Publications

{Colors can have an influence on the emotional processing of affective stimuli. In this work, we investigate if the emotional processing of images from the International-Affective-Picture-System (IAPS; Lang et al., 2008) are modulated when experienced through tinted lenses. The experiment (N\textequals20) presented IAPS images (Valence: neutral, pleasant, unpleasant) for a duration of 3s and participants wore tinted lenses (Tint: none, Blue, Red, Yellow, Green). During image presentation, we measured EEG/ERP and Skin-Conductance-Response (SCR). We found that phasic SCR was significantly diminished with Red. In the EEG, we observed an ERP component that differentiated for the factors of Valence and Tint, which was similar to the late-positive-potential (i.e., LPP; 500-1500ms; FCz). An ANOVA of the mean voltage potential returned significant main effects for Valence (F(2,38)\textequals15.0, p\textless.05, $\omega$\mbox{$^2$}\textequals.41) and Tint (F(4,76)\textequals3.46, p\textless.05, $\omega$\mbox{$^2$}\textequals.11) and their interaction (F(8,152)\textequals2.12, p\textless.05, $\omega$\mbox{$^2$}\textequals.05). With neutral images, yellow tinted lens resulted in largest LPP compared to none. With pleasant and unpleasant images, red tinted lens diminished the LPP response relative to none. To conclude, red tinted lenses appear to suppress an arousal response to high valence images. Interestingly, individuals with meditation experience have similarly been reported to reduce LPP responses to negative images (Sobolewski et al., 2011).}

{Absolute quantification is a challenge with many paths to reach the final goal of quantifying metabolites in absolute units (e.g. Molarity and molality). Utilizing an external reference standard (ERF) is an attractive method for quantifying in vivo metabolites due to the ability for direct comparison between a known concentration of a metabolite and the in vivo data. A major concern in utilization of an ERF is the differences in coil loading between in vivo and in vitro measurements. To that end, this work describes a method to calibrate and adjust the transmitter voltage in order to maximize signal detection independently of coil load.}

{Unmanned Aerial Vehicles\textquoteright (UAVs) ability to reach places not accessible to humans or other robots and execute tasks makes them unique and is gaining a lot of research interest recently. Initially UAVs were used as surveying and data collection systems, but lately UAVs are also efficiently employed in aerial manipulation and interaction tasks. In recent times, UAV interaction with the environment has become a common scenario, where manipulators are mounted on top of such systems. Current applications has driven towards the direction of UAVs and humans coexisting and sharing the same workspace, leading to the emerging futuristic domain of Human-UAV physical interaction. The research in this thesis initially addresses the delicate problem of external wrench (force/torque) estimation in aerial vehicles which is executable during flight without any additional sensors. Thereafter a novel architecture is proposed, allowing humans to physically interact with a UAV through the employment of sensor-ring structure and the developed external wrench estimator. The methodologies and algorithms to distinguish forces and torques derived by physical interaction with a human from the disturbance wrenches (due to e.g., wind) are defined through an optimization problem. Furthermore, an admittance-impedance control strategy is employed to act on them differently. This new hardware/software architecture allows for the safe human-UAV physical interaction through exchange of forces. But at the same time, other limitations such as the inability to exchange torques due to the underactuation of quadrotors and the need for a robust controller become evident. In order to improve the robust performance of the UAV, an adaptive super twisting sliding mode controller that works efficiently against parameter uncertainties, unknown dynamics and external perturbations is implemented. In addition, a novel fully actuated tilted propeller hexarotor UAV is designed along with the exact feedback linearization controller and the tilt angles are optimized in order to minimize power consumption, thereby improving the flight time. This fully actuated hexarotor could reorient while hovering and perform 6DoF (Degrees of Freedom) trajectory tracking. Eventually, the external wrench observer, interaction techniques, hardware design, software framework, the robust controller and the different methodologies are put together into the development of Human-UAV physical interaction with fully actuated Hexarotor UAV. This framework allows humans and UAVs to exchange forces as well as torques, becoming the next generation platform for the aerial manipulation and human physical interaction with UAVs.}

{Chemical exchange saturation transfer (CEST) and chemical exchange sensitive spinlock (CESL) were shown to have potential to provide molecular information for diagnosing a wide range of diseases. However, the lack of standardized acquisition protocols and freely available post-processing software prohibited the widespread application of these promising techniques until now. In this work, we present a modularly designed CEST/CESL preparation block that is easy to operate and can be used with arbitrary MRI readouts. Further, we developed and provide a C++ based open-source software that offers many CEST/CESL specific functionalities for the post-processing of the acquired data.}

Scanning transmission x-ray microscopy has been used to image electric currents in superconducting films at temperatures down to 20 K. We detect significant deviations from a regular current path driven by macroscopic geometrical constraints. The magnetic stray field of supercurrents in a thin YBaCuO film is mapped into a soft-magnetic coating of permalloy. The so-created local magnetization of the ferromagnetic film can be detected by dichroic absorption of polarized x rays. To enable high-quality measurements in transmission geometry, the whole heterostructure of ferromagnet, superconductor, and single-crystalline substrate has been thinned to an overall thickness of less than 1 µm. With this technique, local supercurrents can be analyzed in a wide range of temperatures and magnetic fields. The less than 100 nm spatial resolution of the magnetic signal together with simultaneously obtained nanostructural data allow the correlation of local supercurrents with the micro- and nanostructure of the superconducting film.

{This special interest group addresses the status quo of HCI research with regards to research practices of transparency and openness. Specifically, it discusses whether current practices are in line with the standards applied to other fields (e.g., psychology, economics, medicine). It seeks to identify current practices that are more progressive and worth communicating to other disciplines, while evaluating whether practices in other disciplines are likely to apply to HCI research constructively. Potential outcomes include: (1) a review of current HCI research policies, (2) a report on recommended practices, and (3) a replication project of key findings in HCI research.}

{Motor-based theories of facial expression recognition propose that the visual perception of facial expression is aided by sensorimotor processes that are also used for the production of the same expression. Accordingly, sensorimotor and visual processes should provide congruent emotional information about a facial expression. Here, we report evidence that challenges this view. Specifically, the repeated execution of facial expressions has the opposite effect on the recognition of a subsequent facial expression than the repeated viewing of facial expressions. Moreover, the findings of the motor condition, but not of the visual condition, were correlated with a nonsensory condition in which participants imagined an emotional situation. These results can be well accounted for by the idea that facial expression recognition is not always mediated by motor processes but can also be recognized on visual information alone.}

{Cholinergic neuromodulation is involved in all aspects of sensory processing and is crucial for processes such as attention, learning and memory, etc. However, despite the known roles of acetylcholine (ACh), we still do not how to disentangle ACh contributions from sensory or task-evoked changes in functional magnetic resonance imaging (fMRI). Here, we investigated the effects of local injection of ACh on fMRI and neural signals in the primary visual cortex (V1) of anesthetized macaques by combining pharmaco-based MRI (phMRI) with electrophysiological recordings, using single electrodes and electrode arrays. We found that local injection of ACh elicited two distinct profiles of fMRI and neurophysiological activity, depending on the distance from the injector. Near the injection site, we observed an increase in the baseline blood oxygen-level-dependent (BOLD) and cerebral blood flow (CBF) responses, while their visual modulation decreased. In contrast, further from the injection site, we observed an increase in the visually induced BOLD and CBF modulation without changes in baseline. Neurophysiological recordings suggest that the spatial correspondence between fMRI responses and neural activity does not change in the gamma, high-gamma, and multiunit activity (MUA) bands. The results near the injection site suggest increased inhibitory drive and decreased metabolism, contrasting to the far region. These changes are thought to reflect the kinetics of ACh and its metabolism to choline.}

{Introduction: For understanding proper functioning of the neural circuits, one has to consider dynamical changes in brain activation unfolding over time: distinct networks can be topographically similar, but differ from each other in terms of temporal dynamics (Pavlova, 2017). Time is a key to understanding the organization of functional brain networks, since brain topography alone does not allow us to understand neural communication in the brain. In this work, we analyzed temporal dynamics of the BOLD response to point-light biological motion. This analysis was motivated by a desire to characterize the functional role of the brain areas playing in unison at different time points and, thus, making up diverse neural circuits engaged in body motion processing. Our previous work (Pavlova et al. 2017) was limited to the analysis of the temporal dynamics of the BOLD response within the brain areas exhibiting significant activation during processing of upright and inverted body motion (BM). Here, we used several methods for uncovering ensembles of regions over the whole brain exhibiting similar temporal dynamics Methods: By using whole-brain coverage, we conducted fMRI recording at field strength of 9.4 tesla during processing of point-light BM. Participants were administered a 2-AFC task: they indicated whether an upright walker or control configurations (the same displays shown upside-down that participants were not informed about) were presented. We used several methods for uncovering ensembles of brain regions exhibiting similar temporal dynamics: (1) Temporal contrasts analysis (four 5s bins). Independent of display condition, temporal contrasts of each bin with respect to the weighted average of 3 other bins were evaluated, either with positive (for a relative increase) or negative (for a relative decrease) weights resulting in 8 temporal contrasts. Then t-values were calculated for each of these temporal contrasts at each peak of activation from the parameter estimates extracted for each individual participant (Pavlova et al. 2017). Significance (P \textless 0.01, unc.) was reached for 4 out of all 8 temporal contrasts in: Bin1inc [1, \textminus1/3, \textminus1/3, \textminus1/3], Bin2dec [1/3, \textminus1, 1/3, 1/3], Bin3dec [1/3, 1/3, \textminus1, 1/3], and Bin4inc [\textminus1/3, \textminus1/3, \textminus1/3, 1]. Here the whole brain search was performed for these bin-patterns. For each voxel t-values for all 4 possible temporal bin-patterns were calculated, and then the pattern with greatest t-value was assigned to this voxel. (2) Independent component analysis (ICA) based on temporal pattern (bin size, 1.3s \textequals TR). This analysis ((Calhoun et al., 2001) was motivated by a desire to uncover the temporal brain networks with finer temporal characteristics across the whole event epoch. (3) Kmeans clustering based on temporal pattern analysis (bin size, 5s). The contrast images were used to search for clusters of typical time courses by the kmeans clustering method (Lloyd, 1982), which partitions data into k mutually exclusive clusters with squared Euclidean metric to determine distances. The number of clusters was set to K\textequals6 cluster centers and the algorithm was repeated 10 times with different randomly chosen centroid seeds using the k-means++ algorithm. The solution with the lowest within-cluster sums of point-to-centroid distances was selected as the final outcome. Results: All three methods resulted in rather similar distributed large-scale networks playing in harmony. For example, the outcome of the temporal contrast analysis is represented in Figure 1. The network with an increase in Bin 1 is rather sparse, and comprises a few portions of the occipital and frontal cortices, whereas the large-scale network with an increase in Bin 4 is likely involved in decision making and executive functions. Conclusions: The benefits and disadvantages of each of 3 methods for uncovering of temporal dynamics of the large-scale networks are discussed. Future work should be focused on interactions between brain regions making up the social brain.}

{The prevalence of automation and user-adaptive systems has created a demand for human-machine interfaces that are designed to be aware and sensitive to the cognitive states of the user. We argue that the unitary concept of mental workload is insufficient in capturing the benefits that computing systems can deliver to their users. More specifically, we demonstrate with examples from our own research how to target more specific and robustly defined cognitive processes, with the use of non-obtrusive methods such as gaze-tracking, electroencephalography, and physiological measurements. Our examples, particularly in the evaluation of auditory notification design and in situ displays, will demonstrate that it is advantageous to target specific cognitive processes and mechanisms in accordance to the design purpose of a given interface, within the constraints of contemporary models of psychology and neuroscience.}

{This study investigates different parallel transmission (PTx) pulse design methods to find a universal PTx-pulse that excites the same local pattern with a 90 degree flip-angle across different heads. Thus, it abandons prospective the need for time-consuming subject specific B1+mapping and PTx-pulse calculation, during the scan session. The best results were achieved by solving a minimax optimization problem were the maximum normalized root mean square error (NRMSE) over all subjects was minimized. The resulting pulse created magnetization profiles with a maximum NRMSE of around 0.049 across all volunteers.}

{Faces that move contain rich information about facial form, such as facial features and their configuration, alongside the motion of those features. During social interactions, humans constantly decode and integrate these cues. To fully understand human face perception, it is important to investigate what information dynamic faces convey and how the human visual system extracts and processes information from this visual input. However, partly due to the difficulty of designing well-controlled dynamic face stimuli, many face perception studies still rely on static faces as stimuli. Here, we focus on evidence demonstrating the usefulness of dynamic faces as stimuli, and evaluate different types of dynamic face stimuli to study face perception. Studies based on dynamic face stimuli revealed a high sensitivity of the human visual system to natural facial motion and consistently reported dynamic advantages when static face information is insufficient for the task. These findings support the hypothesis that the human perceptual system integrates sensory cues for robust perception. In the present paper, we review the different types of dynamic face stimuli used in these studies, and assess their usefulness for several research questions. Natural videos of faces are ecological stimuli but provide limited control of facial form and motion. Point-light faces allow for good control of facial motion but are highly unnatural. Image-based morphing is a way to achieve control over facial motion while preserving the natural facial form. Synthetic facial animations allow separation of facial form and motion to study aspects such as identity-from-motion. While synthetic faces are less natural than videos of faces, recent advances in photo-realistic rendering may close this gap and provide naturalistic stimuli with full control over facial motion. We believe that many open questions, such as what dynamic advantages exist beyond emotion and identity recognition and which dynamic aspects drive these advantages, can be addressed adequately with different types of stimuli and will improve our understanding of face perception in more ecological settings.}

{Design recommendations for notifications are typically based on user performance and subjective feedback. In comparison, there has been surprisingly little research on how designed notifications might be processed by the brain for the information they convey. The current study uses EEG/ERP methods to evaluate auditory notifications that were designed to cue long-distance truck drivers for task-management and driving conditions, particularly for automated driving scenarios. Two experiments separately evaluated naive students and professional truck drivers for their behavioral and brain responses to auditory notifications, which were either auditory icons or verbal commands. Our EEG/ERP results suggest that verbal commands were more readily recognized by the brain as relevant targets, but that auditory icons were more likely to update contextual working memory. Both classes of notifications did not differ on behavioral measures. This suggests that auditory icons ought to be employed for communicating contextual information and verbal commands, for urgent requests.}

{The goal of this study is to develop 3D high resolution MRF scans and partial volume analysis methods to assist detection and characterization of epileptogenic foci in patients with drug refractory epilepsy undergoing presurgical evaluation. In addition to providing quantitative T1/T2 values for tissue characterization, we hypothesize that quantitative maps also provide better sensitivity in detecting subtle epileptic lesions. To this end, high resolution T1 and T2 maps, as well as gray matter, white matter fractions maps and tissue cluster maps, were used to detect and characterize epileptic lesions that were difficult to identify from the weighted images and from conventional voxel-based post-processing analysis based on T1-weighted images.}

{One major challenge of social interaction research is to achieve high experimental control over social interactions to allow for rigorous scientific reasoning. Virtual reality (VR) promises this level of control. Pan and Hamilton guide us with a detailed review on existing and future possibilities and challenges of using VR for social interaction research. Here, we extend the discussion to methodological and practical implications when using VR.}

{A number of neuroimaging studies have consistently reported significant activations in human somatosensory cortices during observation of touch actions. However, it is still debated which brain region is mainly associated with the processing of observed touch (e.g. primary somatosensory cortex; S1, secondary somatosensory cortex; S2, posterior parietal cortex; PPC). In this fMRI study, we searched for brain regions exhibiting neural activity patterns encoding visually evoked roughness intensities. Fifteen healthy volunteers with no deficits in tactile and visual processing participated. They first explored a set of differently colored sandpapers with their right index fingertip outside of the MR room. During the fMRI experiment, video clips of tactile explorations of the sandpaper set were presented and the participants were asked to recall the perceived roughness intensity as vividly as possible. The neural representations of the roughness intensities could be successfully decoded from the brain signals elicited by the video clips in the absence of any intrinsic tactile content. In particular, a random-effects group analysis revealed that four brain regions encoded the different roughness intensities distinctively: The bilateral PPC, the primary visual cortex (V1), and the ipsilateral S1. Although we found brain activations in ipsilateral S1, we cannot confirm the S1 engagement because the majority of previous studies have reported brain activations in contralateral S1. Significant decoding accuracies in V1 may be attributed to differences of visual contents in the presented video clips. Therefore, among the three brain regions mentioned above, our findings supported the hypothesis that especially the PPC plays an important role in the processing of observed touch.}

{The creation or streaming of photo-realistic self-avatars is important for virtual reality applications that aim for perception and action to replicate real world experience. The appearance and recognition of a digital self-avatar may be especially important for applications related to telepresence, embodied virtual reality, or immersive games. We investigated gender differences in the use of visual cues (shape, texture) of a self-avatar for estimating body weight and evaluating avatar appearance. A full-body scanner was used to capture each participant\textquotesingles body geometry and color information and a set of 3D virtual avatars with realistic weight variations was created based on a statistical body model. Additionally, a second set of avatars was created with an average underlying body shape matched to each participant\textquotesingles height and weight. In four sets of psychophysical experiments, the influence of visual cues on the accuracy of body weight estimation and the sensitivity to weight changes was assessed by manipulating body shape (own, average) and texture (own photo-realistic, checkerboard). The avatars were presented on a large-screen display, and participants responded to whether the avatar\textquotesingles weight corresponded to their own weight. Participants also adjusted the avatar\textquotesingles weight to their desired weight and evaluated the avatar\textquotesingles appearance with regard to similarity to their own body, uncanniness, and their willingness to accept it as a digital representation of the self. The results of the psychophysical experiments revealed no gender difference in the accuracy of estimating body weight in avatars. However, males accepted a larger weight range of the avatars as corresponding to their own. In terms of the ideal body weight, females but not males desired a thinner body. With regard to the evaluation of avatar appearance, the questionnaire responses suggest that own photo-realistic texture was more important to males for higher similarity ratings, while own body shape seemed to be more important to females. These results argue for gender-specific considerations when creating self-avatars.}

{Recent work indicates that the central nervous system forms multisensory perceptions differently depending on inferred signal causality. In accordance with these findings, we hypothesize that multisensory perception of traveled distance in the horizontal plane conforms to such Causal Inference (CI). Participants (n\textequals13) were seated in the Max Planck Cablerobot Simulator, and shown a photo-realistic rendering of the simulator hall via a Head-Mounted Display. Using this setup, they were presented various unisensory and (incongruent) multisensory visual-inertial horizontal linear surge motions, differing only in amplitude (i.e., distance). Participants performed both a Magnitude Estimation and a Two-Interval Forced Choice task. We modeled the responses in the tasks according to a CI model, as well as competing models (Cue Capture, Forced Fusion), and compared the models based on their fits. The data indicate that distance is somewhat underestimated for both the visual and inertial unisensory channels, and that differential thresholds increase with physical distance -in accordance with a Weber\textquoterights law. Preliminary findings on model comparisons favor different models in different individuals, with an overall preference for the CI model. However, the data also suggest that different priors may be needed to account for differences between the tasks.}

{The von Economo and fork neurons are atypical cortical projection neurons. They occur in insular and cingulate cortical areas that interface homeostatic bodily states with high-order brain functions. Their recent discovery in the macaque monkey offers a unique opportunity to examine the neurocellular basis of interoception in cognitive processes including perceptual awareness and perhaps language.}

{It is currently not well understood whether people experience themselves to be located in one or more specific part(s) of their body. Virtual reality (VR) is increasingly used as a tool to study aspects of bodily perception and self-consciousness, due to its strong experimental control and ease in manipulating multi-sensory aspects of bodily experience. To investigate where people self-locate in their body within virtual reality, we asked participants to point directly at themselves with a virtual pointer, in a VR headset. In previous work employing a physical pointer, participants mainly located themselves in the upper face and upper torso. In this study, using a VR headset, participants mainly located themselves in the upper face. In an additional body template task where participants pointed at themselves on a picture of a simple body outline, participants pointed most often to the upper torso, followed by the (upper) face. These results raise the question as to whether head-mounted virtual reality might alter where people locate themselves making them more \textquotedbllefthead-centred\textquotedblright.}

{We investigated the following three questions: 1) Where do people locate themselves on their body? 2) How precisely can people locate their body parts? and 3) Do people locate themselves differently in terms of their perceived compared to their physical body dimensions? Alsmith and Longo (2014) asked participants to point directly to themselves with a physical pointer. They found pointing to be to two distinct locations, upper face and upper torso. To investigate the robustness of their findings, we used virtual reality (VR), because this allows for systematic control over experimental variables and easy manipulation of visual information. In a VR headset and on a large-scale immersive display, participants rotated a pointer in their sagittal plane instructed to "Point directly to you", but also to nine of their body parts (feet, knees, hips, waist, shoulders, chin, nose, eyes and top of the head) previously measured for their physical heights. From the pointed-to body parts a perceived body was constructed, to which the self-locations were alternatively scaled/normalized. Pointing to self relative to the physical body was frequently found for all body regions above mid-torso, as well as above the head (Supplement, left graph). Participants pointed precisely to many body parts, but not to feet and knees, nor to the top of the head. Relative to the perceived body, pointing to self resembled more the results from the earlier physical setup, that is participants pointed mainly to upper torso and the face (Supplement, right graph). These results suggest, that a) people do not have one specific location where they locate themselves, and b) people do not accurately point to their entire body in the vertical plane.}

{Whole-brain metabolite mapping with volumetric coverage is a valuable tool for the evaluation of metabolite levels across different regions in the human brain. Previously, several whole-brain 1H MRSI studies have been performed at 3T e.g. 1. 2 using the EPSI sequence 3. However, so far, similar studies have not been done at higher field strengths despite the many advantages of higher fields for spectroscopy applications. In this study, we combine a robust in/plane acceleration technique with dynamic slice-wise B0 shim updating and present whole-brain metabolite maps acquired with a high resolution at 9.4T.}

{In this work, we present high resolution whole-brain metabolite maps acquired at 9.4T for the first time. By combining a robust acceleration method with dynamic B0 slice-wise shim updating, we achieved high quality metabolite maps from 10 slices of the brain with a nominal voxel size of \textasciitilde80$\mu$L in \textasciitilde25 minutes.}

{Understanding the underlying mechanisms of the human brain in health and disease will require models with necessary and sufficient details to explain how function emerges from the underlying anatomy and is shaped by neuromodulation. Here, we provide such a detailed causal explanation using a whole-brain model integrating multimodal imaging in healthy human participants undergoing manipulation of the serotonin system. Specifically, we combined anatomical data from diffusion magnetic resonance imaging (dMRI) and functional magnetic resonance imaging (fMRI) with neurotransmitter data obtained with positron emission tomography (PET) of the detailed serotonin 2A receptor (5-HT2AR) density map. This allowed us to model the resting state (with and without concurrent music listening) and mechanistically explain the functional effects of 5-HT2AR stimulation with lysergic acid diethylamide (LSD) on healthy participants. The whole-brain model used a dynamical mean-field quantitative description of populations of excitatory and inhibitory neurons as well as the associated synaptic dynamics, where the neuronal gain function of the model is modulated by the 5-HT2AR density. The model identified the causative mechanisms for the non-linear interactions between the neuronal and neurotransmitter system, which are uniquely linked to (1) the underlying anatomical connectivity, (2) the modulation by the specific brainwide distribution of neurotransmitter receptor density, and (3) the non-linear interactions between the two. Taking neuromodulatory activity into account when modeling global brain dynamics will lead to novel insights into human brain function in health and disease and opens exciting possibilities for drug discovery and design in neuropsychiatric disorders.}

{This workshop discusses the balance between safety and productivity as automated vehicles turn into \textquotesinglemobile offices\textquotesingle: spaces where non-driving activities are performed during one\textquotesingles daily commute. Technological developments reduce the active role of the human driver that might, nonetheless, require occasional intervention. To what extent are drivers allowed to dedicate resources to non-driving work-related activities? To address this critical question, the workshop brings together a diverse community of researchers and practitioners that are interested in questions as follows: what non-driving activities are likely to be performed on one\textquotesingles way to work and back; what is a useful taxonomy of these tasks; how can various tasks be studied in experimental settings; and, what are the criteria to assess human performance in automated vehicles. To foster further dialogue, the outcome of the workshop will be an online blog where attendees can contribute their own thoughts: https://medium.com/the-mobile-office.}

We study both experimentally and theoretically the dynamics of chemically self-propelled Janus colloids moving atop a two-dimensional crystalline surface. The surface is a hexagonally close-packed monolayer of colloidal particles of the same size as the mobile one. The dynamics of the self-propelled colloid reflects the competition between hindered diffusion due to the periodic surface and enhanced diffusion due to active motion. Which contribution dominates depends on the propulsion strength, which can be systematically tuned by changing the concentration of a chemical fuel. The mean-square displacements obtained from the experiment exhibit enhanced diffusion at long lag times. Our experimental data are consistent with a Langevin model for the effectively two-dimensional translational motion of an active Brownian particle in a periodic potential, combining the confining effects of gravity and the crystalline surface with the free rotational diffusion of the colloid. Approximate analytical predictions are made for the mean-square displacement describing the crossover from free Brownian motion at short times to active diffusion at long times. The results are in semi-quantitative agreement with numerical results of a refined Langevin model that treats translational and rotational degrees of freedom on the same footing.

Finding optimal feedback controllers for nonlinear dynamic systems from data is hard. Recently, Bayesian optimization (BO) has been proposed as a powerful framework for direct controller tuning from experimental trials. For selecting the next query point and finding the global optimum, BO relies on a probabilistic description of the latent objective function, typically a Gaussian process (GP). As is shown herein, GPs with a common kernel choice can, however, lead to poor learning outcomes on standard quadratic control problems. For a first-order system, we construct two kernels that specifically leverage the structure of the well-known Linear Quadratic Regulator (LQR), yet retain the flexibility of Bayesian nonparametric learning. Simulations of uncertain linear and nonlinear systems demonstrate that the LQR kernels yield superior learning performance.