The concurrent performance of two tasks, i.e., dual tasking (DT), is a common and ubiquitous every day phenomena. For example, people frequently walk while talking on a cellphone or drive while talking to a passenger. Often, the performance of one or more of these simultaneously performed tasks may deteriorate when another task is carried out at the same time, even in healthy young adults. This reduction in performance is referred to as the DT deficit or DT cost and is typically much higher in patients with Parkinson's disease (PD) than in young adults or age-matched controls. In PD, this DT cost impairs the gait pattern, as manifested, for example, in increased gait variability, exacerbating instability and fall risk.

In the proposed study, would be evaluated the effects of tDCS on dual tasking performance following tDCS.

The researchers expect that stimulation of the Pre Frontal Cortex (PFC) (using tDCS) will increase DT performance and prefrontal activation.

tDCS intervention: Noninvasive tDCS will be delivered by study personnel uninvolved with any other study procedures. In the study will be used a battery-driven electrical stimulator. Stimulation and sham condition will be performed based on previous studies. Briefly, the anode will be placed over the PFC and the cathode over the right supraorbital region. The real tDCS condition will consist of 20 min of continuous stimulation at target intensity of 1.5 mA. This amount of stimulation is safe for healthy young and older adults and has been shown to induce acute beneficial changes in cortical excitability and cognitive functions. For the sham condition, an inactive stimulation protocol would be followed, as compared with an 'off-target' active protocol, in order to minimize participant risk. After each session, subjects will complete a side effects questionnaire. The efficacy of tDCS blinding will also be assessed after the final session, by asking each subject to judge whether they received real or sham tDCS, as well as their certainty of this judgment. Pre- and post-tDCS assessments will include:

fMRI: All of the MR images will be acquired on a 3.0 T scanner using an 8-channel head coil. T1-weighted brain volume (BRAVO) acquisitions will evaluate gray matter (GM) volume and thickness, markers of brain atrophy. This sequence will measure the ratio of GM within the PFC to overall GM, which will then be used to quantify the level of activation within the PFC. T2* echo planner imaging acquisition will be used for all the DT paradigms including intrinsic functional connectivity. Intrinsic connectivity will be examined while subjects are not engaged in any particular task and are requested to lie still with their eyes open (i.e., resting state). To examine task related changes versus more generalized patterns of DT activations, the type of the cognitive task or the nature of the motor task will be different in each task. The researchers will specifically examine the contribution of a secondary task involving working memory (arithmetic processing vs. attention), conflict monitoring, and motor planning on DT related activations.

fNIRS : fNIRS will be used to investigate the role of the frontal lobe in DT walking and how it is affected by tDCS [Mirelman et al. 2014]. The fNIRS system (Oxymon MKIII; Artinis Medical Systems) consists of flexible circuit board that carries the near-infrared light sources and detectors. The fNIRS sources and detectors pairs will be placed over the left (Fp1) and right (Fp2) frontal cortex regions of the forehead, as previously reported.

Gait assessment: Gait parameters will include both spatial and temporal parameters obtained using body fixed wearable sensors (accelerometers and gyroscopes) [Weiss et al. 2015;Ben et al. 2015]. Parameters will include (but are not limited to) gait speed, stride length and stride time as well as rhythmicity measures such as stride to stride variability and gait regularity.

The UPDRS, fall history and fear of falling will also be assessed (e.g., Falls Efficacy Scale International, FES-I) to further characterize the cohort and explore possible confounds.

Cognitive assessment: A detailed computerized cognitive battery that has been used extensively at TASMC in PD and other cohorts [Dwolatzky et al. 2003;Hausdorff et al. 2006;Springer et al. 2006;Yogev et al. 2005;Aarsland et al. 2003] will quantify several cognitive domains including working memory, executive function, verbal function, problem solving, a global cognitive score, and attention.

Sample size: Based on the effects of tDCS on DT walking outcomes in other cohorts [Leite et al. 2014;Zhou et al. 2014], the research group consider a conservative change of 15% in HbO2 levels after tDCS, as compared to sham, 18 subjects per group will provide >80% power. In order to allow for potential inter-subject variability and to address secondary questions (e.g., effect of disease severity), would be to assess 30 participants in each group.

Data collection:

A research assistant will assist participants filling in the electronic questionnaires and will conduct the non electronic ones (these would be later transcribed to excel sheets by research assistants).

A post-doc fellow and a PhD student will run the MRI scans and the tDCS sessions together with one-two research assistants. The participants will receive a reminder (by phone and or email) one day prior to each session. Participation will be monitored by the research assistants.

Inclusion Criteria:

1. Diagnosis of idiopathic PD (defined by the UK Brain Bank criteria)
2. Hoehn and Yahr score between 1.5-3
3. Taking anti-parkinsonian medications.

Exclusion Criteria:

1. Mini Mental State Exam (MMSE) score =< 24
2. Brain surgery in the past including implanted DBS
3. Major depression (DSM-IV Criteria)
4. Cerebral Infarction with Residual Deficits Diagnosis
5. Neurological diseases (except from PD)
6. Orthopaedic or cardiovascular diseases that may affect walking and cognitive abilities.