Modulating Brain Activity to Improve Cognitive-motor Function in Alzheimer's Disease

Beyond the profound impact on memory, Alzheimer's disease (AD) neuropathology, even in its early stages, affects the prefrontal lobes leading to executive dysfunction and mobility disturbances. Prefrontal cortex functions, including executive control, attention, and working memory, are known to decline with the progression of AD. In older adults, better performance on executive cognitive tasks is associated with greater activation of the left dorsolateral prefrontal cortex (dlPFC). Reduced activation within the dlPFC is believed to play a role in both the executive and physical functioning declines seen in AD, significantly contributing to loss of functional independence. In mild AD, an individual's state of executive functioning is a sensitive predictor of the ability to stand and walk safely, especially when performing additional cognitive tasks (i.e., dual tasking). Therefore, the investigators contend that by facilitating the excitability of the left dlPFC, some of the early cognitive and mobility impairments of AD may be reduced, ultimately leading to more functional independence, increased physical activity, and improved quality of life.

tDCS provides a noninvasive means of facilitating the excitability of the prefrontal cortex and its connected neural networks, and thus holds promise as a therapy to improve the executive control of cognition and mobility in older adults with mild AD. tDCS modulates cortical excitability by passing low-level currents through electrodes placed upon the scalp over the dlPFC. These currents induce electrical fields within the brain that in turn polarize neuronal populations and alter their likelihood of firing. The research team demonstrated in older adults aged 65 years and older with executive dysfunction and slow gait that 10 sessions of 20-minutes of tDCS targeting the left dlPFC improved cognitive and physical functioning for at least two weeks following the intervention. Considerable evidence, including our preliminary studies, now suggest that multi-session tDCS interventions targeting the dlPFC may induce measurable and meaningful improvements in cognitive and/or mobility outcomes in relatively healthy adults and in those with mild-to-moderate executive dysfunction. Still, the size and duration of tDCS-induced benefits in older adults with executive dysfunction have not been established. Moreover, to date, tDCS delivery has not attempted to account for interpersonal differences in older adults, particularly the high inter-individual variance in skin, skull, brain, and cerebrospinal fluid and how each of these characteristics impacts the current flow. Such personalization is now possible with the current flow modeling the investigators propose.

The overall aim of the study is to conduct a pilot, randomized sham-controlled trial to determine the feasibility and effects of a 10-session personalized tDCS intervention targeting the left dlPFC on cognitive function, dual task standing and walking, and other metrics of mobility in 24 older adults with mild AD living in supportive housing. The investigators will include personalized current flow modeling approach using baseline structural MRIs to determine the tDCS electrode placement and stimulation parameters to optimize current flow to each participant's brain. The investigators do not expect tDCS to revere the structural brain changes that result from AD, but instead maximize the function of remaining, intact brain neurons and frontal networks, and thereby improve functional outcomes in people suffering from the neurodegenerative process.

The investigators hypothesize that, in older adults 65 years and older with mild AD, a personalized tDCS intervention targeting the left dlPFC, as compared to sham, will mitigate dual task costs to the control of gait and standing posture and enhance executive functioning.