Depression in Parkinson's disease: an EEG frequency analysis study

Although depression is a common finding in Parkinson's disease (PD), its neurobiological mechanism is still unknown. The purpose of this study was to determine whether there are specific spectral electroencephalographic (EEG) characteristics that distinguish depressed from non-depressed PD patients. The study was performed in 24 patients with idiopathic PD whose antiparkinson medication was stopped 24h beforehand. They were divided into two groups of 12 patients each, one with depressive symptomatology, and one without. The groups did not differ with respect to age, sex distribution, and disease severity and duration. All recordings were conducted using a 16-channel electroencephalograph, and artifact-free EEG was processed using a Fast Fourier Transformation. The EEG of depressed PD patients showed significantly less absolute and relative power in spectral band 7.5-10Hz (alpha1), and slightly more relative power in spectral band 10.513Hz (alpha2), while there was no difference in other spectral bands. Topographic analysis of the alpha1 absolute power revealed that, while in non-depressed patients this activity has a clear occipital maximum (and thus corresponds to the standard background activity), in depressed patients its maximum was shifted anteriorally toward the parietal region. Topographic analysis of the significance of the difference between the groups in the relative power of alpha1 and alpha2 bands revealed opposite gradients, posterior to anterior and anterior to posterior directions, respectively. The spectral EEG characteristics of the depressed PD patients not only differed from the spectral EEG characteristics of non-depressed PD patients, but they were also different from the usually reported spectral EEG characteristics of depressed patients without neurological disease. We propose that our data are sufficient to raise the possibility for the existence of a distinctive neurobiological substrate of depression in PD. This is not just a simple addition of two neurobiological substrata, one of depression (as it is determined in non-neurological patients) and one of PD, but rather a complex product of their interaction.