Line bisection in normal adults: direction of attentional bias for near and far space.

The direction of attentional bias in forty normal adults was assessed using a computer generated line bisection task. A 4 (viewing distance)x4 (line length)x2 (cursor starting position) repeated measures factorial design was employed. As predicted, differences in bisection performance as a function of viewing distance were observed. The findings confirmed that scanning direction (contingent upon the starting position of the cursor), but not line length, significantly modulated this effect. The direction of bias across near and far space was further clarified yielding a progressive shift from a leftward bias in near space to a rightward bias in far space. A significant interaction of distance, line length and starting position revealed differential effects for left and right starting positions as a function of viewing distance and line length. More specifically, a leftward start witnessed deviations shifting progressively from left-to-right as distance and line length increased though no comparable pattern was observed for rightward starts. The results provide important behavioural support for the suggestion that dissociated neural systems may be responsible for attending and acting in near and far space and that other lateralised functions (such as scanning strategies) can influence hemispheric activation. The findings have relevant theoretical implications as well as important implications for the clinical assessment of unilateral neglect using a standard line bisection task, both of which are discussed.