Optimizing tDCS to Improve Dual Task Gait and Balance

The objective of this study is to determine the acute effects of single sessions of optimized tDCS, conventional tDCS, and sham stimulation on dual task standing and walking in older adults who are free of overt disease yet who present with poor baseline dual task performance.

Standing and walking are almost always completed in unison with other cognitive tasks such as talking, reading or making decisions. The ability to perform this important type of "dual tasking" is critical to daily activities and dependent upon one's capacity to effectively activate appropriate brain networks that include the left dorsolateral prefrontal cortex (dlPFC). Transcranial direct current stimulation (tDCS) is a safe, noninvasive technology that can selectively modulate brain excitability (i.e., the likelihood of activation) by passing low-level currents between electrodes placed upon the scalp. We have demonstrated through a series of studies that a single, 20-minute exposure of 'conventional' tDCS targeting the left dlPFC-administered via two large sponge electrodes-reduces dual task costs to metrics of standing postural control and gait, when tested immediately following stimulation. Still, we and others have also observed relatively high between-subject variability in the effects of this conventional bipolar form of tDCS. We contend that this variability in effectiveness arises in part from relatively diffuse and unspecific current flow when using large sponge electrodes, in combination with individual variability in head and brain anatomy that significantly alters current flow and the generated electric field in the target brain region.

In this project, we will 1) apply recent advances in tDCS modeling and administration to model the electric fields generated by conventional tDCS in older adults using their individual structural brain MRIs, and 2) develop and test an multi-channel tDCS montage designed to optimize current flow to the left dlPFC (i.e., 'optimized' tDCS). Our Specific Aim is to examine the immediate after-effects of conventional tDCS, optimized tDCS, and sham stimulation on dual task standing and walking in older adults. Our study population will be older men and women without overt disease or illness, yet with poor baseline dual task performance defined as a dual task cost (i.e., reduction) to gait speed of at least 10% induced by simultaneously performing a serial subtraction task when walking. We hypothesize that across participants, the effect of conventional tDCS on dual task standing and walking performance will correlate with a specific component of the electric field generated over the left dlPFC target. We also hypothesize that optimized tDCS will induce A) greater effects on dual task standing and walking performance as compared to conventional tDCS and sham stimulation, and B) these effects will be more consistent across individuals as compared to conventional tDCS.

This project will provide important insights into tDCS "dosage" that will enable us and many other researchers to better understand, control, and optimize this form of noninvasive brain stimulation to individual head and brain anatomy. It is also expected to demonstrate that optimized tDCS, as compared to the conventional approach, significantly improves the size and consistency of observed benefits to dual task standing and walking in vulnerable older adults.

Eligibility Criteria:
Inclusion Criteria:

* Men and women aged 65-85 years
* Poor dual task performance, defined as a preferred gait speed that is \>10% slower when walking and simultaneously performing verbalized serial subtractions (i.e., dual tasking), as compared to walking normally (i.e. single tasking)

Exclusion Criteria:

* Unwillingness to cooperate or participate in the study protocol
* An inability to walk or stand for 30 continuous seconds without an assistive device
* A diagnosis of a gait disorder, Parkinson's disease, Alzheimer's disease or dementia, multiple sclerosis, previous stroke or other neurodegenerative disorder
* Self-report of acute illness, injury or other unstable medical condition; Any report of severe lower-extremity arthritis or pain, physician-diagnosis of peripheral neuropathy, or other peripheral neuromuscular disease that may confound the effects of tDCS on gait or postural control
* Use of antipsychotics, anti-seizure, benzodiazepines, or other neuroactive medications
* Severe depression defined by a Geriatric Depression Scale score greater than 11;
* Any report or physician-diagnosis of schizophrenia, bipolar disorder or other psychiatric illness
* Contraindications to MRI or tDCS, including reported seizure within the past two years, use of neuro-active drugs, the risk of metal objects anywhere in the body, self-reported presence of specific implanted medical devices (e.g., deep brain stimulator, medication infusion pump, cochlear implant, pacemaker, etc.), or the presence of any active dermatological condition, such as eczema, on the scalp