The vertical-horizontal illusion in hemi-spatial neglect.

The vertical-horizontal illusion is a robust phenomenon of length mis-estimation between two orthogonal lines. This illusion involves an anisotropy component that makes the vertical line appear longer than the horizontal one and a bisection component that makes the bisected line shorter than the bisecting one. Six patients presenting a moderate left hemi-neglect (N-patients) were compared to four right brain damaged patients without neglect (RH-patients) and with control participants in the perception of various spatial configurations of the vertical-horizontal illusion. Relative to controls, we found that both components of the illusion increased in patients: the anisotropy component rose from 5 to 11% and 10% (for N- and RH-patients, respectively) and the bisection component from 17 to 22% and 20% (for N- and RH-patients, respectively). In addition, we found that an horizontal-'T' figure oriented to the left produced much less bias than the same figure oriented to the right. These results are discussed in light of explanations based on attentional disengagement from an image junction and strength of the representation of objects extending over the neglected side.