Local (focussed) and global (distributed) visual processing in hemispatial neglect.

In the present study we set out to investigate deficits of focussed and distributed attention (and their interaction) in brain-damaged patients. To this purpose, four left brain damaged (LBD) patients without signs of hemispatial neglect and six right brain damaged (RBD) patients with variable signs of hemispatial neglect were tested by means of an experimental paradigm comprising two embedded tasks performed on the same visual array. The first task (i.e. counting the number of shapes, 1-4, briefly displayed) mainly involved distributed attention rather than focussed attention. The second task was a typical target detection task, which emphasized the detailed analysis of each element in the array, thus mainly tapping focussed attention. Results clearly showed that: (1) LBD patients are slightly impaired at directing focussed attention to the contralesional visual hemifield; (2) in comparison to LBD patients, RBD patients with mild neglect show an exaggerated difficulty in orienting focussed attention toward the contralesional side, while they are similarly unimpaired in the deployment of distributed attention, i.e. in global processing; (3) RBD patients with the most severe neglect suffer from a deficit of both local and global visual processing, i.e. of both focussed and distributed attention. Taken together, these observations indicate that focussed and distributed components of visual attentional processing may be differentially affected in left and right brain-damaged patients with and without neglect.