Line bisection by eye and by hand reveal opposite biases

Abstract

The vision-for-action literature favours the idea that the motor output of an action-whether manual or oculomotor-leads to similar results regarding object handling. Findings on line bisection performance challenge this idea: healthy individuals bisect lines manually to the left of centre and to the right of centre when using eye fixation. In case that these opposite biases for manual and oculomotor action reflect more universal compensatory mechanisms that cancel each other out to enhance overall accuracy, one would like to observe comparable opposite biases for other material. In the present study, we report on three independent experiments in which we tested line bisection (by hand, by eye fixation) not only for solid lines, but also for letter lines; the latter, when bisected manually, is known to result in a rightward bias. Accordingly, we expected a leftward bias for letter lines when bisected via eye fixation. Analysis of bisection biases provided evidence for this idea: manual bisection was more rightward for letter as compared to solid lines, while bisection by eye fixation was more leftward for letter as compared to solid lines. Support for the eye fixation observation was particularly obvious in two of the three studies, for which comparability between eye and hand action was increasingly adjusted (paper-pencil versus touch screen for manual action). These findings question the assumption that ocular motor and manual output are always inter-changeable, but rather suggest that at least for some situations ocular motor and manual output biases are orthogonal to each other, possibly balancing each other out.

Ocular drift along the mental number line

“…On the other hand, such small effects are in line with the size of the effects often observed in line bisection tasks, both with and without number processing (e.g. Leonards, Stone, & Mohr, 2013 ; Nuthmann & Matthias, 2014 ).…”

Negative correlation between leftward bias in line bisection and schizotypal features in healthy subjects

“…Another important aspect to be considered is that spatial pseudoneglect is not a discrete measure, as it depends on several variables, such as the segment length and spatial dislocation of the stimulus (Balconi et al, 2012 ), the hand (Leonards et al, 2013 ), and the direction of performance of the endpoint task, i.e., left-to-right or right-to-left (Urbanski and Bartolomeo, 2008 ). Following this line of research, some studies have investigated the influence of different psychiatric disorders in LB performance.…”