Spatial bias: attentional and intentional influence in normal subjects

Normal subjects often demonstrate a spatial bias on line bisection (LB) and cancellation (CA) tasks. We wanted to learn if horizontal spatial bias found in normal subjects may be dissociable into sensory-attentional (ATT) and motor-intentional (INT) subgroups similar to those described in brain-injured patients with spatial neglect. We studied the influence of ATT and INT factors on the spatial bias observed in normal subjects using a new technique that uncouples the direction of action from the direction of attention. This technique also allowed us to test both LB and CA tasks on the same individuals. Our results show that ATT bias significantly influenced performance on an LB task, whereas performance on a CA task was influenced by biases in both the ATT and INT systems. In addition, the overall bias on these two tasks reflects an interaction between the biases induced by the ATT and INT systems that may be in the same or different directions.