Neural signatures of actively controlled self-motion and the subjective encoding of distance.

Navigating through an environment requires knowledge about one's direction of self-motion (heading) and traveled distance. Behavioral studies showed that human participants can actively reproduce a previously observed travel distance purely based on visual information. Here, we employed EEG to investigate the underlying neural processes. We measured in human observers event-related potentials (ERPs) during visually simulated straight-forward self-motion across a ground plane. The participants' task was to reproduce (active condition) double the distance of a previously seen self-displacement (passive condition) using a gamepad. We recorded the trajectories of self-motion during the active condition and played it back to the participants in a third set of trials (replay condition). We analyzed EEG activity separately for four electrode clusters: frontal (F), central (C), parietal (P), and occipital (O). When aligned to self-motion on- or offset, response modulation of the ERPs was stronger, and several ERP-components had different latencies in the passive as compared to the active condition. This result is in line with the concept of predictive coding, which implies modified neural activation for self-induced vs. externally induced sensory stimulation. We aligned our data also to the times when subjects passed the (objective) single distance d_obj and the (subjective) single distance d_sub. Remarkably, wavelet-based temporal-frequency analyses revealed enhanced theta-band activation for F, P, and O-clusters shortly before passing d_sub. This enhanced activation could be indicative of a navigation related representation of subjective distance. More generally, our study design allows to investigate subjective perception without interfering neural activation due to the required response action. Significance statement Human observers can accurately judge traveled distance when actively reproducing a passively observed displacement, the neural basis of which is not well understood. We measured with EEG in human observers the neural signature of subjective distance estimations during visually simulated self-motion. We found different latencies and larger amplitudes of relevant event-related potentials for the passive-viewing as compared to the active-reproduction condition; a finding which is in line with the predictive coding framework. Remarkably, in our time-frequency analysis we found enhanced EEG-power especially in the theta-band when participants passed the subjective required distance.