Compression and lateral shift of mental coordinate systems in a line bisection task

Subjects (N = 18) were required to imagine how far lines of three different lengths which were presented to them previously would extend to the left or to the right of a vertical mark. They were asked to indicate the midpoint of the imagined line length. The frequency of deviations to the left or to the right of the geometrical midpoint of the line which had to be visualized was dependent on the length of the line and on the direction towards which the subjects imagined the line to extend. Whereas a compression of imagined half space compared to geometric half space was observed both for the left and right side, imagined half space on the right side of the visual field was shifted in addition more towards the periphery. It is suggested that the latter result is related to properties of left hemisphere functioning which could explain the occurrence of a typical symptom of unilateral neglect after a right hemisphere lesion.