Alzheimer disease constricts the dynamic range of spatial attention in visual search.

A cued visual search task was used to examine the dynamic range over which spatial attention affects target identification during visual search. Precues varied in validity (valid, invalid, or neutral) and in precision (cue size) of target localization. Participants were "young-old" (65-74 years) and "old-old" (75-85 years) elderly adults and individuals in the mild stage of dementia of the Alzheimer type (DAT). For all participants, search was speeded as the precision with which a precue surrounding the location of a subsequently appearing target increased (precue size decreased). The cue size effect was evident in both feature and conjunction search, but was greatly reduced in both old-old and DAT groups compared to the young-old. However, whereas all non-demented adults showed a progressive modulation of search efficiency over the entire range of cue sizes, the dynamic range of spatial attention was restricted to the most precise cue in the DAT group. The restriction in the dynamic range of spatial attention may represent an underlying component of the impairment in perceptual and memory functioning found in early-stage DAT.