Thesis defense Rick Helmich (Donders Series 54)

Cerebral reorganization in Parkinson's disease

winner of the 2012 Dutch Neurofederation PhD thesis prize

Parkinson's disease is characterized by progressive dysfunction of a small region in the brain: the basal ganglia. During his PhD, Rick Helmich I investigated how these focal brain lesions lead to a reorganization of brain function. In Parkinson's disease, more-affected parts of the basal ganglia lost their connections with the rest of the brain, but these routes were taken over by less-affected parts of the basal ganglia. Importantly, such compensatory adaptations may delay clinical symptoms. Helmich also observed altered brain activity during voluntary movements in Parkinson's disease. For example, patients showed increased activity in visual brain regions. This suggests that they use visual information to guide their movements. Furthermore, when Parkinson patients had to switch between movements, they relied on frontal brain regions instead of the basal ganglia. This made their movements slower. However, when Parkinson patients could earn money during the switching task, they performed as well as healthy subjects. Thus, reward anticipation can be used to compensate for switching problems, presumably because it involves unaffected parts of the basal ganglia. Finally, Helmich found that Parkinson patients with tremor are better in making voluntary movements than patients without tremor. Tremor was produced by several brain regions that either turned the tremor on (like a light switch) or controlled the severity of the tremor (like a dimmer). The activation of this ‘tremor network' changed the activity of different brain regions involved in voluntary movements. This may explain why Parkinson patients with tremor generally have a better prognosis than Parkinson patients without tremor.