Horizontal line bisections in upper and lower body space

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 = 1.3345 mm and 0.4225 mm, respectively; p = .011). In upper body space, bisection errors did not differ from zero, but in lower body space, errors tended to deviate leftward (M = 1.3345 mm, differs from null hypotheses at p = .0755). Our results are consistent with dorsal stream/right hemisphere activation when performing a spatial localization task in lower versus upper body space. (JINS, 2000, 6, 455–459.)