Performance consistency of normal observers in forced-choice tachistoscopic visual line bisection

Abstract

Pseudoneglect (PN) refers to the leftward error exhibited by normal observers on line bisection tasks (Bowers and Heilman, Neuropsychologia, 18, (1980) 491–8). Although a thorough review of the literature has shown PN to be relatively robust (Jewell and McCourt, Neuropsychologia, 38, (2000) 93–110), controversy remains concerning the reliability of the phenomenon, with some studies reporting a relatively high incidence of normal subjects with rightward bisection errors. The present experiment assesses the consistency of bisection performance in normal young observers. Right-handed subjects ( N=22) participated in a tachistoscopic forced-choice line bisection task. Each subject participated in 7–16 experimental sessions separated by at least 24 h (total bisection measurements=317). Individual bisection performance could thus be evaluated with respect to within-subject variability measures. An eyetracker recorded gaze position during the task in one session. A highly significant mean group bisection error of −0.26° ( P<0.001) was obtained (left negative), and individual subject means ranged from −0.55° to +0.03°. Of the 317 total bisection measurements, 9% (28) deviated rightward. Significant ( P<0.05) mean leftward errors occurred in 91% (20/22) of subjects. Mean bisection error in two subjects was not significantly different from zero. No subject possessed a significant rightward error. Mean gaze deviation from screen (and line) center ranged from ±0.9°, and was positively correlated ( P<0.05) with bisection error. It is concluded that forced-choice tachistoscopic line bisection measures are highly reliable; a mean correlation of +0.87 exists between mean error based on 15 trials and means estimated from a random sample of only two trials. The incidence of true rightward bisection error in the population of normal right-handed subjects is thus estimated to be less than 5%.