A ruler for measuring representational space Alessio Toraldo Istituto di Psicologia della Facolta Á di Medicina, Universita Á di Milano, Italy In the last decade a number of theories have been formulated hypothesizing a ``distortion'' of representational space in neglect patients, in order to explain their impairment on tasks investigating the perception of horizontal extents. Trying to compensate for the relative lack of formalization in this field of research, a mathematical model of what a spatial distortion should be is formulated on the basis of theoretical considerations and of some neurophysiological evidence; furthermore, a psychophysical task is proposed that estimates the amount of dis- tortion and is able to overcome some of the limits in the currently used methods. On this task, 32 normal subjects showed two different kinds of deviation from the ideal performance, neither of which is attributable to perceptual factors. Out of five neglect patients, two showed reliable distortion, another two showed a trend towards distortion but in opposite directions with respect to one another), and one showed no sign of distortion. In conclusion, a new method for estimating distortion suggested that the explorative impairmentÐneglectÐcan dissociate from the metric oneÐdistortion, as one would expect on grounds of a evolutionarily plausible) separation between the two operations of exploring and measuring space. It has been known for decades that the normal) human perceptual system does not, in general, build an ideal representation of the metrics of the visual world ``visual anisotropy''; see, for example, Heller, 1997; Howard & Templeton, 1966). Nevertheless, there are conditions in which the normal visual system does provide an accurate representation of the physical metrics. This is apparent, for example, when normal observers are required to explore, under free viewing conditions, an array of landmarks that are distributed along a horizontal straight line either real or virtual) of moderate length around 200 mm) that is placed at VISUAL COGNITION, 2003, 10 5), 567±603 Please address all correspondence to: A. Toraldo, International School for Advanced Studies SISSA), Via Beirut 2±4, 34014 Trieste, Italy. Email: toraldo@sissa.it This work was supported by grants from the Italian Ministry of University and Scientific Research and formed a partial contribution towards a PhD from the University of Pavia. I am grateful to Luigi Burigana, Edoardo Bisiach, David Milner, and Mark Mon-Williams for their thoughtful comments on earlier drafts of the present paper. I am also indebted to David Milner, Justin Harris, and Valerie Lesk for their kind help in revising the English. This paper is dedicated to the memory of Fabio Migliorini. # 2003 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/13506285.html DOI:10.1080/13506280244000212