No abstract

This study investigated the relative performance of Alzheimer's disease (AD) patients and normal controls on directed and divided attention reaction time (RT) tasks that involved the use of global-local stimuli (e.g., a large '1' made from small '2s'). Relative to normals, AD patients displayed disproportionately greater impairment on the divided attention task compared to the directed attention task. On the divided attention task, when the target remained at the same global-local level across consecutive trials, the AD patients displayed a greater facilitation effect than did the controls when responding to the second stimulus. However, when the target changed levels across consecutive trials (i.e., from the global to the local, or vice versa) the AD patients' RTs to the second stimulus were disproportionately slower than were the controls' RTs. These results demonstrated that AD patients are impaired in disengaging and shifting attention across levels of perceptual organization within the same stimulus.

Shifts in attention were examined in patients with Huntington's disease (HD) using a divided attention paradigm that involved the presentation of global-local stimuli. The HD patients' pattern of performance was compared to the previously reported results of groups of patients with Alzheimer's disease (AD; Filoteo et al., 1992) or Parkinson's disease (PD; Filoteo et al., 1994). Across consecutive trials of the divided attention task, a visual target could appear at either the same global-local level or at a different level. When the target changed levels across consecutive trials, the AD patients demonstrated an impairment in disengaging attention from the level at which the last target appeared, whereas the PD patients were impaired in maintaining their attention at the previously attended level. In contrast to these patterns of performances, the HD patients did not demonstrate a significant impairment in shifting attention between hierarchical levels. Both the AD and the PD patients' abnormal shifting ability was significantly related to the number of errors they made in identifying target stimuli; however, the pattern of the relationship was qualitatively distinct. These results suggest that different attentional mechanisms may underlie AD and PD patients' visual-perceptual deficits. The HD patients' shifting ability was not related to the number of errors they made in identifying target stimuli, suggesting that a different mechanism may account for the visual-perceptual impairments exhibited by these patients.