Orientation bias in unilateral neglect: representational contributions

Left-neglect patients bisect horizontal lines to the right of true center. Longer lines are bisected further to the right than shorter lines. This line-length effect might be explained by an increase in the rightward bias of attention because longer lines extend further ipsilesionally. Alternatively, neglect patients might be limited in their abilities to internally represent horizontal magnitudes. Patients might orient further rightward with longer lines because these lines have longer representations. If the line-length effect occurs on lines of identical objective length but they are represented differently, then central mechanisms must contribute to the orientation bias. We constructed two types of lines that were perceived by normal subjects as having different lengths, but were of identical extents. Three neglect patients bisected lines perceived as longer, further to the right than lines perceived as shorter. These results demonstrate that relative magnitudes of internal representations contribute to the degree of bias in neglect patients.