Horizontal line bisections in upper and lower body space ANNA M. BARRETT, 1 J. BRENT CROSSON, 2 GREGORY P. CRUCIAN, 2 and KENNETH M. HEILMAN 2,3 1 Department of Medicine, Division of Neurology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 2 Department of Neurology, College of Medicine, University of Florida, Gainesville, Florida 3 Neurology Service, Veterans Affairs Medical Center, Gainesville, Florida (Received June 3, 1997; Revised June 2, 1999; Accepted June 15, 1999) Abstract Whereas the ventral cortical visual stream is important in object recognition, the dorsal stream is specialized for spatial localization. In humans there are also right and left hemisphere asymmetries in visual processing: the left hemisphere being more important in object recognition and the right in specifying spatial locations. Based on these dorsal–ventral and right–left where–what dichotomies, one would expect that the dorsal right hemisphere systems would be most activated during spatial localization tasks, and this activation may induce a leftward spatial bias in lower space. To determine if visual stimuli in upper and lower body space evoke different hemispheric activation, we had 12 normal participants bisect horizontal lines above and below eye level. Participants erred leftward in lower body space relative to upper body space ( M 5 1.3345 mm and 0.4225 mm, respectively; p 5 .011). In upper body space, bisection errors did not differ from zero, but in lower body space, errors tended to deviate leftward ( M 5 1.3345 mm, differs from null hypotheses at p 5 .0755). Our results are consistent with dorsal stream0right hemisphere activation when performing a spatial localization task in lower versus upper body space. ( JINS, 2000, 6, 455– 459.) Keywords: Attention, Attentional bias, Perception, Body space INTRODUCTION Object recognition and spatial location may be mediated by anatomically segregated intra- and interhemispheric sys- tems. After visual stimuli are processed by the calcarine or primary visual cortex, they undergo further processing in visual association areas. There are multiple visual associa- tion areas that perform different forms of specialized pro- cessing. However, in general, there are two major visual processing streams: a ventral occipital–temporal what stream and a dorsal occipital–parietal where stream (Ungerleider & Mishkin, 1982). In humans, the left hemisphere may be more linked to object recognition and object imagery (Cor- ballis, 1989; Farah, 1991; McCarthy & Warrington, 1990) and the right hemisphere to three-dimensional spatial local- ization (Benton et al., 1978; DeRenzi et al., 1971; Hannay et al., 1976). This has led theorists such as Bryden and Un- derwood (1990) and Liotti and Tucker (1995) to propose that the dorsal and ventral visual pathways are asymmetric in the cerebral hemispheres, with the right hemisphere dom- inant in dorsal visual functions and the left in ventral visual processing. Dorsal and ventral hemisphericity may influence how stimuli appearing above and below eye level are processed. The left hemisphere’s ventral visual systems may be pri- marily directed to the upper visual field and the right hemi- sphere’s dorsal areas to the lower visual field. Visual tasks that present object stimuli in the four visual quadrants have supported this suggestion (Drain & Reuter-Lorenz, 1996; Previc & Blume, 1993). When experimenters examine hemi- spheric specialization by using the spatial location of visual stimuli located in one visual field (right or left, up or down), these stimuli are also located in a specific portion of body- or viewer-centered space. In these studies, the body space where stimuli appear may represent an important factor, even when dissociated from visual field by head and 0or eye move- ment. Cortical processing of stimuli appearing in upper or lower body space may be more strongly associated with one hemisphere, regardless of the viewer’s eye position. This hypothesis predicts that visual stimuli presented in lower body space may activate the right hemisphere’s dorsal stream. Conversely, visual stimuli presented in upper body space Reprint requests to: A.M. Barrett, M.D., Division of Neurology, P.O. Box 850, Penn State University College of Medicine, Hershey, PA 17033. E-mail: abarrett@psghs.edu Journal of the International Neuropsychological Society (2000), 6, 455–459. Copyright © 2000 INS. Published by Cambridge University Press. Printed in the USA. 455