Implicit Sequence Learning in People with Parkinson's Disease

Abstract

People with Parkinson's disease (PD) have known deficits in striatal dopamine, and compared to age-matched healthy older adults, increased striatal deficits, and difficulties with maintaining postural stability and complex language comprehension, behaviors that involve sequencing. Because sequencing abilities are important for daily life and for various types of rehabilitation, this decline with PD should be well understood, yet the literature on the effects of PD in sequence learning is mixed. Some of the confusion in the literature may be due to variation in participant characteristics, but also to task differences, such as the extent of motor movement required. The present studies examined implicit sequence learning using the Triplets Learning Task (TLT) in a PD group. There were 29 medicated PD participants, and 30 healthy older adults were recruited as a control group. Study 1 compared learning over several blocks of training in the TLT. Two learning measures indicated that people with PD learned, but they showed less implicit sequence learning than Controls. This group difference in one analysis was carried largely by late training, when learning is thought to be particularly dependent on the striatum. Additionally, when first and second halves of blocks were examined separately, the PD group showed less learning than Controls in the second halves of blocks, when learning is thought to be striatal-dependent. In Study 2, we examined relationships between learning and PD-related characteristics. Using learning in the second half block, we found that in our PD group, learning positively correlated with Age, Disease duration and Levodopa levels. These three variables were positively correlated with one another, and therefore it was difficult to disentangle their individual effects. Together, these studies suggest that people with PD were able to learn implicit sequences, though to a lesser degree than Controls, and Levodopa medication may have helped moderate this sequencing deficit. Findings of group differences in learning in sections of training suggest that particular points in training may be more sensitive to sequencing or striatal deficits. Thus, the TLT and these learning analyses could be used to better characterize clinical populations that may have hippocampal or striatal deficits.