Vertical line quadrisection: "what" it represents and who gets the upper hand.

Background: Normal adults demonstrate a slight upward bias (vertical pseudoneglect) when attempting vertical line bisection. The mechanism for this bias is unknown. Activation of the allocentric (object-centered) ventral visual system during attempted bisection may induce this bias. This object-centered ventral stream may also mediate focal attention. As compared to bisection, when normal participants perform a vertical line quadrisection task that requires more focal attention, they may have a greater upward bias.

Methods: Sixteen participants bisected and quadrisected vertical lines.

Results: The mean upward bias (deviation error) for the quadrisection tasks was significantly higher than the mean error for the line bisections.

Conclusions: These results are consistent with the hypothesis that activation of the ventral stream by a task that requires focal allocentric attention can induce an upward vertical bias that is greater than the upward bias observed with allocentric line bisection, a task that requires more global attention.