Context and crossover in unilateral neglect

Patients with left-neglect bisect horizontal lines to the right of true cen ter. Their bisection bias is affected by line length, so that longer lines are bisected further to the right. Patients often crossover and bisect very short lines to the left of true center. We tested the hypothesis that the context in which lines are apprehended accounts for the crossover phenomeno n. We replicated previous findings that a line is bisected further leftward when it is preceded by a longer line and further rightward when it is prec eded by a shorter line. These contextual effects occur with relatively shor t and relatively long target lines. Bisection patterns in two different ser ies of lines, one ranging from 10 to 150 mm, and the other from 110 to 250 mm, were investigated. If crossover bisections were simply due to contextua l effects then left-sided errors would be observed on bisections of the sho rter lines of both series. Our findings did not support this hypothesis. Cr ossover bisections occurred only with objectively short lines, those shorte r than 40 mm. Even though we found significant contextual effects on line b isection biases, these effects per se do not account for the crossover phen omenon. Rather, our data suggest that the absolute length of the line is as sociated specifically with the crossover phenomenon. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.