thirty-six undergraduate students participated in the study (eight male, mean age 23.1 years). All had normal vision and were right-handed, as confirmed by the Edinburgh Handedness Inventory (Oldfield, 1971).
Apparatus, stimuli, & procedure
Participants completed the tasks in the following order: (i) a pre-adaptation number line task; (ii) visuomotor adaptation to either a 15° leftward or rightward lateral shift, or to control spectacles with no lateral shift; (iii) a post-adaptation number line task identical to the
Visuomotor adaptation
All participants successfully adapted within 50 pointing trials. To establish whether participants in the left-shifting and right-shifting prism conditions adapted equally, the distance by which the finger missed the target was measured at the start of the adaptation phase (i.e. the first pointing movement made whilst wearing the prisms) and at the start of the de-adaptation phase (the first pointing movement made without the prisms – post-adaptation). These measurements provide an index of (i)
Discussion
Like Longo and Lourenco (2007), participants overestimated the leftward space on the mental number line – consistent with the effect of pseudoneglect. The fact that such a bias is observed for mental representations such as numbers (Göbel et al., 2006, Longo and Lourenco, 2007) and letters (Nicholls & Loftus, 2007) suggests that pseudoneglect occurs independently of stimulus input. While pseudoneglect for physical lines may be related to left-to-right eye movements (Chokron et al., 1998,
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