Attentional dynamics and visual perception: mechanisms of spatial disorientation in Alzheimer's disease.

Visuospatial disorientation forces Alzheimer's disease patients to abandon independent activities. We found previously that limitations of ambulatory and vehicular navigation are linked to impaired visual motion processing in Alzheimer's disease. We now hypothesize that these perceptual impairments reflect temporal constraints on visual attention. We evaluated attentional, perceptual and neuropsychological capacities in 14 Alzheimer's disease patients and 12 age-matched older normal controls. The temporal dynamics of visual attention were measured using rapid serial visual presentation (RSVP) to assess the attentional blink. Visual processing for spatial orientation was assessed using perceptual thresholds for optic flow, the visual motion seen during observer self-movement. Alzheimer's disease patients show an exaggerated attentional blink during RSVP, identifying the first of two targets but missing the second target depending on the number of intervening distractors. They also show a unique form of attentional masking in which they miss the first target but identify the second, again depending on the number of intervening distractors. Both types of RSVP errors are correlated with selectively elevated optic flow thresholds in Alzheimer's disease patients. This suggests that temporal constraints on visual perception might impair optic flow analysis and contribute to spatial disorientation in Alzheimer's disease. These findings are consistent with two-stage models of visual perception, suggesting that the working memory mechanisms in the second stage provide feedback control of input to category-specific perceptual processors in the first stage.