Dissertation

Neural substrates associated with context-dependent learning

Lee, Ya-Yun Alice

2013

Overview

In the first study, we recruited individuals with Parkinson's disease (PD), known to have striatum impairments, to test the hypothesis that striatum is a potential neural substrate for context-dependent learning. Ten individuals with PD and 10 age-matched non-disabled adults were recruited into the PD group and the Control group. The study was conducted over two consecutive days approximately 24 hours apart. On the first day, participants practiced a finger sequence task consisting of 3 numerical sequences. Unknown to the participants, each sequence was embedded within a specific colored circle and a specific location on the computer screen. On the second day, the participants were tested under two testing conditions: SWITCH and SAME conditions. Under the SWITCH condition, the context associated with each sequence was changed from that of practice; while under the SAME condition, the sequence-context association remained the same as practice. The primary outcome measure was total time accuracy cost (TTAC), which took both movement speed and accuracy into account. From the second day testing conditions, switch cost was calculated to indicate context-dependent learning. Switch cost was the TTAC performance difference between the SWITCH and SAME conditions normalized by the SAME condition (100 % × [SWITCH − SAME] / SAME). A larger switch cost value would indicate greater context-dependent learning. The results showed that individuals with PD and non-disabled adults demonstrated comparable learning of the finger sequence task when tested under the SAME condition. Study 2 was designed to investigate whether the frontostriatal circuit is associated with context-dependent learning. To indicate the integrity of the frontostriatal circuit, set-shifting ability was tested. The participants in Study 1 also participated in this study. After completion of the finger sequence task on the second day, the participants were given the trail making test (TMT) to assess their set-shifting ability. The result of the TMT was correlated with the TTAC switch cost obtained from Study 1. Findings from Study 2 showed that TTAC switch cost was positively correlated with the result of the TMT in people with PD, suggesting that an individual with PD who had greater difficulty performing set-shifting also demonstrated greater context-dependency. However, this relationship was not observed in non-disabled adults. The results of Study 2 suggested that context-dependent learning could be related to the integrity of the frontostriatal circuit. To test the hypothesis that the DLPFC plays a specific role in context-dependent learning, 30 non-disabled adults (age-matched to the participants with PD in Study 1) were recruited for Study 3. The participants were recruited into the Control group, the rTMS DLPFC group and rTMS Vertex group. The participants in the Control group were the same participants in Study 1. Similar to the procedures of Study 1, all participants practiced the finger sequence task on the first day. Before the SWITCH and SAME testing conditions on the second day, the participants in the rTMS DLPFC and rTMS Vertex groups received 1 Hz rTMS over the left DLPFC or the Vertex for 20 minutes. The rTMS DLPFC group demonstrated a reduced TTAC switch cost when compared to the Control group or the rTMS Vertex group, suggesting that perturbation to the DLPFC reduced context-dependent learning compared to the control conditions. Overall, findings from these three studies suggest that the neuronal interaction between the DLPFC and the striatum within the frontostriatal circuit have specific roles in context-dependent learning. While impairment of the striatum as exists in PD leads to a heightened context-dependent learning, decreased neuronal excitability of the DLPFC reduces context-dependency. Given these results, it is reasonable to hypothesize that the DLPFC is relatively over-activated in people with PD in order to compensate for the impaired striatum. This over-activation of the DLPFC with excessive encoding may be the cause of greater context-dependency observed in PD. (Abstract shortened by UMI.)

• Publisher: ProQuest Dissertations & Theses
• Subject: Cognitive psychology / Kinesiology / Neurosciences / Physical therapy
• ISBN: 9781303693885, 1303693887