Understanding the effects of noninvasive brain stimulation on dual-task EEG patterns in older adults

• Main Author: Finnerty, Emma Kate
• Other Authors: Manor, Brad
• Language: en_US
• Published: 2020
• Subjects: Gerontology; Balance; Noninvasive brain stimulation; Dual-task; Older adults; tDCS
• Online Access: https://hdl.handle.net/2144/41110

INTRODUCTION: Age-related functional declines in the body and brain pose significant challenges to mobility and postural control. Older individuals are at increased risk for injury from a fall. Declines in gait and balance control make older adults more likely to suffer a fall or recurrent falls. According to the Centers for Disease Control and Prevention (CDC), falls are the leading cause of injury related death among adults age 65 years and older. (Important Facts about Falls | Home and Recreational Safety | CDC Injury Center, 2019) One out of three older adults falls annually and the likelihood of falling increases with age. (Stevens et al., 2008). In the US alone, the number of individuals living aged 65 and older is estimated at 46 million persons and is expected to reach 74 million by 2030. (Healthy Aging in Action: Advancing the National Prevention Strategy, 2019) Fall death rates in the United States increased by 30% from 2007 to 2016, and if this trend continues, it is expected that by 2030 there will be 7 deaths due to falls every hour. (Important Facts about Falls | Home and Recreational Safety | CDC Injury Center, 2019) Interventions designed to improve gait and balance control in the geriatric population can mitigate fall risk and positively impact these trends. OBJECTIVE: Gait and balance control, traditionally regarded as automatic motor processes, have since been determined to be complex motor functions reliant on executive function. (Hausdorff et al., 2005; Woollacott & Shumway-Cook, 2002) Normal walking and balance control are attentionally demanding and require shifting of attentional resources to frontal brain regions in order to maintain upright stance. This ability to dual-task is impaired in older adults. A single session of transcranial direct current stimulation (tDCS), a form of noninvasive brain stimulation, targeting the excitability of the left dorsolateral prefrontal cortex (l-dlPFC) has been found to reduce dual-task costs to gait and balance in both young and healthy older adults. (Manor et al., 2016; Zhou et al., 2014) However, little is known about how tDCS influences electroencephalogram (EEG) patterns and if changes in EEG are associated with functional outcomes. The specific aims of this study were to determine whether 1, 20-minute session tDCS targeting the l-dlPFC reduces the slow-wave/fast-wave frequency power ratio in EEG and absolute EEG power and whether these reductions are associated with changes in measures of postural control. METHODS: The data from this study was analyzed as part of a larger clinical trial testing multiple tDCS stimulation montages in combination with batteries of cognitive, gait, and balance assessments. Twenty-two older adults (median age=71 years) who were free of overt illness or disease were included in the analysis. Participants were outfitted with wireless movement sensors and a wireless 32-electrode EEG cap configured according to the 10-20 system. Participants completed a dual-task of serial subtraction by 3’s from a randomized three-digit number while standing for 60 seconds. EEG was simultaneously recorded during the 60 second trials. One, 20-minute tDCS stimulation targeting the l-dlPFC followed the balance assessment. EEG and dual-task assessments were repeated following the stimulation. EEG was not recorded simultaneously with tDCS. EEG data was processed and analyzed with Cartool EEG software. (Brunet et al., 2011) Spectral analysis of the EEG power values pre and post stimulation was conducted using a paired t-test. Power ratios of slow wave (4-8Hz) to fast wave (12-30Hz) were calculated for pre and post stimulation and analyzed for significant changes. Additionally, absolute power values in theta and beta frequency range were calculated. Postural sway velocity and postural sway area were also assessed and analyzed for changes following stimulation. RESULTS: Spectral analysis showed significant reductions in absolute power values across low theta frequency ranges following stimulation. This significant reduction in power was localized, but not exclusive, to frontal electrodes measuring activity of the l-dlPFC in the 4-8Hz frequency range. Most notably electrode F3, which has been found to correspond to the location and activity of the l-dlPFC using both the 10-20 electrode placement system and MRI guided neuronavigation. (Beam et al., 2009; De Witte et al., 2018) In addition to a significant reduction in power values, there was a reduction in slow-fast EEG ratios following stimulation. The percent reduction in EEG ratio was associated with a reduction in postural sway area (m2/s4) and sway velocity (m/s). CONCLUSION: tDCS is used to facilitate the excitability of cortical neurons. The l-dLPFC is a critical component of executive function. Due to the role of executive function in mediating attentional requirements of gait and balance, the l-dlPFC was chosen as a target to enhance dual-tasking capabilities, and thereby improve gait and postural control. The reduction in the slow wave-fast wave ratio and theta power indicates that participants had higher power in the fast wave relative to the slow wave after tDCS administration. The reduction in slow wave power may be indicative of less cognitive attentional effort required to complete a simultaneous dual-task involving postural control. This is supported by the associated reductions in postural sway following tDCS stimulation. These results further current research of tDCS as a viable intervention for improving balance and cognition in older adults and offers additional information about optimizing the efficacy of noninvasive brain stimulation to improve functional outcomes in this population.