Automatic and voluntary orienting of attention in patients with visual neglect: horizontal and vertical dimensions.

The present study was aimed at testing internally and externally-controlled mechanisms of covert orienting in patients with visuo-spatial neglect. Internally-controlled orienting was tested by presenting central informative cues. Externally-controlled orienting was tested by presenting peripheral non-informative cues. We also tested for the presence of vertical neglect in patients with horizontal neglect, and tried to assess whether altitudinal neglect is an attentional deficit. Finally we examined whether altitudinal neglect manifests itself only in the visual field contralateral to the lesion or, as has been shown for horizontal neglect, whether it is also present in the ipsilesional visual field. The results showed that patients with neglect have a deficit of externally-controlled covert orienting in the visual field opposite to that of the lesion. Further, the impairment appeared to be more pronounced in the lower than in the upper visual field and to be mainly evident in the visual field contralateral to the lesion. The deficit could, however, be partially compensated for by the use of internally-controlled covert orienting. These findings seems to support the dual-mechanisms hypothesis which maintains that automatic and voluntary orienting are subserved by separate mechanisms possibly located in different parts of the brain.