Cortical mechanisms and modulation of somatosensation in older adults with foot sole somatosensory impairments

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PROPOSAL SUMMARY My career goals are to 1) understand the supraspinal sensorimotor pathways through which age- and age- related conditions diminish the functionality of the locomotor control system, and 2) use this information to develop new therapies to improve gait and balance in older adults. I hope to achieve this goal by transitioning into an independent scientist and leading an research program in the fields of aging and balance control. In older adults, diminished lower-extremity somatosensation is highly prevalent and a primary contributor to poor balance, reduced mobility, and increased risk of falling. The vast majority of research and clinical efforts to date have attempted to improve somatosensation by restoring the function of peripheral elements of the somatosensory system. However, somatosensation is also dependent upon the capacity to activate the appropriate cortical networks in response to a given stimulus, which is also altered in older adults. Therapeutic strategies aimed at enhancing the excitability of the somatosensory cortical network thus offer untapped potential to improve foot-sole somatosensation in this population. Our preliminary studies suggest that a single session of traditional transcranial direct current stimulation (tDCS), which produces a diffuse electric field over the primary somatosensory cortex, improves foot-sole somatosensation in older adults. In this K01 Award, we will first work to identify the specific cortical network that is responsive to walking-related foot-sole stimulation in older adults with and without foot-sole somatosensation (Aim 1). We will use a block-design functional MRI paradigm with a custom-designed, MRI-compatible foot sole stimulation system to apply individualized pressures to the participant’s foot soles that mimic those experienced when they walk, yet while they lay motionless in the scanner. Based upon this knowledge obtained from Aim 1, we will develop a novel multi- target tDCS intervention targeting the identified cortical network and test the effects of a single session of this intervention on foot-sole somatosensation, balance and mobility in older adults with mild-to-moderate foot-sole somatosensory impairments (Aim 2). We will then use participant brain MRIs and electric field modeling to establish the “dose-response” relationship between on-target current intensity induced by tDCS and its acute effect on the cortical response to foot sole stimulation (Aim 3). Through this work we will learn about how chronic lower-extremity somatosensory impairments influence the cortical processing of sensory feedback involved in the control of balance and mobility, demonstrate that such processing can be modulated by tDCS, and thus, obtain critical information needed to design a larger more definitive trial to test the potential for tDCS to improve foot sole sensation, balance, and mobility in this population. This career development award will provide me with unique training experiences in neurophysiology of somatosensation in aging, the clinical care of these older adults, and increase my expertise in advanced neuroimaging and brain stimulation techniques, which, taken together, will greatly facilitate my efforts to transition into an independent academic scientist.