Neuropsychologia 47 (2009) 2008–2012 Contents lists available at ScienceDirect Neuropsychologia journal homepage: www.elsevier.com/locate/neuropsychologia Note Eye rotation does not contribute to shifts in subjective straight ahead: Implications for prism adaptation and neglect Roger Newport ∗ , Catherine Preston, Rachel Pearce, Roxanne Holton School of Psychology, University of Nottingham, University Park, Nottingham NG7 2RD, UK article info Article history: Received 19 September 2008 Received in revised form 4 February 2009 Accepted 6 February 2009 Available online 14 February 2009 Keywords: Prism adaptation Rehabilitation Visuospatial neglect Aftereffect SSA abstract Prism adaptation has received much attention in recent years as a potential method for the rehabilitation of visual neglect. Recent theories as to the underlying mechanisms include oculomotor resetting and pathological realignment of subjective straight ahead (SSA). Typical prism adaptation procedures involve both ocular rotation and manual correction making the precise mechanisms and contribution of these to the amelioration of neglect difficult to determine. This experiment separated the contributions of ocular rotation and manual error reduction to SSA realignment in normal participants by shifting the eye alone, the hand alone or both together. Rotating the eye alone did not contribute to SSA realignment whereas shifting the hand did. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Despite its apparent simplicity, prism adaptation (PA) remains an important tool for research into one of our most fundamental brain functions providing an observable insight into the remark- able plasticity of the human motor system. Prism lenses displace visual information reaching the eye such that participants wearing prisms initially misreach to visual targets in the direction of visual displacement. Healthy participants easily adapt to this and very rapidly become accurate, but when the prisms are subsequently removed the aftereffects of adaptation causes misreaching in the opposite direction. This aftereffect is most commonly measured either by visual open-loop pointing (VOL) (the target is visible, but the hand is not) or indication of subjective straight ahead (SSA) (pointing straight ahead of the nose/body midline with the eyes closed). Prism adaptation has received much attention in recent years as a potential method for the rehabilitation of visual neglect (see Luauté, Halligan, Rode, Rossetti, & Boisson, 2006 for a review). Neglect is a frequent behavioural outcome following right hemi- sphere stroke in which patients fail to respond appropriately to left-sided events or stimuli, often ignoring them altogether (Heilman & Valenstein, 1979). For neglect patients, the aftereffect of rightward PA improves performance on a range of behavioural mea- sures, from line bisection to wheelchair navigation (Jacqui-Courtois, ∗ Corresponding author. Tel.: +44 115 846 7925; fax: +44 115 951 5324. E-mail address: roger.newport@nottingham.ac.uk (R. Newport). Rode, Pisella, Boisson, & Rossetti, 2008; Rossetti et al., 1998). Some studies in normal participants have shown that mild neglect-like symptoms can be induced by adaptation to leftward displacing prisms, causing participants to bisect lines slightly to the right of centre for example (e.g. Michel et al., 2003). Despite the excitement caused by its apparent success, the precise mechanisms underlying the amelioration of neglect by PA are not fully understood while doing so would provide a major benefit for patient rehabilitation strategies. Two recent theories include oculomotor resetting and a pathological shift in subjective straight ahead. Serino et al. (Angeli, Benassi, & Ladavas, 2004; Serino, Angeli, Frassinetti, & Làdavas, 2006; Serino, Bonifazi, Pierfederici, & Làdavas, 2007) suggest that the improvement in neglect symptoms following prism adaptation might occur as a result of what they term oculomotor resetting. By this account, a leftward deviation of the eye is prompted by the incremental leftward deviations of the arm that occur during prism exposure. According to this theory, having missed the target to the right on the first trial, patients must correct their reaches further and further leftward until accuracy is achieved. Because the eye and hand are yoked during goal-directed reaching (e.g. Carey, Coleman, & Della Sala, 1997; Fisk & Goodale, 1985; Jackson, Newport, Mort, & Husain, 2005), the eye also devi- ates leftwards—potentially ameliorating scanning behaviour and prompting leftward orienting of attention. Thus, leftward reduction of reaching error during prism exposure promotes leftward devia- tion of the oculomotor system, which, in turn, improves leftward performance on a number of measures of neglect after exposure. At first sight, however, this idea seems slightly at odds with typi- cal prism exposure behaviour in which the eyes deviate in the same 0028-3932/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2009.02.017