Visuospatial attention in line bisection: stimulus modulation of pseudoneglect

Neglect and pseudoneglect are asymmetries of spatial attention which are of ten assumed to possess a fundamental theoretical and neurological relations hip to each other, although this assumption has never been directly tested and there is as yet no unifying quantitative theory. A total of 217 subject s participated in five experiments demonstrating that both the magnitude an d direction of bisection errors in normal subjects (pseudoneglect) are modu lated by stimulus factors that similarly influence the magnitude and direct ion of neglect. Stimulus positional uncertainty did not abolish pseudonegle ct, 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 byproduc t of covert directional scanning of the line stimulus in iconic or short-te rm Visual memory. Finally, bisection errors are influenced by the direction of contrast gradients imposed on line stimuli, such that perceived line mi dpoint is drawn toward the lower-contrast line end. The magnitude and direc tion of pseudoneglect are modulated by stimulus factors that also influence the magnitude and direction of neglect. Both phenomena are succinctly desc ribed as biases in attention (i.e., neglect is a right-bias, whereas pseudo neglect is a left-bias). The two phenomena are modulated by stimulus factor s 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 l ines. Azimuthal line position: an increasing bias accompanies increasing le ftward placement for both phenomena. Line aspect ratio: there is a decreasi ng bias with increasing line height for both phenomena. Line elevation: the re is a decreasing bias with increasing elevation for neglect, and an incre asing bias with increasing elevation for pseudoneglect. The only case in wh ich a factor's influence on the two phenomena is discrepant is for elevatio n, and this difference is explicable. Taken together these congruencies str ongly 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. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.