Mental representation of a line when patients with left unilateral spatial neglect bisect it: A study with an endpoint reproduction task

The line bisection bias as a deficit of proportional reasoning - evidence from number line estimation in neglect

This study aimed to investigate whether neurological patients presenting with a bias in line bisection show specific problems in bisecting a line into two equal parts or their line bisection bias rather reflects a special case of a deficit in proportional reasoning more generally. In the latter case, the bias should also be observed for segmentations into thirds or quarters. To address this question, six neglect patients with a line bisection bias were administered additional tasks involving horizontal lines (e.g., segmentation into thirds and quarters, number line estimation, etc.). Their performance was compared to five neglect patients without a line bisection bias, 10 patients with right hemispheric lesions without neglect, and 32 healthy controls. Most interestingly, results indicated that neglect patients with a line bisection bias also overestimated segments on the left of the line (e.g., one third, one quarter) when dissecting lines into parts smaller than halves. In contrast, such segmentation biases were more nuanced when the required line segmentation was framed as a number line estimation task with either fractions or whole numbers. Taken together, this suggests a generalization of line bisection bias towards a segmentation or proportional processing bias, which is congruent with attentional weighting accounts of line bisection/neglect. As such, patients with a line bisection bias do not seem to have specific problems bisecting a line, but seem to suffer from a more general deficit processing proportions.

Full-field input generated from right visual field information for healthy participants reproduces performance simulating left unilateral spatial neglect in line bisection

Patients with left unilateral spatial neglect (USN) typically place the subjective midpoint to the right of the objective centre when bisecting a horizontal line. This pathological phenomenon may be explained as a result of greater dependence on the right endpoint in the external reference frame (Koyama et al., Brain Cogn, 35, 1997, 271; McIntosh et al., Cogn Brain Res, 25, 2005, 833). Ishiai et al. (Brain, 112, 1989, 1485) reported that once patients with USN fixated on a certain point on the right part of the presented line, they persisted with this point and marked the subjective midpoint there without leftward searches. Ishiai et al.'s interpretation was that the patients saw a totalised line representation that extended equidistantly to the right and left sides, based on the information of the attended rightward extent from the subjective midpoint. Accordingly, we used virtual reality goggles (VRG) and devised a mirror-image viewing (MV) condition that showed a full-field view based on the right visual field information to test whether healthy participants would thereby show neglect-like bisection performance. The participants were 30 healthy adults (22-37 years old; 15 women and 15 men). In this condition, 96.7% (29/30) of participants were judged to exhibit USN-like performance of line bisection, indicating the effectiveness of MV condition to simulate USN. The novelty of the present study lies in the use of a task-specific intervention of neglect-like visuospatial processing during line bisection without attempting to modify the direction of spatial attention. This approach may contribute to the understanding of the pathological visuospatial processing of USN.

Endpoints and viewpoints on spatial neglect

In this issue of the Journal of Neuropsychology, Abe and Ishiai (2022) report an experiment designed to probe the subjective experience of line bisection in neglect. A re-analysis of their data can also offer insights into how best to characterise neglect performance for this and other tasks. We show that sensitive measures of neglect can be obtained by quantifying the difference in the influence (or 'weighting') that each endpoint has on the response. The right endpoint is dramatically more influential than the left in people with neglect performing line bisection and endpoint reproduction tasks. This supports the view that neglect may limit the ability to simultaneously represent two locations, so that the response is determined primarily with respect to the right endpoint. We also discuss Abe and Ishiai's conclusion that bisection responses in neglect are accompanied by the subjective experience of a complete line extending equally to either side of the chosen midpoint.