The source of the memory impairment in Parkinson's disease: acquisition versus retrieval

Memory deficits are common in persons with Parkinson's disease (PD) even without the presence of a frank dementia. These memory deficits have traditionally been attributed to inability of patients to retrieve information from long-term memory, referred to as the "retrieval failure hypothesis." However, some studies additionally document problems in recognition memory, noted to be inconsistent with the retrieval failure hypothesis. Given the neuroanatomical abnormalities observed in the hippocampus of PD patients and the role of the hippocampus in learning new information, the current study was designed to specifically examine learning abilities in a nondemented PD sample through the application of a learning paradigm, the Open Trial Selective Reminding Test. We examined 27 patients with PD without dementia and 27 age-, gender-, and education-matched healthy controls (HCs) with a neuropsychological test battery designed to assess new learning and memory. Results indicated a significant difference between the groups in terms of their ability to learn a list of 10 semantically related words. However, once the groups were equated on learning abilities, no significant difference was noted between the PD and HC participants in recall or recognition of the newly learned material. The memory deficit observed in nondemented PD patients is thus largely the result of a deficit in learning new information. This finding should be used to guide treatment for memory deficits in persons with PD, and future research should seek to identify novel means of improving new learning in this population.