The influence of center of mass effect on the distribution of spatial attention in the vertical and horizontal dimensions.

Normal subjects attend toward the middle of grouped items (center of mass effect). In order to learn if mass effect could influence performance on line bisection tasks and if the spatial orientation of the line (vertical vs. horizontal) could influence center of mass effect, we administered bisection tasks to 16 normal subjects using either lines composed of two unequal segments (one thick and one thin) or unsegmented lines. When the longer segment was to the right, left, up, or down, subjects erred by deviating their bisection toward the longer segment (center of mass effect). Our results demonstrate that the center of mass effect can be seen with bisection tasks and is greater in the vertical than in the horizontal dimension.