## 1 Introduction  
The rehabilitation of patients with gait-related health problems plays an important role in medical health care [1, 2]. Stroke, aging, overweight, medication, and diseases such as diabetes mellitus type II or mild cognitive impairments, as well as other life circumstances such as accidents at work or at home can lead to impairments in human gait [3, 4]. Therefore, problems in gait control are not always diagnosed properly because the factors contributing to changes in human gait patterns are multi-factorial. Importantly, subtle impairments in gait are often not detected in traditional clinical assessments and observations of gait. The physical symptoms of impaired gait can be diagnosed by health professionals including physiotherapists, neuropsychologists or doctors by means of well-validated behavioral tools and neuropsychological tests [5]. However, examinations based on the neural correlates of human gait are still not included in sufficient detail in the rehabilitation routine. Lately, there has been a huge interest in the neural correlates of human locomotion [6]. Methodologically, the analysis of event-related brain potentials (ERPs) and the analysis of changes in oscillatory neural activity, as measured via electroencephalography (EEG), have become important non-invasive methods for investigating human locomotion, including gait [7]. The EEG, and the analysis of ERPs in particular, allows for the investigation of changes in brain activity related to certain external or internal sensory events in the time range of milliseconds (ms). The electrophysiological correlates of arm, finger, or limb movements, or the recognition of own vs. other-generated movements, for example, have been well investigated [8; 9; 10; 11; 12; 13; 14]. Moreover, processes involved in movement control during motor imagery have been investigated intensively [13]. For human motion and movement control, two types of EEG measures are of particular interest. First, the modulation of event-related brain potentials (ERPs, [7; 10]) and, second, the modulation of changes in oscillatory activity, such as changes in event-related (de)synchronization (ERDS/ERS) [15]. Whereas ERPs are phase-locked to a particular event of interest, event-related (de)synchronization describes frequency band-specific changes in EEG rhythms that appear due to changes in externally or internally paced cognitive, affective, and motor activity. Locomotion-related changes in ERD/ERS are well investigated in the literature [15; 16]. However, studies investigating the modulation of gait- event-related potentials (gERPs) during walking are still underrepresented in the literature. Regarding ERP measurement during walking or running, previous studies used dual task paradigms (e.g., solving a cognitive task (primary task)) while walking or running on a treadmill (secondary task). However, these studies aimed at analyzing ERPs of the primary task while simultaneously canceling out brain activity from the secondary task (e.g., walking or running) as a source of noise elicited by the movement of walking or running [10; 17; 18]. Research that looks at gait-specific ERP modulation elicited during spontaneous or instructed walking is still sparse. This research, however, is of relevance for clinical research interested in improving the diagnosis and treatment of gait disorders with a primarily neural origin (e.g., stroke patients, Parkinson's disease) or secondary neural origin (e.g., amputees, traumatic injuries, mentally disabled patients, elderly people with a risk of falls, or overweight people with diabetes type II disorder). These disorders can be accompanied by CNS-related cortical gait impairments. One of the main reasons for this lack of research about gait-specific neural correlates is due to technical reasons. Another reason concerns physiological movement artifacts produced by the gait itself [18]. Yet, another reason concerns assumptions about the control of highly automated movement in the human brain.