Visual neglect in horizontal and radial space: when left goes right, proximal goes distal.

Hemi-spatial neglect can manifest in both the horizontal and radial spatial plane. However, debate exists over how closely the two forms of neglect relate. Here we compared the ability of a neglect patient to bisect stimuli in horizontal versus radial orientation. When oriented horizontally, single lines were mis-bisected to the right, yet when surrounded by visual distracters, the lines were mis-bisected to the left. A leftward bias also emerged when horizontally aligned strings of symbols were bisected. Unexpectedly, an analogous pattern of bias appeared when the stimuli were bisected in radial orientation; stimuli that induced a leftward bias now induced a proximal bias, while stimuli that induced a rightward bias induced a distal bias. Spontaneous reversals in radial bias have not been previously reported, and given that they were coupled with the horizontal reversals, suggest that the spatial boundaries of horizontal and radial neglect are strongly constrained by common stimulus configurations.