# Visual Demands of Walking Are Reflected in Eye-Blink-Evoked EEG-Activity
## 1 Introduction
It is established that eye blinks do not occur randomly. Instead, they are functionally timed to segment periods of meaningful visual information. For instance, blinks tend to occur at the end of a sentence when reading [33], at the end of a scene when watching movies [34], or after a decision that has been made [35]. Interestingly, this behavior has also been observed during auditory information processing, suggesting that it might be a general trans-modal reflex [36]. Relying on these findings, we can assume that eye blinks mark a moment when the processing of a chunk of information is finalized. The re-opening of the eye, therefore, marks the moment when new input enters the eye, comparable to the presentation of an external stimulus. Thus, event-related EEG that is time-locked to the blink could be treated similarly to event-related activity evoked by an external stimulus. Indeed, some studies have demonstrated that blink-evoked potentials are modulated by the cognitive demands of everyday tasks, either in a work-place simulation [4] or while a pedestrian navigates in a city [11]. To summarize, eye blinks can serve as discrete events that could allow us to directly estimate the cognitive load of a dual-task walking situation from both ERPs and ERSPs. Blinks mark the onset of incoming visual information by which processing should be affected depending on the level of gait complexity. To verify this, we re-analyzed two existing data sets. In both experiments, participants had to either stand, walk in a meadow, or walk across an obstacle course while performing auditory tasks [12; 13]. Blinking behavior and blink-evoked EEG-activity were analyzed for both experiments. Blinks were first identified as independent components in the EEG data, decomposed using an independent component analysis (ICA). Using Gaussian-fit-based template matching (see Section 2), individual eye blinks were identified from the relevant independent component and their peak amplitudes served as epoch markers for ERPs and ERSPs. The following predictions were evaluated in the epoch-averaged data. First, walking on more demanding ground should demand enhanced attentional allocation that would be reflected in larger amplitudes in the posterior N1 and parietal P2 components. The same can be expected for fronto-central N2, a known correlate of executive control that is sensitive to task demands [37]. Additionally, the parietal P3, which is similarly connected to cognitive resource allocation, typically decreases with increasing task demands [38]. Highly complex visual input during walking is predicted to generate similar effects. Some studies suggest that high scene complexity could decrease the amplitudes of N1, N2, and P3 amplitude [39], indicating attentional narrowing towards relevant information and the suppression of irrelevant information. The former predictions could reflect the role of elementary processes during walking, while the latter could highlight the systemic aspects of information processing. Both sets of predictions are evaluated in our analyses of eye-blink-evoked ERPs. The same set of predictions may hold true for ERSPs, in particular, Theta and Alpha activity. A purely function-based view would predict that more demanding tasks reduce levels of Alpha power, since more task engagement is involved [40; 41]. However, Alpha activity could also increase with more complex scenes [42]. Additionally, this effect can be interpreted in terms of increased inhibitory processing for irrelevant information in favor of relevant signals.