Cognitive estimation in mild Alzheimer's disease.

In everyday life, we often estimate rather than know. We investigated in an experimental approach modality-specific cognitive estimation in patients with mild Alzheimer's disease (AD). Estimation of weight, size, number, and time prior and subsequent to observation of a moving object was assessed in healthy controls (HC; n = 49; 62.5 +/- 7.8 years (mean +/- standard deviation); MMSE 29.2 +/- 1.1) and patients with AD (NINCDS-ADRDA, DSM IV; n = 42; 75.0 +/- 9.5 years; p < 0.001 to HC; MMSE 22.8 +/- 2.9; p < 0.001 to HC). In HC none of the estimation tasks correlated with age or general intellectual ability. AD patients were impaired for estimation of time and weight while estimation of number, size, and distance was not impaired. Estimation of time that a marble will need to roll down a marble track was associated with lower scores for verbal fluency and higher scores for clock drawing in the AD group and estimation of time subsequent to observation was associated with higher scores in clock drawing. Time estimation for moving objects as well as the ability to correct oneself on observation is impaired in patients with very mild AD. This argues for caution in tasks such as car driving already in this stage. Errors in estimation of time observed indicate temporo-parietal impairment while errors on estimation prior to observation of the moving object indicate additional frontal lobe impairment.