Impaired shifting of visuospatial attention in Alzheimer's disease as shown by the covert orienting paradigm: Implications for visual construction disability

Objective: To investigate impaired shifting of visuospatial attention in Alzheimer's disease (AD) compared with age-matched controls. Method: An attention shifting was examined in 20 AD patients and 10 age-matched normal subjects by choice reaction time (CRT) and covert orienting paradigm. Visuospatial functions tests were also performed. For covert orienting, a peripheral spatial cue method was used, with stimulus-onset (SOA) between the cue and the target time varying from 250 to 2100 ms. Results: The CRT showed no difference between the AD and normal groups. However, the RTs costs plus benefits were greater in the AD than normal group for two SOA conditions independent of dementia severity. Individual profiles in the time course of cue validity revealed two AD subgroups, i.e., a normal pattern for the cue validity of time course, and an abnormal, 'extinction-like' pattern. The latter had a particular difficulty in performing visual construction and spatial attention. Conclusions: Focusing attention was relatively intact in AD. However, shifting of visuospatial attention was impaired in AD compared with normal controls. There was a subgroup whose deficits were not only in 'disengagement,' but their voluntary shifting of attention was affected. These subgroups may show clinically severe visuospatial symptoms in more advanced stage.