Visual Search - Effect of Alzheimer's

Effect of Alzheimer's

Dementia of the Alzheimer type (DAT) in patients results in a significant benefit with spatial cueing, but this benefit is only obtained for cues with high spatial precision. In fact, the reduction in the dynamic range of spatial attention is so clear that only the smallest cue used in a study by Parasuraman et al., (2000) facilitated visual search speed in DAT patients.

Abnormal visual attention may underlie certain visuospatial difficulties in patients with Alzheimer's disease (AD). Patients of AD have hypometabolism and neuropathology in parietal cortex, and given the role of parietal function for visual attention, patients with AD may have hemispatial neglect. Hemispatial neglect is a deficit in attention and awareness of one side of space after damage to one hemisphere of the brain. In AD, hemispatial neglect on visual search tasks may relate to difficulty in disengaging attention in visual search.

An experiment conducted by Tales et al., (2000) concerned the ability of patients with AD to perform various types of efficient visual search tasks. They found that people with AD were significantly impaired overall in visual search tasks. Their results showed that search rates on the "pop-out" tasks were similar for both AD and control groups, however, people with AD searched significantly slower compared to the control group on the conjunction task. One interpretation of these results is that the visual system of AD patients has a problem with feature binding, such that it is unable to communicate efficiently the different feature descriptions for the stimulus. Features are typically analyzed in functionally and anatomically separate cortical areas in the brain, and an impairment of "binding" would result in an impaired ability to compare across these features. The binding of features is thought to be mediated by areas such as the temporal and parietal cortex, and these areas are known to be affected by AD-related pathology. In conjunction search, healthy people are thought to employ grouping strategies among the distractors in order to reduce the need for attention shifting, thereby improving search efficiency. "Grouping" is different from "binding". Grouping is an ability to jointly represent similar items distributed across space, whereas binding is an ability to jointly represent the different characteristics of a single item in visual space. It might be possible for AD patients to have a reduction in the efficiency of basic visual processing, such as grouping necessary to form a grouping strategy during visual search tasks. This would therefore result in a greater need for attention shifting in order to detect the target among the distractors. A third possibility for the impairment of people with AD on conjunction searches, is that there may be some damage to general attentional mechanisms in AD, and therefore any attention-related task will be affected, including visual search.

Tales et al., (2000) detected a double dissociation with their experimental results on AD and visual search. Earlier work was carried out on patients with Parkinson's disease (PD) concerning the impairment patients with PD have on visual search tasks. In those studies, evidence was found of impairment in PD patients on the "pop-out" task, but no evidence was found on the impairment of the conjunction task. As discussed, AD patients show the exact opposite of these results: normal performance was seen on the "pop-out" task, but impairment was found on the conjunction task. This double dissociation provides evidence that PD and AD affect the visual pathway in different ways, and that the pop-out task and the conjunction task are differentially processed within that pathway.