Line quadrisection errors in normal subjects

Many studies have investigated line bisection behaviors in normal individuals and patients with hemispatial neglect. However, little is known about what happens when subjects attempt to fractionate line into more than two equal components (e.g., line quadrisection). Thirty right handed normal subjects were asked to place a mark either 25% (left quadrisection) or 75% (right quadrisection) of the distance along on a 240 mm line. On average, they placed the left quadrisection mark significantly to the left (- 4.2+/-6.7 mm) from the true quadrisection point but they were relatively accurate on the right quadrisection task (1.0+/-6.7 mm). However, comparison of actual quadrisection performances with those of expected performance based on subjective midpoint disclosed that both right and left quadrisection marks deviate toward each end of the line. Individual data analysis also support this trend because majority of subjects showed the lateral deviation in either or both quadrisection tasks. Therefore, in the left quadrisection task the pseudoneglect (leftward bias) in bisection and the lateral bias are additive, resulting in a robust left lateral deviation, whereas in the right quadrisection, these two biases nullify each other, resulting in accurate performance.