We further explored distribution of the assortment of PD-related concepts in the published documents (journal articles and patents) (Figure ). Aging is the most widely explored concept, it has been discussed in nearly 1/3 of the PD-related published documents (Figure ). Aging is a leading risk factor for developing PD, as it is for most neurodegenerative diseases, including Alzheimer's disease, Huntington's disease, and frontotemporal lobar dementia. Whilst PD involves a complex range of symptoms, a key brain region affected by critical cell loss in PD is the substantia nigra, which is also the main reason for the motor symptoms associated with this disease. Specifically, it is the dopaminergic neurons of the pars compacta within the substantia nigra that are lost. The substantia nigra neurons are specific dopaminergic neurons in certain ways: they are pigmented (contain the pigment neuromelanin), exhibit autonomous pacemaking activity and increased susceptibility to oxidative stress. They are also thought to be particularly susceptible to the mitochondrial dysfunction which accumulates within them with advancing age. Thus, understandably, brain, neuron, and substantia nigra are also in the top concepts considered in the PD research (Figure ).