Context and crossover in unilateral neglect.

Patients with left-neglect bisect horizontal lines to the right of true center. 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 phenomenon. 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 preceded by a shorter line. These contextual effects occur with relatively short and relatively long target lines. Bisection patterns in two different series 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 contextual effects then left-sided errors would be observed on bisections of the shorter lines of both series. Our findings did not support this hypothesis. Crossover bisections occurred only with objectively short lines, those shorter than 40 mm. Even though we found significant contextual effects on line bisection biases, these effects per se do not account for the crossover phenomenon. Rather, our data suggest that the absolute length of the line is associated specifically with the crossover phenomenon.