Apathy is a pathology of voluntary action or goal-directed behavior (GDB) and the underlying mechanism(s) responsible for apathy may be seen as dysfunctions occurring at the level of elaboration, execution and control of GDB. Apathy is often present after direct lesions of the prefrontal cortex (PFC), but it is also a common clinical feature of basal ganglia diseases. It can be observed in neurodegenerative diseases such as Parkinson's disease or as progressive supranuclear palsy and Huntington's disease. Apathy is also frequently encountered after focal lesions of specific structures of the basal ganglia such as the caudate nuclei, the internal pallidum and the medial-dorsal thalamic nuclei. Apathy is therefore one of the clinical consequences of the disruption of the PFC-basal ganglia axis, one of the functional systems involved in the generation and control of self-generated purposeful behaviour. From this perspective, a prefrontal-like syndrome (including apathy as one of its clinical manifestations) can be encountered following diseases that mainly involve the basal ganglia. This suggests that apathy can also be the consequence of a 'prefrontal-like' syndrome due to lesions mostly affecting the basal ganglia. Therefore, we try to define the apathy importance in PD, and even in different clinical phenotype profiles of PD.
