In older adults, falls are costly, consequential and correlated with both physical and cognitive decline. Most falls occur when standing or walking. Many activities require people to stand or walk while performing tasks like talking or making decisions. Such "dual tasking" interferes with the control of standing and walking. This interference, or "cost," is exaggerated in older adults with previous falls and is predictive of future falls. Neuroimaging evidence indicates that standing and walking,especially when dual tasking, activate distributed brain networks including the left dorsolateral prefrontal cortex (dlPFC)-a brain region sub-serving executive function. Thus, strategies that facilitate activation of the left dlPFC and its connected neural networks hold promise to mitigate dual task costs, improve physical and cognitive function, and ultimately, reduce falls.

Transcranial direct current stimulation (tDCS) provides a noninvasive means of selectively modulating cortical excitability. The investigators have shown in younger and older adults that a 20-minute session of tDCS designed to increase excitability of the left dlPFC reduces dual task costs and improves mobility when tested just after stimulation. The investigators have since completed a pilot, sham-controlled trial of a 2-week, 10-session tDCS intervention targeting the left dlPFC in 20 older adults with slow gait and mild-to-moderate executive dysfunction. The intervention was successfully double-blinded and well-attended. tDCS, compared to sham, reduced dual task costs and induced trends towards improved mobility and executive function over a 2-week follow-up. The investigators thus contend that tDCS targeting the left dlPFC holds promise to improve the control of standing and walking-and ultimately reduce falls-in older adults. Still, the size and duration of tDCS-induced benefits to older adult "fallers" have not been established. Moreover, to date, tDCS delivery has attempted to optimize current flow based on a "typical" brain and has thus not accounted for individual differences in skin, skull, cerebrospinal fluid and brain tissue in the aging brain. Such personalization is now possible with the current flow modeling the investigators propose.

The Overall Aim is to compare, in older adults with previous falls, the effects of a personalized tDCS intervention designed to target the left dlPFC on the dual task costs to standing and walking, and other physical and cognitive factors that are on the causal pathway to falls and important to everyday function. The investigators will conduct a randomized, sham-controlled, double-blinded trial with assessments at baseline and post-intervention (immediate, 3-, 6-month follow-up) in 120 non-demented men and women (60 per arm) aged 60 or older who are at risk of falls and report mobility and balance problems and a fear of falling, yet have no major neural or musculoskeletal disorders that explain their falls. The tDCS intervention will comprise 20, 20-minute sessions of tDCS over a 4-week period.

The investigators hypothesize that, in older adults at risk of falls and over a 6-month follow-up, a personalized tDCS intervention targeting the left dlPFC, as compared to sham, will mitigate dual task costs to the control of standing and walking and enhance other metrics of both physical and cognitive function.