Contributions of object- and space-based mechanisms to line bisection errors

In two experiments, normal adults divided a horizontal line segment and an equal spatial interval that did not contain a line into eight equal-appearing segments by means of successive bisections. In the first experiment, subjects' average initial bisections erred to the left of objective center for both stimuli. Their subsequent bisections produced similar errors for the line-present stimulus, as the bisection of each progressively smaller line segment was placed to the left of true center. However, this pattern did not occur when bisecting the empty interval. The finding that the presence of a line influences bisection errors implicates an 'object-based' mechanism in the genesis of line bisection errors and suggests that this mechanism varies in its operation with visual field location. In the second experiment, subjects successively bisected longer line and interval stimuli which were presented either centered on the subjects' midlines or displaced to the right or left. Bisections tended to be placed farther to the left for the left stimuli and farther to the right for the right stimuli, with little or no bias for the centrally located stimuli. Repeated measures with the centrally located stimulus demonstrated strong individual differences in bisection biases. Errors were also found to be correlated for the line-present and line-absent stimuli in both experiments, suggesting the additional contribution of a mechanism that is not object-based.