Processing of illusion of length in spatial hemineglect: a study of line bisection G. Vallar a, b, *, R. Daini a,b , G. Antonucci b, c a Universita Á degli Studi di Milano-Bicocca, Dipartimento di Psicologia, Edi®cio U-6, Piazza dell'Ateneo Nuovo 1, 20126, Milan, Italy b IRCCS S. Lucia, Rome, Italy c Universita Á di Roma `La Sapienza', Dipartimento di Psicologia, Rome, Italy Received 11 November 1998; received in revised form 20 September 1999; accepted 24 September 1999 Abstract Bisection of horizontal lines and of the Brentano form of the MuÈller-Lyer illusion was investigated in six right brain-damaged patients with left spatial hemineglect, and in six control subjects. Patients bisected the lines to the right of the objective mid- point. Comparable illusory eects on line bisection were however found in both patients and control subjects. Relative to the baseline condition, in both groups the subjective midpoint was displaced towards the side expanded by the illusion, both leftwards and rightwards. By contrast, line length and spatial position of the stimulus had dierential eects. In neglect patients, the rightward bisection error increased disproportionately with line length, and when the stimulus was located in the left, neglected, side of egocentric space. Control subjects showed no such eects. The suggestion is made that the visual, non- egocentric, processes underlying these illusory eects of length may be spared in patients with left spatial neglect. The possible neural basis of this dissociation is discussed. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction A main and widely investigated manifestation of neglect for the contralesional side of space is the ipsile- sional displacement of the subjective mid-point of a horizontal line in the bisection task. Right brain- damaged patients with left neglect set the perceived mid-point to the right of the objective centre of the segment [4,5,20,46,49]. A number of interpretations have been put forward to account for this rightward directional error. Accord- ing to some early views, patients with unilateral neglect, due to a higher-order non-sensory disorder, fail to take into consideration the left side of the hori- zontal segment, or part of it, at least for the purpose of computing the subjective mid-point. Suggestions have been made that the left-to-right distribution of spatial attention may be defective, with the left side of the stimulus being comparatively less attended than the right [46]. The disorder may aect the internal rep- resentation of the stimulus, with a representational sco- toma concerning the left side [4]. More recently, attempts have been made to elucidate in more detail the pathological mechanisms underlying the rightward bisection error made by patients with left neglect. These patients may perceive objects in extra-personal space in a disordered fashion, which primarily concerns their horizontal, left-to-right, extent. The left side, or part of it, might not be unat- tended, or its internal representation lost or cut, but compressed rightwards, with all spatially de®ned seg- ments being available. Halligan and Marshall [19], who performed an experiment in which a right brain- damaged patient with left neglect was required to esti- mate visually the spatial position pointed to by an arrow, suggested that points in space may be patho- Neuropsychologia 38 (2000) 1087±1097 0028-3932/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0028-3932(99)00139-6 www.elsevier.com/locate/neuropsychologia * Corresponding author. Tel.: +39-02-64-486-810. E-mail address: giuseppe.vallar@unimib.it (G. Vallar).