Background: In studies of mild cognitive impairment (MCI) in Parkinson disease (PD), patients

Normal maintenance of human motivation depends on the integrity of subcortical structures that link the prefrontal cortex with the limbic system. Structural and functional disruption of diff erent networks within these circuits alters the maintenance of spontaneous mental activity and the capacity of aff ected individuals to associate emotions with complex stimuli. The clinical manifestations of these changes include a continuum of abnormalities in goal-oriented behaviours known as apathy. Apathy is highly prevalent in Parkinson's disease (and across many neurodegenerative disorders) and can severely aff ect the quality of life of both patients and caregivers. Diff erentiation of apathy from depression, and discrimination of its cognitive, emotional, and auto-activation components could guide an individualised approach to the treatment of symptoms. The opportunity to manipulate dopaminergic treatment in Parkinson's disease allows researchers to study a continuous range of motivational states, from apathy to impulse control disorders. Parkinson's disease can thus be viewed as a model that provides insight into the neural substrates of apathy.

Cognitive defects associated with cortical pathology may be a marker of dementia in Parkinson's disease (PD). There is a need to improve the diagnostic criteria of PD dementia (PDD) and to clarify the cognitive impairment patterns associated with PD. Current neuropsychological batteries designed for PD are focused on fronto-subcortical deficits but are not sensitive for cortical dysfunction. We developed a new scale, the Parkinson's Disease-Cognitive Rating Scale (PD-CRS), that was designed to cover the full spectrum of cognitive defects associated with PD. We prospectively studied 92 PD patients [30 cognitively intact (CogInt), 30 mild cognitive impairment (MCI), 32 PDD] and 61 matched controls who completed the PD-CRS and neuropsychological tests assessing the cognitive domains included in the PD-CRS. Acceptability, construct validity, reliability, and the discriminative properties of the PD-CRS were examined. The PD-CRS included items assessing fronto-subcortical defects and items assessing cortical dysfunction. Construct validity, test-retest and inter-rater reliability of PD-CRS total scores showed an intraclass correlation coefficient >0.70. The PD-CRS showed an excellent test accuracy to diagnose PDD (sensitivity 94%, specificity 94%). The PD-CRS total scores and confrontation naming item scores-assessing "cortical" dysfunction-independently differentiated PDD from non-demented PD. Alternating verbal fluency and delayed verbal memory independently differentiated the MCI group from both controls and CogInt. The PD-CRS appeared to be a reliable and valid PD-specific battery that accurately diagnosed PDD and detected subtle fronto-subcortical deficits. Performance on the PD-CRS showed that PDD is characterized by the addition of cortical dysfunction upon a predominant and progressive fronto-subcortical impairment.