Olfactory-induced brain activity in Parkinson's disease relates to the expression of event-related potentials: a functional magnetic resonance imaging study

Olfactory disorders are common in patients with idiopathic Parkinson's disease (IPD). In IPD patients with hyposmia olfactory event-related potentials (ERPs) are typically found to be delayed or absent. Altered ERPs in IPD patients may also be consistent with reduced neuronal activity in the medial temporal lobe following olfactory stimulation, as demonstrated by functional magnetic resonance imaging (fMRI). We analyzed ERPs and fMRI scans of hyposmic IPD patients (n=18) to gain further insight about the brain regions involved in generation of olfactory ERPs. Patients were separated into two groups (n=9 per group), based on the detectability (+) or non-detectability (−) of ERPs. Central activation during olfactory stimulation was examined using fMRI. Both ERP+ and ERP− patients showed activity in brain areas relevant to olfactory processing, such as the amygdala, parahippocampal regions, and temporal regions (BA 37, 21/22). Comparison of both groups revealed higher activation in ERP+ patients, especially in the amygdala, parahippocampal cortex, inferior frontal gyrus (BA 47), insula, cingulate gyrus, striatum, and inferior temporal gyrus. The relationship between the expression of olfactory ERPs and cortical activation patterns seen during olfactory stimulation in fMRI in IPD patients supports the idea that ERPs are a sensitive marker of neurodegeneration in olfactory regions. In accordance with current neuropathological staging concepts, olfactory ERPs may be reflecting pathological changes in olfactory regions, independent of the typically observed nigro-striatal degeneration in IPD. Reduced activation of primary olfactory areas in the ERP-group may reflect a severe disruption of olfactory processing in these patients.