Every-day life means being part of a complex environment and performing complex tasks that usually involve a combination of motor and cognitive skills. However, the process of aging or the sequelae of neurological diseases such as atypical Parkinson's disease (APD) compromises motor-cognitive interaction necessary for an independent lifestyle. While motor-cognitive performance has been identified as an important goal for sustained health across different clinical populations, little is known about underlying brain function leading to these difficulties and how to best target these motor-cognitive difficulties in the context of rehabilitation and exercise interventions. The challenge of improving treatments of motor-cognitive difficulties (such as dual-tasking and navigation) is daunting, and an important step is arriving at a method that accurately portrays these impairments in an ecologically valid state. The investigators aim therefore to explore brain function during complex walking in healthy and APD by investigating the effects of age and neurological disease on motor-cognitive performance and its neural correlates during three conditions of complex walking (dual-task walking, navigation and a combination of both) using non-invasive measures of brain activity (functional near infrared spectrometry, fNIRS) and advanced gait analysis in real time in older healthy adults and people with APD.