Centripetal versus centrifugal bias in visual line bisection: Focusing attention on two hypotheses

A variety of stimulus factors have been shown to influence the degree of le ftward displacement of perceived line midpoint (i.e., pseudoneglect), which typifies the performance of normal subjects in line bisection tasks [M.E. McCourt & G. Jewell: Neuropsychologia 37, 843-855 (1999); G. Jewell & M.E. McCourt: Neuropsychologia 38, 93-110 (2000)]. One such factor is the positi on of lines within the visual field, where two conflicting patterns of bise ction error have been reported. Some authors report a centrifugal pattern o f error, where perceived line midpoint shifts away from the vertical midlin e, regardless of line position, i.e., relatively leftward for leftward disp laced lines and vice versa. Others have reported a centripetal pattern of b isection error, where perceived line midpoint is always displaced centrally , toward the vertical midline, regardless of line position. There is no sat isfactory explanation for these discrepant findings. An experiment using a tachistoscopic forced-choice line bisection protocol is described which dis closes that neurologically normal right-handed subjects (N=82) typically di splay a centrifugal pattern of bisection error when lines are azimuthally d isplaced over a relatively small range, whereas a centripetal pattern is ob served when lines are displaced over a wider range. Results from ancillary control experiments, in which eye position was measured during testing, con firm that systematic differences in gaze direction do not occur as a functi on of line position, and thus cannot account for the different patterns of bisection error. We conclude that stimulus context significantly modulates the strategy with which observers deploy spatial attention. When line posit ion is constant, or varies over a narrow range, observers hold attention st eady and widen its aperture to accommodate the relevant range of spatial lo cation. Centrifugal bisection error is thus produced by the asymmetric cuei ng effect of laterally displaced lines, according to the activation-orienta tion theory [M. Kinsbourne: Acta Psychologica 33, 193-201 (1970)]. When the range of line position exceeds the aperture of focal attention, we hypothe size that observers adopt a strategy in which attention is dynamically scan ned in the direction of azimuthally displaced lines. The effects of attenti onal scanning on line bisection performance are quite robust. The centripet al scanning proposed to occur for widely displaced lines is consistent with the centripetal pattern of bisection error in this condition.