To halve and to halve not: an analysis of line bisection judgements in normal subjects.

Normal adults were tested in a series of three experiments to examine the influences of spatial location and cueing upon line bisection judgements. Judgements in all three experiments were strongly influenced by cueing with a letter at one or other end of the line. The spatial location of the line (in left, central or right body space) also had a minor effect in Experiments 1 and 2, where evidence was additionally found for a small constant error when lines were presented centrally. It is argued from the results of Experiments 2 and 3, where no explicit bisection response was required, that perceptual/attentional factors, rather than an orienting bias, play the major role in mediating the cueing effect. It is concluded that there is a substantial attentional effect upon judgements of extent, whereby paying less attention decreases perceived relative line length. However the constant error and the effect of spatial location, whilst mainly perceptual in nature, may also be partly determined by premotor orienting biases caused by differential hemispheric activation.