EXPRESS: Beyond bias: a registered examination of the validity of using line bisection to measure non-lateralised attention.

Line bisection is a task widely used to assess lateral asymmetries of attention, in which participants are asked to mark the midpoint of a horizontal line. The Directional Bisection Error (DBE) from the objective midpoint of the line is the traditional measure of performance. However, an alternative method of studying bisection behaviour, the end-point weightings method, has been proposed. This method produces two measures of performance: end-point weightings bias (EWB) and end-point weightings sum (EWS). Whilst EWB measures attentional asymmetry, it has been suggested that EWS quantifies the total (non-lateralised) attention allocated to the task. If EWS provides a valid index of non-lateralised attention, then changes in tonic and phasic arousal should systematically affect EWS. In this manuscript, we formally test this prediction, using time on task to manipulate tonic arousal, and unpredictable auditory tones, presented simultaneously with line stimuli, to manipulate phasic arousal. Our registered analyses revealed that neither of our manipulations for tonic or phasic arousal significantly influenced EWS. Therefore, the null hypotheses cannot be rejected. An exploratory analysis of all trials and conditions revealed a significant reduction in EWS with time spent on task. However, the lack of any significant effect of alerting tone on EWS suggests that EWS may not be a valid measure of generalised attention to the task.