In this study, normal control subjects bisected lines in near and far space. By dissociating perceptual and action space, we sought to fractionate the effects of perceptual-attentional (PA) and motor-intentional (MI) bias.
Previous investigators found that normal subjects made greater leftward errors when they bisected lines in near versus far space (Dellatolas et al., 1996
; Varnava et al., 2002
). Our results are consistent with these prior reports. Additionally, the near leftward errors made by our subjects were primarily accounted for by feedback-dependent, PA “where” bias. Varnava et al. found both slight leftward and rightward bias for line bisections performed in their furthest conditions (120 cm from subjects) for their longest line lengths (7.59 degrees). The direction of the bias for these conditions depended on which side of the screen the cursor started. However, PA and MI bias were not dissociated in that study, so it is unclear whether their results were related to perceptual-attentional or motor-intentional processing. In the current study, the distance for far space was further away than any of the conditions used by Varnava et al., (175 cm) and the line length used was longer than the longest lengths used in that study. Additional experiments will be needed to determine whether our failure to replicate rightward bias in far space may be partly related to the line length or distance we used.
We investigated the effect of an asymmetric visual distractor, an experimenter standing to one side of the line to be bisected, which we believe may constitute a bottom-up type influence on perceptual-attention. As hypothesized, there was no effect of this distractor on MI “aiming” bias, but PA “where” bias was altered in the direction of the distractor, with leftward error increasing in the presence of a left distractor, and decreasing in the presence of a right distractor. Previous studies reported that attentional cuing shifted line bisection error in the direction of the cue (Milner, Brechmann, & Pagliarini, 1992
; Nichelli, Rinaldi, & Cubelli, 1989
; Reuter-Lorenz, Kinsbourne, & Moscovitch, 1990
; McCourt, Garlinghouse & Reuter-Lorenz, 2005
), or when cues were presented on both sides, toward the cued which was engaged first (Fischer, 1994
). However, in this study, a static attentional distractor also influenced line bisection performance.
In prior cuing studies, the cues may have altered the perceived length of the line, augmenting it (Milner et al., 1992
; Nichelli et al., 1989
) or shortening it (Chieffi & Ricci, 2002
) at one end. In the present study, distractors were spatially removed and distinct from the line. Thus, our stimuli not only activated bottom-up attentional systems more strongly than cues used in prior studies, but may have been less likely to act upon central representations of the line itself than those used in previous research. Because they were experimenters, the saliency of the current of distractors may have been an important aspect of the influenced spatial bias. For example, Tamietto et al. (2005)
found that emotional face cues reduced line bisection errors relatively more than neutral face cues, suggesting that the increased saliency of the affective cues biased attention and not just perceived line length. Similarly, an actual human may have a similar direct effect on attention and spatial bias. Finally, detection of distractors in our study also was not confounded by the over-learned scanning eye movements involved in reading, which has affected bias in other studies of cuing in the line bisection task (Fischer, 1994
We hypothesized that a top-down motor instruction to begin trials pointing to one side of the workscreen, would affect MI “aiming” bias. Indeed, leftward MI bias increased for trials starting on the left as compared with trials starting on the right. This is consistent with prior studies investigating the effects of directional scanning on line bisection, in which responses shifted toward the scanning startside (Bradshaw, Nathan, Nettleton, Wilson, & Pierson, 1987
; Chokron, Bartolomeo, Perenin, Helft, & Imbert, 1998
). An important consideration for the current study, however, is that the direction of scanning eye movements was dissociated from the direction of laser pointer movement in half of the trials. In mirror-reversed trials, when the subjects started on the left side of the screen, the pointer appeared on the right side of the video monitor. Thus, scanning eye movements may have been reversed in direction. Because MI “aiming” bias was shifted in the direction of startside, independent of whether eye movements were cued in the same or the opposite direction, this supports MI bias as being tied to early, ballistic motor-preparatory processing, to limb as opposed to eye movement, or both.
The current findings support and validate the PA/MI “where” versus “aiming” construct distinction by demonstrating that they indeed may be separable bias components. In this experiment, the effect of the visual distractor was primarily PA, while the effect of startside was primarily MI. Thus, the bias components did not interact with each other, and were independently affected by the two manipulations of distractor and startside. This double dissociation provides strong support to PA and MI bias as indicative of unique, discrete processes.
The current study is limited to the extent that the line bisection task is generalizable to other tasks and behaviors. It will be important to determine whether the same spatial bias dissociations are attainable in other tests of spatial processing, including other common bedside tests such as cancellation tasks. The particular features of the current paradigm also warrant further investigation. For example, the type of object used for a distractor may have an important effect on the magnitude of bias exhibited. In the current study, a second experimenter stood as a distractor, but it is unclear whether inanimate objects with differing degrees of relevance to the participant may have differential effects on spatial bias.
It is important to understand whether PA and MI spatial bias may contribute differently to acquired spatial neglect, as this implies that further analysis of subgroups may be important in treatments with purported small group effect sizes (Marshall, Halligan, & Robertson, 1993
; cf. Barrett et al., 2006
), and that “recovered” neglect may differ categorically depending upon whether both PA and MI bias decrease. Distinct neural systems may mediate visuomotor processing in near versus far space (Weiss et al, 2000
), which may be selectively impaired by brain injury. Dynamic or multitasking conditions, which are frequently encountered in daily life and understudied in neglect, may differentially affect one type of bias. This may affect patients acutely or in chronic stages, and may even affect patients with other attentional disorders. It is known that distraction adversely affects attentionally demanding activities such as driving in young healthy subjects (Chen, Baker, Braver & Li, 2000
). Further studies of young, aged, and brain-injured subjects, including both experimental and functional tasks, are needed in order to clarify whether the current constructs provide a meaningful way of analyzing life-relevant spatial errors.