Prefrontal cortical activity during uneven terrain walking in younger and older adults Provisionally Accepted

Jungyun Hwang¹ Chang Liu^{2, 3} Steven P. Winesett^{4, 5} Sudeshna A. Chatterjee⁶ Anthony D. Gruber II⁴ Clayton W. Swanson^{1, 4} Todd M. Manini⁷ Chris J. Hass⁵ Rachael D. Seidler^{3, 5, 8} Daniel Ferris^{2, 3} Arkaprava Roy⁹ David J. Clark^{1, 4*}

• ¹Department of Neurology, College of Medicine, University of Florida, United States
• ²J. Crayton Pruitt Family Department of Biomedical Engineering, Herbert Wertheim College of Engineering, University of Florida, United States
• ³McKnight Brain Institute, J. Crayton Pruitt Family Department of Biomedical Engineering, Herbert Wertheim College of Engineering, University of Florida, United States
• ⁴RR&D Brain Rehabilitation Research Center, United States Department of Veterans Affairs, United States
• ⁵Department of Applied Physiology and Kinesiology, College of Health and Human Performance, University of Florida, United States
• ⁶Department of Physical Therapy and Rehabilitation Sciences, College of Nursing and Health Professions, Drexel University, United States
• ⁷Department of Health Outcomes and Biomedical Informatics, College of Medicine, University of Florida, United States
• ⁸Norman Fixel Institute for Neurological Diseases, Department of Neurosurgery, College of Medicine, University of Florida, United States
• ⁹Department of Biostatistics, College of Public Health and Health Professions, University of Florida, United States

Walking in complex environments increases the cognitive demand of locomotor control; however, our understanding of the neural mechanisms contributing to walking on uneven terrain is limited. We used a novel method for altering terrain unevenness on a treadmill to investigate the association between terrain unevenness and cortical activity in the prefrontal cortex, a region known to be involved in various cognitive functions. Prefrontal cortical activity was measured with functional near infrared spectroscopy while participants walked on a novel custom-made terrain treadmill surface across four different terrains: flat, low, medium, and high levels of unevenness. The assessments were conducted in younger adults, older adults with better mobility function and older adults with worse mobility function. The primary hypothesis was that increasing the unevenness of the terrain would result in greater prefrontal cortical activation in all groups. Secondary hypotheses were that heightened prefrontal cortical activation would be observed in the older groups relative to the younger group, and that prefrontal cortical activation would plateau at higher levels of terrain unevenness for the older adults with worse mobility function, as predicted by the Compensation Related Utilization of Neural Circuits Hypothesis. The results revealed a significant main effect of terrain, indicating a significant increase in prefrontal cortical activation with increasing terrain unevenness during walking in all groups. A significant main effect of group revealed that prefrontal cortical activation was higher in older adults with better mobility function compared to younger adults and older adults with worse mobility function in all pooled terrains, but there was no significant difference in prefrontal cortical activation between older adults with worse mobility function and younger adults. Contrary to our hypothesis, the older group with better mobility function displayed a sustained increase in activation but the other groups did not, suggestive of neural compensation. Additional findings were that task-related increases in prefrontal cortical activation during walking were lateralized to the right hemisphere in older adults with better mobility function.. These findings support that compared to walking on a flat surface, walking on uneven terrain surfaces increases demand on cognitive control resources as measured by prefrontal cortical activation.