Visuospatial attention in line bisection: stimulus modulation of pseudoneglect.

Neglect and pseudoneglect are asymmetries of spatial attention which are often assumed to possess a fundamental theoretical and neurological relationship to each other, although this assumption has never been directly tested and there is as yet no unifying quantitative theory. A total of 217 subjects participated in five experiments demonstrating that both the magnitude and direction of bisection errors in normal subjects (pseudoneglect) are modulated by stimulus factors that similarly influence the magnitude and direction of neglect. Stimulus positional uncertainty did not abolish pseudoneglect, indicating that bisection judgements are made within an object-centered frame of reference. Backward masking line stimuli had no influence on the magnitude of pseudoneglect, signifying that pseudoneglect is not a byproduct of covert directional scanning of the line stimulus in iconic or short-term visual memory. Finally, bisection errors are influenced by the direction of contrast gradients imposed on line stimuli, such that perceived line midpoint is drawn toward the lower-contrast line end. The magnitude and direction of pseudoneglect are modulated by stimulus factors that also influence the magnitude and direction of neglect. Both phenomena are succinctly described as biases in attention (i.e., neglect is a right-bias, whereas pseudoneglect is a left-bias). The two phenomena are modulated by stimulus factors as follows. Line length: there is an increased bias with increasing line length for both phenomena, and a cross-over to an reversed bias for short lines. Azimuthal line position: an increasing bias accompanies increasing leftward placement for both phenomena. Line aspect ratio: there is a decreasing bias with increasing line height for both phenomena. Line elevation: there is a decreasing bias with increasing elevation for neglect, and an increasing bias with increasing elevation for pseudoneglect. The only case in which a factor's influence on the two phenomena is discrepant is for elevation, and this difference is explicable. Taken together these congruencies strongly support the notion that neglect and pseudoneglect are phenomena that are twin manifestations of parameter changes in a unitary set of underlying hemispheric attentional asymmetries.