Abstract Combining multiple sources of information al- lows the human nervous system to construct an approxi- mately Euclidean representation of near (personal) space. Within this space, binocular vergence is an impor- tant source of egocentric distance information. We inves- tigated how the nervous system determines the signifi- cance (weight) accorded to vergence information when other (retinal) distance cues are present. We found that weight decreases with (1) increasing discrepancy be- tween vergence information and other cues and (2) re- duced vergence demand. The results also provided evi- dence that the nervous system represents vergence relat- ed distance information in units of nearness (the recipro- cal of distance). Key words Binocular · Vergence · Distance perception · Cue weight · Human Introduction Humans (and other primates) are extremely proficient in reaching to pick up objects. Such skilled behaviour is marked by an ability to correctly judge the distance to be reached before onset of movement. Prehensile behaviour occurs within a region (<1.5 m) generally reported as be- ing approximately Euclidean in nature (see Cutting 1997 for a review of the empirical evidence) and described as ‘personal’ space (Cutting 1997). In order to achieve an Euclidean representation of this space, people integrate multiple sources of information or ‘cues’ (Cutting 1997; Brunswick 1952; Landy et al. 1995). A number of cues potentially contribute to the representation of distance (Cutting 1997), but this paper is restricted to considering ocular vergence. Since at least the time of Descartes it has been appreciated that observers could obtain an esti- mate of the distance of a binocularly fixated point from knowledge of vergence angle. Some recent studies have explored the use of binocular vergence in distance per- ception (see Tresilian et al. 1999; Mon-Williams and Tresilian 1999). The results of these studies have indicat- ed that binocular vergence is a valuable source of ego- centric distance information. Nonetheless, little progress has been made in documenting how vergence is integrat- ed with other cues to provide the Euclidean representa- tion required for skilled interaction with objects in per- sonal space. Many studies have reported that cue integration is equivalent to computation of the weighted mean of partic- ipating cues (Landy et al. 1995; Bruno and Cutting 1988; Dosher et al. 1986; Young et al. 1993) – only at threshold have significant exceptions been found (Bradshaw and Rogers 1996). Thus, weighted average computation is a well established model of cue combination (Landy et al. 1995; Howard and Rogers 1995). The model requires that: (1) each cue is associated with a multiplicative scale factor or weight (the sum of the weights being one) and (2) cues are represented in a common unit so that they can be add- ed together in a meaningful manner (Landy et al. 1995). We investigated the role of two factors hypothesised as major contributors to the determination of cue weight: (1) the cue’s intrinsic reliability, which is related to such fac- tors as its signal to noise ratio (Massarro 1988; Young 1971; Von Holst 1973). Previous work suggests that the reliability of vergence information decreases with increas- ing egocentric distance – implying a constant level of ad- ditive noise or a constant uncertainty in the measure of vergence angle (Cutting 1997; Collett et al. 1991). (2) The degree to which the cue conflicts with information provid- ed by other available cues – its discrepancy (Landy et al. 1995; Maloney and Landy 1989). To investigate the role of fixation distance and dis- crepancy, vergence was manipulated with ophthalmic prisms. In order to unequivocally interpret the prismatic J.R. Tresilian Human Movement Studies, University of Queensland, St. Lucia, Queensland 4072, Australia M. Mon-Williams ( ✉ ) School of Psychology, University of St. Andrews, St. Andrews, Fife KY16 9JU, Scotland e-mail: mon@st-andrews.ac.uk Tel: +44 1334 462074, Fax: +44 1334 463042 Exp Brain Res (2000) 132:362–368 Digital Object Identifier (DOI) 10.1007/s002210000333 RESEARCH ARTICLE James R. Tresilian · Mark Mon-Williams Getting the measure of vergence weight in nearness perception Received: 2 July 1999 / Accepted: 24 December 1999 / Published online: 31 March 2000 © Springer-Verlag 2000