When the whole is more than the sum of the parts: evidence from visuospatial neglect

One possible pathological mechanism underlying the rightward bisection error of right-brain-damaged patients with left spatial neglect is a leftward relaxation of the spatial representational medium. This view was originally based on the finding that patients with left neglect, required to extend horizontal segments, in order to double their original length, may exhibit a relative left overextension of the drawn lines (Bisiach et al., 1994). We investigated this putative distortion of representational space using a 16 cm 'line segmentation' task (Experiment I). Were the representation of space relaxed contralesionally, a progressive increase from right to left of the size of the drawn segments would be expected. Right-brain-damaged patients with left unilateral neglect (N=12) performed the segmentation task with no left versus right differences, as right-brain-damaged patients without neglect (N=8), and neurologically unimpaired control subjects (N=10), did. Experiments 2 and 3 explored the effects of sample length (1, 2, 4, and 8cm), by which the 16cm lines had to be segmented. Neglect patients produced longer left-sided segments only for the 8 cm sample (i.e. half of the length of the segment). This set of experiments suggests an impairment in the segmentation task only with the larger (8 cm) sample, when a more global level of processing may be involved. Experiment 4 assessed this hypothesis by a 'part/whole' bisection task, using 8 cm lines, presented either embedded in a longer 16 cm line or in isolation. Neglect patients made a larger rightward bisection error when the segment was not embedded. The suggestion is made that the lateral distortion of the representation of space in neglect patients (i.e., a leftward relaxation of the spatial medium) concerns tasks where a more 'global' representation of the visual stimulus has to be set up. The different demands of the segmentation and bisection tasks are discussed.