Stimulus contrast and word reading speed in Alzheimer's disease.

The oral word reading speed of patients with Alzheimer's disease (AD) and healthy young and older control participants was evaluated across a broad range of stimulus contrast levels in two experiments. The impact of stimulus repetition on reading speed also was examined. It was found that the older adult participants, and particularly the AD patients, were more sensitive to contrast reductions. Each subject group was able to read repeated words more rapidly than novel words but this repetition effect emerged only at lower stimulus contrast levels. It was concluded that AD patients have feature extraction speeds comparable to non-demented older adults but only when the stimuli are presented at a relatively high contrast. These findings suggest that the automatic encoding processes involved in word recognition remain intact in mildly demented AD patients given stimuli of sufficient strength.