Assessing the weights of visual neglect: a new approach to dissociate defective symptoms from productive phenomena in length estimation

Right hemisphere damage often provokes signs of visual neglect, characterized by a prominent left-right imbalance in information processing. Neglect patients are biased towards right-sided objects and ignore left-sided events. Left-right imbalance may not only result from left neglect but also from right attraction, which have been considered as, respectively, defective and productive phenomena in neglect patients. However, the relative contributions of these two mechanisms to the final left-right imbalance remain uncertain. Using a novel experimental paradigm, we were able to separately test the contribution of left neglect and right attraction to neglect behavior. We used horizontal and vertical lines implemented in L shapes in a line extension task. The use of L shapes oriented either to the left or to the right made it possible to measure the left bias by comparing the length of a left-sided horizontal line to that of a centered vertical line, and to measure the right bias by comparing a centered vertical to a rightwards horizontal line. Our results showed that, in this experimental set, the left-right discrepancy is supported more by left neglect than by right attraction, with important implications about the role of left-right competition on the deployment of left neglect and right attraction.