Brain and Cognition 45, 52–63 (2001) doi:10.1006/brcg.2000.1255, available online at http://www.idealibrary.com on The Influence of Unimanual Response on Pseudoneglect Magnitude Mark E. McCourt, Pierre Freeman, Cindi Tahmahkera-Stevens, and Matthew Chaussee North Dakota State University, Fargo, North Dakota Published online January 17, 2001 Various factors influence the degree of leftward error (pseudoneglect) (Bowers & Heilman, 1980) that typifies the performance of normal subjects in line bisection tasks (Jewell & McCourt, 2000). The results of this experiment show that unimanual responding also exerts a subtle but significant modulating influence on spatial attention, as indexed by the differential magnitude of pseudoneglect. Using a forced-choice tachistoscopic line bisection protocol, 184 subjects (92 male and 92 female) bisected horizontally oriented lines (22.3° wide  0.39° height) presented to central vision in two conditions, in which bisection responses were exe- cuted via button presses using the first two fingers of either the left (LH) or right (RH) hand. Perceived line midpoint deviated significantly leftward of veridical ( p  .05) in both condi- tions. There was no significant influence of subject sex ( p  .05). A significant influence of unimanual response was revealed ( p  .05) where pseudoneglect magnitude was greater in the LH than the RH condition. The results are interpreted within the framework of the activation- orientation theory of attentional asymmetry.  2001 Academic Press Key Words: line bisection; pseudoneglect; spatial attention; unimanual. INTRODUCTION Hemispatial Neglect Hemispatial neglect is typified by a failure of patients to spatially orient in a di- rection contralesional to their (typically right hemispheric) lesions (Heilman, Wat- son, & Valenstein, 1993). Bisection tasks are frequently employed to measure the presence and severity of neglect (Colombo, De Renzi, & Faglioni, 1976; Riddoch & Humphries, 1983; Levine, Warach, Benowitz, & Calvanio, 1986; Ishiai, Furukawa, & Tsukagoshi, 1989; Heilman, Watson, & Valenstein, 1993). Patients with left hemi- neglect generally bisect horizontal lines of moderate length (10° visual angle) sig- nificantly to the right of veridical center, as though being either inattentive to the majority of the left-hand side of the stimulus (via right hemispheric hypoarousal: Heilman & Valenstein, 1979; Mesulam, 1981) or, alternatively, hyperattentive to its This research was supported by a grant (to M.E.M.) from the National Eye Institute (EY 12267-01). The authors thank George Jewell, Matt Garlinghouse, Katie McGrath, and Jennifer Nodes for help with data collection. Address correspondence and reprint requests to Mark E. McCourt, Department of Psychology, North Dakota State University, Fargo, ND 58105-5075. Fax: (701) 231-8426. E-mail: mark_mccourt@ndsu. nodak.edu. 52 0278-2626/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.