Neurostimulation Reveals Context-Dependent Arbitration Between Model-Based and Model-Free Reinforcement Learning.

While it is established that humans use model-based (MB) and model-free (MF) reinforcement learning in a complementary fashion, much less is known about how the brain determines which of these systems should control behavior at any given moment. Here we provide causal evidence for a neural mechanism that acts as a context-dependent arbitrator between both systems. We applied excitatory and inhibitory transcranial direct current stimulation over a region of the left ventrolateral prefrontal cortex previously found to encode the reliability of both learning systems. The opposing neural interventions resulted in a bidirectional shift of control between MB and MF learning. Stimulation also affected the sensitivity of the arbitration mechanism itself, as it changed how often subjects switched between the dominant system over time. Both of these effects depended on varying task contexts that either favored MB or MF control, indicating that this arbitration mechanism is not context-invariant but flexibly incorporates information about current environmental demands.

Measuring visual perspective taking in the brain with avatars and arrows: Which question are we asking?

There is an ongoing debate about the involvement of Theory of Mind (ToM) processes in Visual Perspective Taking (VPT). In an fMRI study (Schurz et al., 2015), we borrowed the positive features from a novel VPT task - which is widely used in behavioral research - to study previously overlooked experimental factors in neuroimaging studies. However, as Catmur et al. (2016) rightly argue in a comment on our work, our data do not speak strongly to questions discussed in the original behavioral studies, in particular the issue of implicit mentalizing. We appreciate the clarification of these interpretational limitations of our study, but would like to point out the differences between questions emerging from behavioral and neuroimaging research on VPT. Different from what Catmur et al. (2016) discuss, our study was not intended as a test of implicit mentalizing. In fact, the terms "automatic" and "implicit mentalizing" were never mentioned in our manuscript. Our study addressed a methodological gap between ToM and VPT research, which we identified in two previous meta-analyses on the topics (Schurz et al., 2013, 2014). With this difference in mind we show that the critical points levelled by Catmur et al. (2016) cease to apply.

Insufficient sleep: Enhanced risk-seeking relates to low local sleep intensity.

OBJECTIVES: Chronic sleep restriction is highly prevalent in modern society and is, in its clinical form, insufficient sleep syndrome, one of the most prevalent diagnoses in clinical sleep laboratories, with substantial negative impact on health and community burden. It reflects every-day sleep loss better than acute sleep deprivation, but its effects and particularly the underlying mechanisms remain largely unknown for a variety of critical cognitive domains, as, for example, risky decision making. METHODS: We assessed financial risk-taking behavior after 7 consecutive nights of sleep restriction and after 1 night of acute sleep deprivation compared to a regular sleep condition in a within-subject design. We further investigated potential underlying mechanisms of sleep-loss-induced changes in behavior by high-density electroencephalography recordings during restricted sleep. RESULTS: We show that chronic sleep restriction increases risk-seeking, whereas this was not observed after acute sleep deprivation. This increase was subjectively not noticed and was related to locally lower values of slow-wave energy during preceding sleep, an electrophysiological marker of sleep intensity and restoration, in electrodes over the right prefrontal cortex. INTERPRETATION: This study provides, for the first time, evidence that insufficient sleep restoration over circumscribed cortical areas leads to aberrant behavior. In chronically sleep restricted subjects, low slow-wave sleep intensity over the right prefrontal cortex-which has been shown to be linked to risk behavior-may lead to increased and subjectively unnoticed risk-seeking. Ann Neurol 2017;82:409-418.

Clarifying the role of theory of mind areas during visual perspective taking: Issues of spontaneity and domain-specificity.

Visual perspective taking is a fundamental feature of the human social brain. Previous research has mainly focused on explicit visual perspective taking and contrasted brain activation for other- versus self-perspective judgements. This produced a conceptual gap to theory of mind studies, where researchers mainly compared activation for taking another's mental perspective to non-mental control conditions. We compared brain activation for visual perspective taking to activation for non-mental control conditions where the avatar was replaced by directional (arrow, lamp) or non-directional (brick-wall) objects. We found domain-specific activation linked to the avatar's visual perspective in right TPJ, ventral mPFC and ventral precuneus. Interestingly, we found that these areas are spontaneously processing information linked to the other's perspective during self-perspective judgements. Based on a review of the visual perspective taking literature, we discuss how these findings can explain some of the inconsistent/negative results found in previous studies comparing other- versus self-perspective judgements.