Mapping spatial neglect symptoms in the brain: a preliminary report from a single case.

Background Following injury to the right parietal cortex, patients typically show unilateral spatial neglect, a complex syndrome associated with a reduced capability to orient attentional resources toward the contralateral side of space [1]. Visual extinction is one of the most prominent symptoms of unilateral spatial neglect. In visual extinction, a transient stimulus presented in the right hemifield ‘extinguishes’ from awareness an homologue stimulus simultaneously presented in the left hemifield. This is thought to be a consequence of the residual activity of the left – non damaged – parietal cortex [1-4]. Surprisingly, however, this hypothesis has been largely unexplored to date in real neglect patients. Here we used functional magnetic resonance imaging (fMRI) to identify brain regions contributing to visual extinction in a stroke patient with unilateral spatial neglect and in four healthy controls. Method During fMRI, all participants were presented with a target stimulus (i.e., a black square of 2x2°; 10° apart from the central fixation point), that equiprobably appeared on either the left, right or both hemifields. Participants pressed one of three response buttons to indicate the hemifield(s) of appearance of the target, left, right, or bilateral. Results Behaviourally, we found 100% accuracy in control subjects, irrespective of the target side, left, right or bilateral. By contrast, the neglect patient systematically failed to detect bilateral targets (0% of accuracy). Importantly, “bilateral” targets were perceived by the patient as “right” targets in the great majority of trials (93,75%). Moreover, the patient showed a decreased capability in detecting left (44%) vs. right targets (100%). The fMRI analysis revealed in the patient a greater activation of the left posterior parietal cortex, namely of the left angular gyrus (x, y, z = -44, -68, 30), when comparing detection of bilateral vs. unilateral (left and right) targets. The same comparison revealed no significant activations in the control group. Conclusions Our findings provides empirical evidence that confirm the crucial role played by the left parietal cortex during visual extinction in spatial neglect, providing important insights for the current models of unilateral spatial neglect.