Comparative measures of represented linear extent: implications for clinical assessment

Although line copying provides a simple and direct perceptual estimate of linear extent, line bisection is the most common clinical test traditionally used to quantify visuospatial neglect. This study aimed to establish whether the two tasks tap similar aspects of linear extent and determine which approximates actual stimulus length. One hundred and forty healthy controls were tested. Misrepresentations when copying were more than double those misrepresentations when bisecting, suggesting that the task demands involved in bisection may be less demanding than those involved in copying. Moreover, bisection performance provided the best approximation to the stimulus length used. It is suggested that the (midpoint) computational requirement of bisection facilitates a more accurate representation of linear extent by engaging implicit cues within the task. The fact that line bisection provides for a more accurate measure of represented linear extent attests to its value as a clinical test for patients with visuospatial neglect.