Radial Character-line Bisection

Abstract

With conventional solid radially oriented-line bisections (SLB), normal subjects tend to bisect distal to the true midpoint (distal bias). This bias is thought to reflect a distal attentional bias in the radial plane. Whereas the right hemisphere, which mediates distributed-global attention, has a distal bias, the left hemisphere, which mediates focal attention, has a proximal bias. When performing conventional radial-line bisections, participants might primarily use right hemisphere-mediated global-distributed attention and thus develop a distal bias. The character-line bisection (CLB) task consists of strings of target and nontarget characters (letters or symbols) that are linearly aligned, and subjects are required to bisect a character that is closest to the midline. Thus, when performing the CLB subjects must use focal attention and detect symbols and both these activities are mediated by their left hemisphere. This left hemisphere activation might either reduce or reverse the distal bias observed on the SLB. The purpose of this study is to test this hypothesis. Fifteen normal subjects performed CLB (letter-line and symbol-line) and SLB tasks. Overall, the subjects revealed a distal bias in both tasks, but the attempted bisections in the CLB tasks were more proximal than those in the SLB tasks. These result are consistent with the hypothesis that the CLB task, which requires both focal attention and the detection of symbols, activates the left hemisphere that has a greater proximal bias than the right hemisphere, thus reducing the distal bias observed with the SLB task.