Acute tryptophan depletion attenuates brain-heart coupling following external feedback

External and internal performance feedback triggers neural and visceral modulations such as reactions in the medial prefrontal cortex and insulae or changes of heart period (HP). The functional coupling of neural and cardiac responses following feedback (cortico-cardiac connectivity) is not well understood. While linear time-lagged within-subjects correlations of single-trial EEG and HP (cardio-electroencephalographic covariance tracing, CECT) indicate a robust negative coupling of EEG magnitude 300 ms after presentation of an external feedback stimulus with subsequent alterations of heart period (the so-called N300H phenomenon), the neurotransmitter systems underlying feedback-evoked cortico-cardiac connectivity are largely unknown. Because it has been shown that acute tryptophan depletion (ATD), attenuating brain serotonin (5-HT), decreases cardiac but not neural correlates of feedback processing, we hypothesized that 5-HT may be involved in feedback-evoked cortico-cardiac connectivity. In a placebo-controlled double-blind cross-over design, 12 healthy male participants received a tryptophan-free amino-acid drink at one session (TRP−) and a balanced amino-acid control-drink (TRP+) on another and twice performed a time-estimation task with feedback presented after each trial. N300H magnitude and plasma tryptophan levels were assessed. Results indicated a robust N300H after TRP+, which was significantly attenuated following TRP−. Moreover, plasma tryptophan levels during TRP+ were correlated with N300H amplitude such that individuals with lower tryptophan levels showed reduced N300H. Together, these findings indicate that 5-HT is important for feedback-induced covariation of cortical and cardiac activity. Because individual differences in anxiety have previously been linked to 5-HT, cortico-cardiac coupling and feedback processing, the present findings may be particularly relevant for futures studies on the relationship between 5-HT and anxiety

The Role of Plant Litter in Driving Plant-Soil Feedbacks

Most studies focusing on plant-soil feedbacks (PSFs) have considered direct interactions between plants, abiotic conditions (e. g., soil nutrients) and rhizosphere communities (e.g., pathogens, mutualists). However, few studies have addressed the role of indirect interactions mediated by plant litter inputs. This is problematic because it has left a major gap in our understanding of PSFs in natural ecosystems, where plant litter is a key component of feedback effects. Here, we propose a new conceptual framework that integrates rhizosphere- and litter-mediated PSF effects. Our framework provides insights into the relative contribution of direct effects mediated by interactions between plants and soil rhizosphere organisms, and indirect effects between plants and decomposer organisms mediated by plant root and shoot litter. We distinguish between three pathways through which senesced root and shoot litter may influence PSFs. Specifically, we examine: (1) physical effects of litter (layer) traits on seed germination, soil structure, and plant growth; (2) chemical effects of litter on concentrations of soil nutrients and secondary metabolites (e.g., allelopathic chemicals); and (3) biotic effects of saprotrophic soil communities that can perform different functional roles in the soil food web, or that may have specialized interactions with litter types, thereby altering soil nutrient cycling. We assess the role of litter in PSF effects via physical, chemical and biotic pathways to address how litter-mediated feedbacks may play out relative to, and in interaction with, feedbacks mediated through the plant rhizosphere. We also present one of the first experimental studies to show the occurrence and species-specificity of litter-mediated feedbacks and we identify critical research gaps. By formally incorporating the plant-litter feedback pathway into PSF experiments, we will further our understanding of PSFs under natural conditions

Essays in Applied Economics

Essay 1 studies physician agency problems, which arise whenever physicians fail to maximize their patients\u27 preferences, given available information. These agency problems are well documented, but the magnitude of their welfare consequences for patients---the losses from suboptimal treatment choice induced by agency---are unclear. I infer patient drug preference from their compliance decisions. I begin by showing that initial prescriptions respond to physician financial incentives to control costs and to pharmaceutical detailing, but compliance does not, pointing to agency problems. I then develop and estimate a model of physician-patient interactions where physician write initial prescriptions, but patients choose whether to comply. Fully eliminating agency problems increases compliance by 6.5 percentage points, and raises patient welfare by 22\% of drug spending. Contracts that better align doctor and patient preferences can improve patient welfare, but attain only half the gains from eliminating agency completely. Although physician agency problems reduce patient welfare, eliminating them is thus likely difficult. Essay 2, co-authored with Alexander M. Gelber and Damon Jones, studies frictions in adjusting earnings to changes in the Social Security Annual Earnings Test (AET) using a panel of Social Security Administration microdata on one percent of the U.S. population from 1961 to 2006. Individuals continue to bunch at the convex kink the AET creates even when they are no longer subject to the AET, consistent with the existence of earnings adjustment frictions in the U.S. We develop a novel estimation framework and estimate in a baseline case that the earnings elasticity with respect to the implicit net-of-tax share is 0.23, and the fixed cost of adjustment is \$152.08. Essay 3 studies the impact of health expenditure risk on annuitization. Theoretical research suggests that such risk can have an ambiguous influence on the annuitization decisions of the elderly. I provide empirical evidence on this linkage, by estimating the impact of supplemental Medicare insurance (Medigap) coverage on the annuity demand of older Americans. Medigap coverage has a strong impact on annuitization: the extensive margin elasticity is 0.39, the overall elasticity of private annuity income with respect to Medigap coverage is 0.56. These results are robust to controls for health, wealth, and preferences, as well as other robustness tests. They suggest that medical expenditure risk has a large impact on underannuitization

Dissociation between medial frontal negativity and cardiac responses in the ultimatum game: Effects of offer size and fairness

In the present study, we examined the role of fairness and offer size on brain and cardiac responses in the ultimatum game (UG). Twenty healthy volunteers played the role of responder in a computerized version of the UG in which the fairness and size of the offers were systematically varied. Both fairness and size of the offer influenced the acceptance rates in a predictable way, leading to fewer accepted unfair and low offers. Only unfair high, but not unfair low offers were accompanied by a medial frontal negativity. An unexpected stronger cardiac deceleration to fairer offers was found, which was not affected by the size of the offers. Cardiac and electrocortical measures showed a different relation with performance, and both measures were correlated only modestly. This dissociation between cardiac responses and brain potentials is discussed in terms of a possible differential sensitivity to effects of stimulus probability and violation of the social rules

Blood Levels of Serotonin Are Differentially Affected by Romantic Love in Men and Women

People who are in love think about their beloved the whole day, which is supposed to be associated with serotonin. The research questions were how peripheral serotonin levels differ between individuals that are in love compared to individuals that are not in love, and how these levels are associated with obsessive thinking about the beloved. Because gender differences exist in romantic love and in the serotonergic system, gender differences were tested. Twenty participants who were in love (10 men) and 20 who were not in love (10 men) completed several questionnaires, and plasma and serum serotonin levels were assessed. The men who were in love had lower serotonin levels than the men who were not in love, while women showed the opposite pattern. The participants that were in love reported obsessive thinking about their beloved for 65% of their waking hours. In women, obsessive thinking about the beloved was associated with an increased serotonin level in serum. The serotonergic system seems to play a role in romantic love, but the effects appear opposite for men and women