Serotonin is involved in a wide range of mental capacities essential for navigating the social world, including emotion and impulse control. Much recent work on serotonin and social functioning has focused on decision-making. Here we investigated the influence of serotonin on human emotional reactions to social conflict. We used a novel computerised task that required mentally simulating social situations involving unjust harm and found that depleting the serotonin precursor tryptophan—in a double-blind randomised placebo-controlled design—enhanced emotional responses to the scenarios in a large sample of healthy volunteers (n = 73), and interacted with individual differences in trait personality to produce distinctive human emotions. Whereas guilt was preferentially elevated in highly empathic participants, annoyance was potentiated in those high in trait psychopathy, with medium to large effect sizes. Our findings show how individual differences in personality, when combined with fluctuations of serotonin, may produce diverse emotional phenotypes. This has implications for understanding vulnerability to psychopathology, determining who may be more sensitive to serotonin-modulating treatments, and casts new light on the functions of serotonin in emotional processing.

Introduction: With the rise of camera phones, selfie-taking has become a normative part of our modern culture. However, little is known about how this behavior may relate to eating disorder (ED) characteristics, particularly in those who already have eating disorder symptoms of clinical severity. The current study investigated how selfie-posting and selfie-taking with no intention of posting online (offline selfies) were related to ED symptoms. Method: A total of 152 females (average age 22.44 years) with ED symptoms of clinical severity completed selfreport questionnaires measuring selfie-frequency (online and offline), frequency of non-selfie photo posting, social networking site use, body dissatisfaction, body checking, ED symptom severity, self-esteem and body avoidance. Responses were collected via an ED social community. Results: No direct relationship, or indirect association via body dissatisfaction, was found between selfie behavior and ED symptom severity. However, the more offline selfies an individual took, the more frequently they body checked, and this, in turn, was related to greater ED symptom severity. Conclusions: These results suggest that offline selfies may be a modern form of body checking. Our findings are the first to imply that offline selfie-taking may be a problematic behavior and a potential maintenance factor for individuals with severe ED symptoms. As newcomers to the media landscape, social networking sites (SNS) have introduced new forms of self-expression and communication to our online world. On communication platforms like SNS, creating personal profiles and interacting with others is facilitated by online functions such as public status updates, private messaging, photo sharing, and group pages. Engaging on SNS has become a popular passtime with approximately 90% of young adults in the US using an SNS account (Perrin et al., 2015). More recent numbers showed that in 2018 more than 95% of the Dutch individuals between 18 and 35 was active on Internet (e.g. social media; Central Bureau for Statistics, 2019). Several studies have found that those who spend more time on SNS also reported more body image concerns (for a review see: Holland & Tiggemann, 2016; Marengo, Longobardi, Fabris, & Settanni, 2018). Focusing on photo-related activity (e.g., posting and commenting on photos), adolescent girls who were more engaged with photos on SNS also reported higher levels of body image disturbance (Meier & Gray, 2014). Similarly, frequent use of 'highly visible' social media, such as Instagram and Snapchat which are image based platforms, has been associated with increased body image concerns (Marengo et al., 2018).This suggests that engaging with image content on social media may be problematic for maintaining a healthy body image. Selfie posting has become a popular activity on SNS, especially amongst young adults, with females being the primary subject of selfies around the world (Souza et al., 2015; Tiggemann & Slater, 2013). We propose that the word 'selfie' has expanded its roots in ...

Serotonin is involved in updating responses to changing environmental circumstances. Optimising behaviour to maximise reward and minimise punishment may require shifting strategies upon encountering new situations. Likewise, autonomic responses to threats are critical for survival yet must be modified as danger shifts from one source to another. Whilst numerous psychiatric disorders are characterised by behavioural and autonomic inflexibility, few studies have examined the contribution of serotonin in humans. We modelled both processes, respectively, in two independent experiments (N = 97). Experiment 1 assessed instrumental (stimulus-response-outcome) reversal learning whereby individuals learned through trial and error which action was most optimal for obtaining reward or avoiding punishment initially, and the contingencies subsequently reversed serially. Experiment 2 examined Pavlovian (stimulus-outcome) reversal learning assessed by the skin conductance response: one innately threatening stimulus predicted receipt of an uncomfortable electric shock and another did not; these contingencies swapped in a reversal phase. Upon depleting the serotonin precursor tryptophan—in a double-blind randomised placebo-controlled design—healthy volunteers showed impairments in updating both actions and autonomic responses to reflect changing contingencies. Reversal deficits in each domain, furthermore, were correlated with the extent of tryptophan depletion. Initial Pavlovian conditioning, moreover, which involved innately threatening stimuli, was potentiated by depletion. These results translate findings in experimental animals to humans and have implications for the neurochemical basis of cognitive inflexibility.

Serotonin is critically involved in flexible decision-making, yet the preponderance of evidence comes from non-human studies. Many insights about the serotonin's function have come from the technique of dietary acute tryptophan depletion (ATD). However, it is unclear whether ATD modulates choice behavior during probabilistic reversal learning (PRL), a widely used model of behavioral flexibility with significant translational and clinical value. To determine whether ATD affects latent choice tendencies, we applied computational models of reinforcement learning to PRL data from healthy human volunteers (n = 62; 29 females) who had received either ATD or placebo in a randomized, double-blind, placebo-controlled procedure. A secondary objective was to ascertain how model parameters related to clinically relevant self-report questionnaires. ATD did not affect the updating ("learning rates") or deployment ("reinforcement sensitivity") of value representations driving choice. However, ATD increased "stimulus stickiness", the tendency to choose a previously chosen visual stimulus again, regardless of reinforcement--a pattern seen in stimulant use disorder (SUD). Greater subclinical obsessive-compulsive symptoms were associated with lower stimulus stickiness, a pattern seen in clinically diagnosed obsessive-compulsive disorder (OCD). Low reinforcement sensitivity was instead related to intolerance of uncertainty and symptoms of depression and anxiety. Individuals with higher trait impulsivity showed lower reward and higher punishment learning rates, which parallels findings in SUD. Collectively, these results point to a role for serotonin in compulsive tendencies. They underscore the utility of computational modelling in illuminating the microstructure of behavior, which could point towards new markers of vulnerability to psychopathology.

The involvement of serotonin in responses to negative feedback is well established. Acute serotonin reuptake inhibition has enhanced sensitivity to negative feedback (SNF), modelled by behaviour in probabilistic reversal learning (PRL) paradigms. Whilst experiments employing acute tryptophan depletion (ATD) in humans, to reduce serotonin synthesis, have shown no clear effect on SNF, sample sizes have been small. We studied a large sample of healthy volunteers, male and female, and found ATD had no effect on core behavioural measures in PRL. These results indicate that ATD effects can differ from other manipulations of serotonin expected to have a parallel or opposing action.

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Serotonin is implicated in aversive processing and updating responses to changing environmental circumstances. Optimising behaviour to maximise reward and minimise punishment may require shifting strategies upon encountering new situations. Likewise, emotional reactions to threats are critical for survival yet must be modified as danger shifts from one source to another. Whilst numerous psychiatric disorders are characterised by behavioural and emotional inflexibility, few studies have examined the contribution of serotonin in humans. We modelled both processes in two independent experiments (N = 97), using instrumental and aversive Pavlovian reversal learning paradigms, respectively.Upon depleting the serotonin precursor tryptophan -in a double-blind randomised placebocontrolled design -healthy volunteers showed impairments in updating both behaviour and emotion to reflect changing contingencies. Reversal deficits in each domain, furthermore, were correlated with the extent of tryptophan depletion. These results translate findings in experimental animals to humans and have implications for the neurochemical basis of cognitive inflexibility.

Background Responding emotionally to danger is critical for survival. Normal functioning also requires flexible alteration of emotional responses when a threat becomes safe. Aberrant threat and safety learning occur in many psychiatric disorders, including posttraumatic stress disorder, obsessive-compulsive disorder, and schizophrenia, in which emotional responses can persist pathologically. While there is evidence that threat and safety learning can be modulated by the serotonin systems, there have been few studies in humans. We addressed a critical clinically relevant question: How does lowering serotonin affect memory retention of conditioned threat and safety memory? Methods Forty-seven healthy participants underwent conditioning to two stimuli predictive of threat on day 1. One stimulus but not the other was subsequently presented in an extinction session. Emotional responding was assessed by the skin conductance response. On day 2, we employed acute dietary tryptophan depletion to lower serotonin temporarily, in a double-blind, placebo-controlled, randomized between-groups design. We then tested for the retention of conditioned threat and extinction memory. We also measured self-reported intolerance of uncertainty, known to modulate threat memory expression. Results The expression of emotional memory was attenuated in participants who had undergone tryptophan depletion. Individuals who were more intolerant of uncertainty showed even greater attenuation of emotion following depletion. Conclusions These results support the view that serotonin is involved in predicting aversive outcomes and refine our understanding of the role of serotonin in the persistence of emotional responsivity, with implications for individual differences in vulnerability to psychopathology.