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Idiopathic Parkinson's disease (PD) has a long preclinical phase during which non-motor symptoms exist but are not specific. The aim of the study is to determine whether subtle gait disorders are prediagnostic markers of PD. Participants were selected from the Baltimore Longitudinal Study of Aging, including 10 prediagnostic PD (8M/2W) and thirty age-matched healthy controls. Prediagnostic PD were assessed for an average of 2.6 ± 1.3 years (range 1.0–5.3 years) before PD diagnosis with: clinical gait and balance assessment (i.e., 6-meter corridor walk at usual and maximal gait speed, semi-tandem stand, tandem stand, and one-leg stand), instrumental gait analysis (VICON®), upper-limb motor skills (i.e., Hand Rapid Alternating Movement Test, Finger Tapping Test, and Purdue Pegboard Test), neuropsychological assessment (i.e., processing speed, attention, executive functions, memory, language, and visuo-spatial skills), and non-motor symptoms (orthostatic hypotension, depression, sleep disorders, and urinary symptoms). Prediagnostic IPD showed slower gait speed due to shorter step length compared to controls. They also showed greater gait speed variability, which was correlated with falls. No statistically significant differences were found between the two groups for balance, upper-limb motor skills and non-motor symptoms. Prediagnostic PD showed lower visuospatial ability (Card Rotation Test) than controls. The set of variables that best discriminate between the two groups were age, clinical gait speed, Trail-Making Test Part B, and Card Rotation Test with 100% sensibility and 95.8% specificity. This is the first prospective longitudinal study demonstrating gait changes in prediagnostic idiopathic Parkinson's disease. Previous published studies have recruited high-risk population for PD, as asymptomatic carriers of LRRK2 mutation [1] or patients with rapid eye movement sleep behavior disorder [2] , [3] . Our findings could potentially be used as prediagnostic markers of IPD to identify those who could benefit from future neuroprotective therapy in order to delay, or even prevent, clinical manifestations.

Introduction Alzheimer's disease (AD) and Parkinson's disease (PD) are neurodegenerative diseases of increasing prevalence worldwide. In the absence of effective therapy, current research is focused on identifying individuals at high risk, who could be targeted in intervention trials. Today, the field of behavioral neuroscience has emerged as one of the most promising areas of research on this topic. Recently, it has been shown that the exacerbation of gait disorders in dual-task conditions, which involve the concurrent performance of a cognitive task while walking, is a characteristic feature of AD and PD. Objectives This review aims: – to summarize evidence supporting or refuting that the dual-task walking paradigm have added value for early detection of older people at risk for progression to AD or PD; – to propose new dual-task paradigms and markers for more accurate and earlier diagnosis of these diseases. Study selection The following inclusion criteria were used in this review: – used self-paced (comfortable) walking as the primary motor task; – reported gait parameters during both single- and dual-task performance, or dual-tasking effects on gait; – employed secondary cognitive tasks during dual-task walking. Results Dual-task-related gait changes precede the traditional clinical symptoms of AD and PD. Inhibition and flexibility are mostly impaired in preclinical AD and PD, respectively. Strategies in dual-task conditions may differentiate between preclinical AD and PD. A number of issues related to the application of dual-task methodology in gait (e.g., effect of type of cognitive task, choice of gait parameters, complementary use of linear and nonlinear measures) are discussed. Conclusion and perspectives We propose novel dual-task paradigms and markers based on the pathophysiological mechanisms that underlie gait disorders in each disease to advance our knowledge in this innovative area of research.