Biases in Attentional Orientation and Magnitude Estimation Explain Crossover: Neglect is a Disorder of Both

Abstract

Crossover refers to a pattern of performance on the line bisection test in which short lines are bisected on the side opposite the true center of long lines. Although most patients with spatial neglect demonstrate crossover, contemporary theories of neglect cannot explain it. In contrast, we show that blending the psychophysical construct of magnitude estimation with neglect theory not only explains crossover, but also addresses a quantitative feature of neglect that is independent of spatial deficits. We report a prospective validation study of the orientation/estimation hypothesis of crossover. Forty subjects (17 patients with and without neglect following unilateral brain injury and 23 normal controls) completed four experiments that examined crossover using line bisection, line bisection with cueing, and reproducing line lengths from both memory and a standard. Replicating earlier findings, all except one subject group exhibited crossover on the standard line bisection test, all groups showed a spontaneous preference to orient attention to one end of the lines, and all groups overestimated the length of short lines and underestimated long lines. Biases in attentional orientation and magnitude estimation are exaggerated in patients with neglect. The truly novel finding of this study occurred when, after removing the line from the bisection task, the direction of crossover was completely reversed in all subject groups depending on where attention was oriented. These findings are consistent with our hypothesis of crossover: (1) crossover is a normal component of performance on line bisection; (2) crossover results from the interplay of biases in attentional orientation and magnitude estimation; and (3) attentional orientation predicts the direction of crossover, whereas a disorder of magnitude estimation, not previously emphasized in neglect, accounts for the quantitative changes in length estimation that make crossover more obvious in neglect subjects. Paradoxically, we observed that the traditional line bisection test is suboptimal for exploring crossover because lines elicit spontaneous orientation responses from subjects that confound experimental manipulations of attention. We conclude that attentional orientation and magnitude estimation are necessary and sufficient to explain crossover and that bias in magnitude estimation is a core component of neglect. HHS Public Access

An investigation of the mechanisms underlying the effects of viewing distance and stimulus length on attentional asymmetries during line bisection

“…Specifically, the bias of attention toward the upper hemispace caused by activation of the ventral system (Drain & ReuterLorenz, 1996;Previc, 1990) should be reversed to a bias toward the lower hemispace by the overestimation of length for very short lines. Figure 3 clearly shows no evidence of a downward bias for the very short lines-contradicting the theory proposed by Mennemeier et al (2005).…”

“…For example, Mennemeier et al (2005) suggested that crossover occurs because of the interplay between two separate processes: (1) magnitude estimation and (2) orientation bias. Magnitude estimation refers to a phenomenon whereby the length of short lines is systematically overestimated, whereas the length of long lines is underestimated (Tegner & Levander, 1991).…”

“…Whereas Kinsbourne (1970) hypothesized a dominant role for the left hemisphere and a rightward attentional bias in the general population, it should be noted that most of the literature reviewed in the introduction of this article suggests the opposite-right hemisphere dominance for spatial attention and a leftward attentional bias. Notwithstanding this point, Mennemeier et al (2005) suggested that once attention is directed to the right, overestimation of the length of short lines will cause the bisection mark to fall short of the true center (rightward bias). Conversely, for long lines, underestimation of length will cause the bisection mark to be placed past the true center (leftward bias).…”

“…The upshot of the theory proposed by Mennemeier et al (2005) is that short lines have the effect of reversing the preexisting attentional bias, whereas long lines exacerbate the existing bias. If this were the case, however, one would expect a crossover effect for the vertical dimension.…”

“…One such model was developed by Rueckert et al (2002). Although Rueckert's model also refers to attentional biases and an under-/overestimation of line length, the model was not discussed by Mennemeier et al (2005)-despite the fact that the results of that study were reviewed. The model proposed by Rueckert et al refers to a leftward attentional bias generated by spatial attention mechanisms located in the right hemisphere.…”