Motor and perceptual factors in pseudoneglect.

An important variant of the traditional line bisection task has involved a mechanical device invented by Bisiach and his colleagues (Bisiach et al. Perceptual and premotor factors of unilateral neglect. Neurology 1990;40:1278-81 [3]). This tool was devised to dissociate motor from perceptual factors in hemi-spatial neglect, by means of a mid-line indicator which moved 'congruently' or 'non-congruently' with the direction of hand movement. In the non-congruent condition, Bisiach was able to demonstrate a reduction, or reversal, of the direction of bisection error in a number of patients with neglect. These errors were interpreted as instances of 'motor' neglect. Bisiach et al. [3] also tested 10 normal subjects, who did not differ on the two conditions of the task. However, the original experiment [3] required the use of the right hand only, and it has since become clear that bisection errors in normal subjects (i.e. pseudoneglect) are more substantial when dextral subjects use their left hands. By using a modified version of the Bisiach Tool we show that there is an effect of the motor versus perceptual condition on this task, but only when subjects use their non-dominant (left) hand.