Symposium 8

Abstract

Patients suffering from left neglect typically bisect horizontal lines to the right of the true midpoint under visual control. When asked to mark the endpoints of a virtual line of a given length on the basis of its midpoint printed on a sheet of paper, the distance they generate between the left endpoint and the midpoint is usually larger than the distance they generate between the latter and the right midpoint. Similarly, when they are asked to extend leftwards a horizontal segment of which the left endpoint is located in the middle of a sheet of paper so as to double its length, the segment they generate is usually longer than the segment they generate in the converse (right-ward) condition. In both cases, the relative error (i.e. the ratio between the left and right virtual or actual segments drawn from the landmark given as midpoint) has the same direction (i.e. rightward) typically found in these patients with canonical line bisection tasks. It has been argued that this finding cannot be accommodated by current interpretations of unilateral neglect. It has also been suggested that is is indicative of a left-right anisotropy of the medium underlying visual space representation. The problem still remains as to the extent to which the observed behaviour is specifically dependent on the involvement of the visual modality. The results of an experiment aimed at assessing the relative role of visual and haptic modalities suggest that the behaviour in question may be found in both modalities, though perhaps to a different degree and subject to double dissociation.