Previous dual task studies have demonstrated minimal costs when healthy individuals simultaneously perform two tasks at their own individual ability levels. Conversely, Alzheimer's disease (AD) patients show dual task decrements, but it is unclear whether the problem arises at the encoding, maintenance, and/or retrieval phases of memory. Two experiments combined digit recall and visuo-motor tracking to investigate dual task effects during encoding, maintenance, and/or retrieval for AD patients compared with healthy adults. The demands of each single task were titrated for the ability of each participant. In Experiment 1, the dual task requirement was present throughout both encoding and retrieval of digit recall and the differential dual task effects on a secondary tracking task were examined post-hoc. In Experiment 2, the impact of dual task during encoding only, during maintenance only, and during retrieval only was examined systematically. The findings suggest that the specific AD deficit reflects impairment of a cognitive function that supports the simultaneous performance of two tasks in the healthy brain, particularly during the encoding and retrieval phases of the memory task
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