Stepping forward, stepping backward: a movement-related cortical potential study unveils distinctive brain activities

Human locomotion is the product of complex dynamic systems, which rely on physical capacities as well as cognitive functions. In our daily life, we mostly experience forward walking, but also backward stepping can occur, as in protective stepping. In this work, we investigated the electroencephalographic (EEG) correlates of cognitive processing underpinning step initiation by means of movement-related cortical potentials (MRCPs) analysis and force-plates recordings. Healthy young volunteers (N = 11) performed self-paced forward- and backward-oriented steps on two force platforms, which were synchronized to simultaneous EEG recordings. MRCPs and their source localization analyses were computed. Results demonstrate the involvement of cognitive processing during step preparation and execution, as showed by the prefrontal activity, which was enhanced in backward stepping. Further, the parietal activity was larger in forward than backward stepping, while motor-related regions were involved in both step directions. Thus, the neural timing and sources of forward and backward stepping suggest a functional distinction of these two actions, which undergo different cortical organizations. Backward stepping requires enhanced cognitive control and can be regarded as an avoidance behavior, while forward stepping would be assimilated to an oriented-to-action behavior mainly localized over parietal areas. In conclusion, preparing body locomotion involves high cognitive processing, with step direction showing different cortical organization and functional specialization.