Long-term retention of implicitly acquired learning in patients with Alzheimer's disease

This study examined retention of procedural learning, using the serial reaction time (SRT) task, over a 1- or 2-week delay in Alzheimer's disease (AD) patients and elderly control (EC) subjects. The SRT task is a four-choice reaction-time task consisting of blocks of 100 trials. A 10-item repeating sequence was embedded in the first four blocks of trials in session one and the first two blocks of session two. Sequence-specific learning was assessed in session one by comparing reaction time (RT) in the fourth block with a repeating sequence to a fifth block in session one in which the stimuli were randomly arranged. After excluding subjects with deficient session one learning, there were eight AD patients and 14 EC subjects who showed robust sequence-specific learning in session one. In these subjects, retention of sequence-specific learning over the 1- to 2-week delay was examined. The AD patients and EC subjects showed an equivalent change in RT across sessions, and all the AD patients lacked any declarative knowledge of the repeating sequence within the task. Individually, two of the eight AD patients appeared to deviate substantially from the others and from the EC subjects in their excess slowing of RT across sessions. Since six AD patients did show retention similar to the EC subjects, it is concluded that at least some AD patients show normal retention of implicitly acquired knowledge over a long delay. Preserved retention in some of the AD patients implies that it is mediated by brain structures that are not affected by the Alzheimer neuropathological process.