Deficits in memory are not a characteristic of PD. Patients with PD display difficulty in retrieving newly learned information from memory stores, as indicated by mild impairments in free recall, but relatively intact recognition and cued recall (49). Patients with PD may also show an increased reliance on serial encoding (recalling words in the order they are presented) and reduced semantic encoding (indexed by recall of groups of words according to semantic category) (50,51). Although retrieval and semantic encoding deficits are evident in group studies of PD, there is diversity in memory profiles of individual patients with PD (52). Remote memory is generally preserved in early PD (53). Prospective memory, or memory for intended, future actions, may be compromised in PD (54,55). Findings regarding performance on measures of nondeclarative memory, which refers to "knowing how" and is a form of remembering that can be expressed only through the performance of task operations, appear to be task-dependent (56). Thus, impairments in the learning of new motor, perceptual, and cognitive skills may or may not be evident (57-60) while priming is typically intact (58,61).
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