## 4 Discussion and Conclusions
### Analysis of gERPs Using Gait Markers  
As described in detail in the Methods, the individual events of the human gait cycle were taken from the data recorded by the pressure plate. After the preprocessing of the raw data, prominent gait events, i.e., the initial contact (IC) of the left foot and right foot, respectively, were clearly identified from the recorded data and were used for EEG epoching and ERP averaging. As a gait event, the IC is characterized by distinct periods of motor preparation and motor execution and, therefore, most likely elicits specific motor cortical potentials probably even during spontaneous walking. In the present study, IC-related ERP analysis included both spontaneous and instructed treadmill walking. As illustrated in Figures 5 and 6, this elicited a prominent increase in negativity, starting immediately after the initial contact (IC) and peaking within the first 100 ms post IC onset, clearly visible in all conditions of instructed and spontaneous walking. These motor-related changes in cortical activity were observed in all three individual participants, as well as in the grand average waveforms including all participants. Descriptively, as illustrated in Figures 5 and 6, the amplitude of the potential was differentially pronounced during spontaneous as compared to, e.g., instructed walking. Asking participants, for instance, to make small or large steps modulated the amplitude of the potential compared to spontaneous walking. This suggests that walking is under the control of cognition and attention [10; 21]. This assumption, that in the present pilot study can only be reported descriptively, should be validated in future studies that use the same experimental set-up but include more volunteers than in the present study for statistical testing. Topographic plots, as well as source imaging plots, revealed another interesting finding. Although the motor potential was most pronounced at the left and right frontocentral and centroparietal electrodes, it was topographically most pronounced at the left central and centroparietal electrodes, C3 and CP1. In the 10-20 EEG system, the electrodes C3 and C4, as well as CP1 and CP2, are close to the primary motor and primary somatosensory cortex. Given that the motor potential was found in all walking conditions, this also suggests the participation of the motor cortex during spontaneous walking. In addition, gait control seems to be more under the control of the left compared to the right cortical hemisphere. This mayhave implications for clinical observations of gait asymmetry and brain lesions in stroke patients [31]. Moreover, activity changes extended to the inferior parietal cortex (BA40), forming part of the cortex's fronto-parietal attention network [32]. However, the assumptions of a stronger involvement of the left hemisphere in walking, as well as the assumptions about the role of attention, should be tested statistically and validated in future studies that include more volunteers for statistical testing. Taken together, the present results, although preliminary, suggest that ERPs of the human gait cycle (such as the IC) can be reliably detected in EEG recordings after temporal synchronization with the gait event and after standardized preprocessing of the EEG data. Moreover, the use of active electrodes seems helpful to obtain data with a sufficient signal to noise ratio. Future studies, using the structural design and experimental set-up of the present study, could provide further evidence for gERPs and their modulation during spontaneous and instructed treadmill walking. As shown in Figure 7, motor potential modulation was accompanied by a decrease in the alpha/mu rhythm prior to the IC and an increase in alpha band activity immediately after the onset of the IC. This is in line with other findings, suggesting event-related desynchronization (ERDS) and event-related synchronization (ERS) before and after movement execution [15].