Implicit Learning in People with Parkinson's Disease

Abstract

Despite the wealth of research investigating the implicit learning abilities of people with Parkinson's disease (PD), the role of the basal ganglia in implicit mnemonic processes remains largely unclear. The present research sought to examine the ability of people with PD to implicitly learn sequential regularities, both in the context of visuomotor pattern learning and the acquisition of categorical knowledge, with a view to minimise the motor demands of the tasks and to objectively assess explicit knowledge. Implicit learning in nondemented patients with PD was compared with matched healthy controls in three separate studies. Visuomotor sequence learning was examined using a verbal version of the serial reaction time (SRT) task, while implicit category learning was assessed using the artificial grammar (AG) task and SRT pattern judgment measures. In the first experiment, PD patients demonstrated preserved AG performance, but showed reduced implicit sequence-specific learning for SRT sequences with different sequential structures, exhibiting the greater impairment for the more complex sequence. A closer examination revealed that the SRT deficit was for higher-order sequential associations as well as the learning of pairwise information. Experiment 2 assessed both visuomotor- and judgment-linked sequence learning on a single SRT task, specifically designed to minimise the likelihood of participants acquiring explicit knowledge. Whereas PD patients failed to demonstrate significant visuomotor sequence learning, particularly early in the task, they were able to successfully categorise patterns according to their sequential status, suggesting these forms of implicit learning may be neurally dissociable. However, PD patients evidenced an implicit category learning deficit on an AG task modified so that participants' learning depended on making use of feedback provided on each trial. The final experiment investigated the implicit learning of a spatial sequence, a stimulus-response sequence, and an integrated spatial/stimulus-response sequence, all of which are usually confounded in the standard SRT task. Whereas both PD and control groups demonstrated robust learning for the individual spatial and response sequences, only control participants evidenced learning for the integrated sequence. Further, in a modified AG task (similar to Experiment 2) for which participant learning levels tended to be very low, only PD subjects displaying significant levels of explicit knowledge were able to perform above chance in the implicit judgment-based phase of the task, while successful performance for individuals from the control group was not dependent on explicit knowledge acquisition. As a whole, the results of this study support the hypothesis of an implicit learning impairment in PD patients, but only for certain implicit tasks, such as visuomotor sequence learning or feedback-based category learning. These deficits were not related to differences in explicit knowledge acquisition and, for the most part, were independent of difficulties associated with executive dysfunction. The implicit learning deficits of PD patients are discussed with regard to the specific role played by the basal ganglia in sequence chunking mechanisms and integrative sequence learning in the SRT, and the learning of cue-outcome relations in feedback-based category learning tasks. Finally, some limitations and open issues of the present studies are considered.