Abstract
Early‐stage nondemented Parkinson's disease (PD~(es)~) patients can learn short but not long sequences as well as controls. We have previously shown that to achieve normal performance, PD~(es)~ patients activated the same right‐sided cortical regions as controls plus the homologous left sided cortex and bilateral cerebellum. In this study, we evaluated two related hypotheses to explain the behavioral abnormalities and the increased bilateral brain activation observed in the PD~(es)~ group. Hypothesis 1 proposed that PD~(es)~ patients recruit regions from a normal bilateral network specialized for sequence learning that healthy controls would activate if performing difficult tasks. Thus, PD~(es)~ patients can learn short sequences as well as controls. Hypothesis 2 proposed that information processing within the network in the PD~(es)~ group is impaired. Thus, PD~(es)~ patients cannot learn as difficult a sequence as controls. To test hypothesis 1, we increased task difficulty and statistical power in the control group and showed that the control and the PD~(es)~ groups activated the same regions. To test hypothesis 2, we analyzed the equal performance data using two partial least squares (PLS) multivariate analyses. The task‐PLS analysis showed that to perform equally with controls, the PD~(es)~ group expressed the normal bilateral network more than the control group. The behavior‐PLS analysis showed that the correlation between learning performance and regional activation was significantly different between the groups. We conclude that PD~(es)~ patients have near normal learning if task difficulty is moderate because they can recruit additional regions from a normal bilateral network specialized for sequence learning. However, when a difficult task would normally require bilateral activation, PD~(es)~ patients fail to learn because information processing within the network is impaired. Hum. Brain Mapp. 20:246–258, 2003. © 2003 Wiley‐Liss, Inc.