Attention biases the perceived midpoint of horizontal lines

In patients with right brain damage and left visual neglect, attention tends to be captured by right-sided objects and cannot easily disengage from them. While these phenomena can account for several clinical and experimental patterns of performance such as biased visual search, its role is more controversial for other neglect-related signs, such as the typical rightward shifts in horizontal line bisection. It is thus important to see whether and how attentional orienting can bias line bisection in normal participants using standard clinical bisection stimuli. In 3 experiments, we explored the Attentional Repulsion Effect (ARE, Suzuki & Cavanagh, 1997) on pre-bisected lines. Normal observers saw horizontal lines with a vertical bisection mark near the center, preceded by a cue to the left or right of the line, or by no cue. On each trial, observers indicated whether they saw the bisection mark to the left or at the right of the midpoint. We plotted the proportion of 'seen-at-right' responses as a function of the mark's actual position. For uncued lines, the point of subjective equality was slightly at the left of the true center, consistent with the pseudoneglect phenomenon. Right-sided cues shifted the apparent bisection point to the left (and vice versa), as predicted by the ARE. Similar results occurred with different task instructions (compare the length of the left-sided line segment to the right-sided segment) and in the presence or absence of central fixation marks. These results obtained in normal participants support attentional accounts of biased line bisection in neglect patients.