Abstract
Implicit sequence learning involves learning about dependencies in sequences of events without intent to learn or awareness of what has been learned. Sequence learning is related to striatal dopamine levels, striatal activation, and integrity of white matter connections. People with Parkinson’s disease (PD) have degeneration of dopamine-producing neurons, leading to dopamine deficiency and therefore striatal deficits, and they have difficulties with sequencing, including complex language comprehension and postural stability. Most research on implicit sequence learning in PD has used motor-based tasks. However, because PD presents with motor deficits, it is difficult to assess whether learning itself is impaired in these tasks. The present study used an implicit sequence learning task with a reduced motor component, the Triplets Learning Task (TLT). People with PD and age- and education-matched healthy older adults completed three sessions (each consisting of 10 blocks of 50 trials) of the TLT. Results revealed that the PD group was able to learn the sequence, however, when learning was examined using a Half Blocks analysis (Nemeth et al., 2013), which compared learning in the 1st 25/50 trials of all blocks to that in the 2nd 25/50 trials, the PD group showed significantly less learning than Controls in the 2nd Half Blocks, but not in the 1st. Nemeth et al. (2013) hypothesized that the 1st Half Blocks involve recall and reactivation of the sequence learned, thus reflecting hippocampal-dependent learning, while the 2nd Half Blocks involve proceduralized behavior of learned sequences, reflecting striatal-based learning. The present results suggest that the PD group had intact hippocampal-dependent implicit sequence learning, but impaired striatal-dependent learning. Thus, sequencing deficits in PD are likely due to striatal impairments, but other brain systems, such as the hippocampus, may be able to partially compensate for striatal decline to improve performance.
Highlights
Implicit sequence learning is the learning of relationships between events that occur sequentially in time, and occurs without intent to learn or awareness of what has been learned (Reber, 1989)
We aimed to address both of these issues by using the Triplets Learning Task (TLT), which was derived from the Serial Reaction Time (SRT), but examines implicit sequence learning with no motor sequencing component (Howard et al, 2008)
These results suggest that participants were not aware of the triplet frequencies seen during training, and any learning that occurred was largely implicit
Summary
Implicit sequence learning is the learning of relationships between events that occur sequentially in time, and occurs without intent to learn or awareness of what has been learned (Reber, 1989) This type of learning allows us to be sensitive to regularities in our environment, and adapt to changes in physical and social cues. Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor impairments (Hoehn and Yahr, 1967) that result from dopamine cell death in the substantia nigra (Fearnley and Lees, 1991), with a resulting depletion of dopamine within the striatum (Kish et al, 1988). These dopamine declines play a Frontiers in Human Neuroscience www.frontiersin.org
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