Systematic review of the effects of acute stress in binge eating disorder.

Binge eating disorder (BED) is characterized by recurrent episodes of eating an excessive amount of food over a discrete time period, while feeling a loss of control over one's eating. Although stress is one of the most commonly reported triggers of binge eating in individuals with BED, there has been little work examining the stress response specifically in individuals with the disorder. In this review, we examine what is known about how individuals with BED respond to acute stressors. A systematic literature search identified 14 relevant articles that report on the effects of experimentally induced stress on objective measures. Dependent measures that have been examined include changes in the levels of hormones such as cortisol and ghrelin, cardiovascular function, ad libitum food intake and eating rate. In this review, we describe the published findings and discuss their implications in the context of the wider literature. Overall, we found partial evidence that BED is associated with a heightened response to stress. Given the inconsistencies between studies, we suggest that reported differences between individuals with and without BED might be driven by factors that are correlated with, but not specific to, BED. We suggest that two priorities for this research area are to identify factors that modulate the stress response in individuals with BED, and to address the underrepresentation of males in this literature.

Observing painful events in others leads to a temporally extended general response facilitation in the self.

Excitability in the motor cortex is modulated when we observe other people receiving a painful stimulus (Avenanti et al., Nat Neurosci 8(7):955-960, 2005). However, the task dependency of this modulation is not well understood, as different paradigms have yielded seemingly different results. Previous neurophysiological work employing transcranial magnetic stimulation (TMS) suggests that watching another person's hand being pierced by a needle leads to a muscle specific inhibition, assessed via motor evoked potentials. Results from previous behavioural studies suggest that overt behavioural responses are facilitated due to pain observation (Morrison et al., Cereb Cortex 17:2214-2222, 2007b; Morrison et al., Cognition 104:407-416, 2007a). There are several paradigmatic differences both between typical TMS studies and behavioural studies, and within behavioural studies themselves, that limit our overall understanding of how pain observation affects the motor system. In the current study, we combine elements of typical TMS experimental designs in a behavioural assessment of how pain observation affects overt behavioural responding. Specifically, we examined the muscle specificity, timing, and direction of modulation of motor responses due to pain observation. To assess muscle specificity, we employed pain and non-pain videos from previous TMS studies in a Go/No-Go task in which participants responded by either pressing a key with their index finger or with their foot. To assess timing, we examined response times for Go signals presented at 0 or 500 ms after the video. Results indicate that observation of another individual receiving a painful stimulus leads to a non-effector specific, temporally extended response facilitation (e.g., finger and foot facilitation present at 0 and 500 ms delays), compared to observation of non-pain videos. This behavioural facilitation effect differs from the typical motor inhibition seen in TMS studies, and we argue that the effects of pain observation on the motor system are state-dependent, with different states induced via task instructions. We discuss our results in light of previous work on motor responses to pain observation.

Effects of intentional movement preparation on response times to symbolic and imitative cues.

Speeded responses to an external cue are slower when the cue interrupts preparation to perform the same or a similar action in a self-paced manner. To explore the mechanism underlying this 'cost of intention', we examined whether the size of the cost is influenced by the nature of the external cue. Specifically, we assessed whether the cost of intention is different for movements made in response to an imitative cue (an on-screen hand movement) compared to those made in response to a symbolic cue. Consistent with previous reports, externally cued responses were significantly slower on trials where participants were preparing to perform an internally driven movement later in the trial. Also as predicted, simple response times to the imitative cue were faster than those made to the symbolic cue. Critically, the cost of intention was similar for each cue type, suggesting that preparing an intentional action influenced responses cued by the symbolic and imitative cues to a similar degree. These findings suggest that the nature of the external cue does not influence the response time delay associated with concurrent intentional preparation. Together with previous findings, the results of the current study shed further light on the potential mechanisms underlying the cost of intention.

Self-selected conscious strategies do not modulate motor cortical output during action observation.

The human motor system is active not only when actions are performed but also when they are observed. Experimenters often manipulate aspects of the action or context to examine factors that influence this "mirror" response. However, little is known about the role of the observer's own top-down intentions and motivation. In this exploratory study, we investigated whether observers are able to exert conscious control over their mirror response, when they are explicitly instructed to either increase or decrease mirroring. Transcranial magnetic stimulation (TMS) was used to elicit motor-evoked potentials (MEPs) in a thumb abductor muscle as participants (n = 13) watched a video of a hand squeezing a rubber ball. The size of these MEPs, relative to the size of MEPs elicited during fixation cross observation, was taken as an index of mirroring. In an initial block of trials, participants were instructed to merely observe the actions presented. After the first block, the concept of mirroring was explained to the participants, and in the second and third blocks participants were instructed to either increase or decrease their mirror response. We did not instruct them about how to achieve this increase or decrease. Our results showed no difference in either facilitation or absolute motor excitability (i.e., nonnormalized MEP size) between the three blocks, indicating that individuals do not seem to be able to exert control over motor excitability during action observation, at least in the absence of a specific and maintained strategy.

Timing and specificity of early changes in motor excitability during movement observation.

Motor resonance is typically considered a modulation of motor activity during action observation, which mirrors the pattern of muscle activation occurring during execution of the observed action. There is preliminary evidence that motor activity accompanying action observation comprises two stages: an initial non-specific modulation, followed by a later change in excitability that is specific to the muscles involved in the action. We built on this previous work, with the aims of (1) clarifying the time-course of muscle specificity during action observation and (2) establishing whether modulation at each time point reflects significant changes (facilitation or suppression) from baseline. Motor-evoked potentials were elicited in two muscles-the first dorsal interosseous (FDI) and abductor digiti minimi (ADM)-while participants watched stimuli showing index or little finger abduction. Our results provide partial support for previous findings of muscle-specific modulation after, but not before, 200 ms. Interestingly, however, this was shown only by facilitation of the FDI during index finger observation; there was no significant facilitation of the ADM when little finger movement was observed. Furthermore, we found no facilitation of corticospinal excitability (specific or non-specific) at time points earlier than 200 ms (100-150 ms). We discuss these results in the context of previous work showing a non-specific increase in excitability at 90 ms during action observation. Together, these findings suggest a fast and short-lived facilitation of corticospinal excitability that occurs at around 60-90 ms and ends by 100-150 ms.

To eat or not to eat? Kinematics and muscle activity of reach-to-grasp movements are influenced by the action goal, but observers do not detect these differences.

Recent evidence suggests that the mirror neuron system responds to the goals of actions, even when the end of the movement is hidden from view. To investigate whether this predictive ability might be based on the detection of early differences between actions with different outcomes, we used electromyography (EMG) and motion tracking to assess whether two actions with different goals (grasp to eat and grasp to place) differed from each other in their initial reaching phases. In a second experiment, we then tested whether observers could detect early differences and predict the outcome of these movements, based on seeing only part of the actions. Experiment 1 revealed early kinematic differences between the two movements, with grasp-to-eat movements characterised by an earlier peak acceleration, and different grasp position, compared to grasp-to-place movements. There were also significant differences in forearm muscle activity in the reaching phase of the two actions. The behavioural data arising from Experiments 2a and 2b indicated that observers are not able to predict whether an object is going to be brought to the mouth or placed until after the grasp has been completed. This suggests that the early kinematic differences are either not visible to observers, or that they are not used to predict the end-goals of actions. These data are discussed in the context of the mirror neuron system.

Discounting of delayed monetary and cannabis rewards in a crowdsourced sample of adults.

Excessive delayed reward discounting (DD) is observed across many addictive disorders. However, research on DD among cannabis users is limited, with even less research on discounting of cannabis rewards. This study examined monetary and cannabis reward discounting among cannabis and noncannabis users. A large sample of adults (N = 2,857) recruited from an online crowdsourcing platform was assessed on demographics and DD of monetary ($10, $100) and cannabis (10 g) rewards. Analyses of variance were used to evaluate magnitude and commodity effects. Hierarchical multiple regression models were run to assess whether cannabis use frequency was associated with discounting rates for monetary and cannabis rewards. A magnitude effect was found for the monetary rewards where $10 was discounted more steeply compared to $100 (p < .0001). A commodity effect was found where discounting was higher for the 10g cannabis reward compared to monetary rewards (ps < .05). Regression models controlling for demographics and other substance use indicated severity of cannabis problems significantly predicted discounting of $100 (ß = .045, p < .05) and 10 g of cannabis (ß = .088, p < .05). Cannabis use frequency was not significantly associated with any DD measures after controlling for other substance use (ps > .05). These results suggest the association between cannabis use and DD is complex and generally small in magnitude. This study adds to the literature on DD and cannabis use and suggests the need for further studies to determine the extent to which cannabis use impacts DD, both chronically and acutely. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Impulsivity as a mediating factor in the association between posttraumatic stress disorder symptoms and substance use.

OBJECTIVE: Posttraumatic stress disorder (PTSD) has been associated with heightened impulsivity and risk-taking behaviors, including higher rates of substance use than individuals without PTSD. Although a number of studies suggest that impulsivity is associated with substance use in PTSD, the specific role of impulsivity in this common pattern of comorbidity remains unclear. The current study investigated associations between PTSD symptoms, substance use patterns, and impulsivity in a sample of adults. METHOD: A total of 2,967 participants were recruited online through Amazon's Mechanical Turk. Participants who did not report at least one Criterion A traumatic event on the Brief Trauma Questionnaire were excluded. The remaining 1,609 trauma-exposed individuals were placed into either the probable PTSD group (n = 406) or the trauma-exposed non-PTSD group (n = 1,203) based on their PTSD Checklist for Diagnostic and Statistical Manual of Mental Disorders, fifth edition (PCL-5) score. Impulsivity was assessed via a delay discounting measure and the brief UPPS-P (urgency, premeditation, perseverance, sensation seeking, and positive urgency) Impulsive Behavior Scale. Alcohol and cannabis were assessed using the Alcohol Use Disorders Identification Test (AUDIT) and Cannabis Use Disorders Identification Test (CUDIT-R) scales, respectively. RESULTS: Probable PTSD participants exhibited steeper (more impulsive) delay discounting and endorsed more impulsive traits than participants in the trauma-exposed non-PTSD group. Moreover, the PTSD group reported significantly higher scores on both the AUDIT and CUDIT-R. Lastly, impulsive personality traits on the UPPS-P partially mediated the association between PTSD and both cannabis and alcohol use. CONCLUSIONS: These findings suggest that trauma-exposed individuals who exhibit elevated PTSD symptoms show heightened impulsivity. It also appears that lower levels of impulsivity may serve as a protective factor among trauma-exposed individuals resilient to the development of PTSD. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Delay Discounting as a Transdiagnostic Process in Psychiatric Disorders: A Meta-analysis.

Importance: Delay discounting is a behavioral economic index of impulsive preferences for smaller-immediate or larger-delayed rewards that is argued to be a transdiagnostic process across health conditions. Studies suggest some psychiatric disorders are associated with differences in discounting compared with controls, but null findings have also been reported. Objective: To conduct a meta-analysis of the published literature on delay discounting in people with psychiatric disorders. Data Sources: PubMed, MEDLINE, PsycInfo, Embase, and Web of Science databases were searched through December 10, 2018. The psychiatric keywords used were based on DSM-IV or DSM-5 diagnostic categories. Collected data were analyzed from December 10, 2018, through June 1, 2019. Study Selection: Following a preregistered Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) protocol, 2 independent raters reviewed titles, abstracts, and full-text articles. English-language articles comparing monetary delay discounting between participants with psychiatric disorders and controls were included. Data Extraction and Synthesis: Hedges g effect sizes were computed and random-effects models were used for all analyses. Heterogeneity statistics, one-study-removed analyses, and publication bias indices were also examined. Main Outcomes and Measures: Categorical comparisons of delay discounting between a psychiatric group and a control group. Results: The sample included 57 effect sizes from 43 studies across 8 diagnostic categories. Significantly steeper discounting for individuals with a psychiatric disorder compared with controls was observed for major depressive disorder (Hedges g = 0.37; P = .002; k = 7), schizophrenia (Hedges g = 0.46; P = .004; k = 12), borderline personality disorder (Hedges g = 0.60; P < .001; k = 8), bipolar disorder (Hedges g = 0.68; P < .001; k = 4), bulimia nervosa (Hedges g = 0.41; P = .001; k = 4), and binge-eating disorder (Hedges g = 0.34; P = .001; k = 7). In contrast, anorexia nervosa exhibited statistically significantly shallower discounting (Hedges g = -0.30; P < .001; k = 10). Modest evidence of publication bias was indicated by a statistically significant Egger test for schizophrenia and at the aggregate level across studies. Conclusions and Relevance: Results of this study appear to provide empirical support for delay discounting as a transdiagnostic process across most of the psychiatric disorders examined; the literature search also revealed limited studies in some disorders, notably posttraumatic stress disorder, which is a priority area for research.

Effects of neuromodulation on cognitive performance in individuals exhibiting addictive behaviors: A systematic review.

BACKGROUND: There is growing interest in non-invasive brain stimulation techniques as treatments for addictive disorders. While multiple reviews have examined the effects of neuromodulation on craving and consumption, there has been no review of how neuromodulation affects cognitive functioning in addiction. This systematic review examined studies of the cognitive effects of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) in individuals exhibiting addictive behavior. METHODS: Articles were identified through searches in PubMed and PsycINFO conducted in October 2017. Eligible studies investigated the effects of tDCS or TMS on cognitive task performance in participants reporting substance use (e.g., alcohol, tobacco, or drugs) or addictive behaviors (e.g., gambling). Tasks were organized into five domains: (1) Inhibitory control, (2) Risk-taking, (3) Impulsive choice (delay discounting), (4) Executive function, and (5) Implicit biases. RESULTS: Twenty-four articles met the inclusion criteria. Fifty-seven percent of studies used tDCS and 43% used TMS, with nearly all studies (96%) targeting the dorsolateral prefrontal cortex. Ten studies reported significant within-subject modulation of cognitive functioning associated with active TMS or tDCS, with the same number reporting no change in cognitive performance. Of four studies that included both an experimental and control participant group, three showed between-group differences in the effects of neuromodulation. CONCLUSIONS: While positive effects in several studies suggest that tDCS and TMS improve cognitive functioning in addiction, there is substantial heterogeneity across studies. We discuss person-related and methodological factors that could explain inconsistencies, and propose individualized stimulation protocols may sharpen the cognitive effects of neuromodulation in addiction.

Effects of neuromodulation on cognitive performance in individuals exhibiting addictive behaviour: a systematic review protocol.

BACKGROUND: Individuals with substance use and addictive disorders often display greater risk-taking behaviour, higher impulsivity, and altered reward processing compared to individuals without these disorders. While it is not known whether cognitive biases precede or result from addictive behaviour, they likely influence addiction-related decision-making, and may facilitate pathological behaviour. There is evidence that cognitive functions-including those shown to be altered in substance use and addictive disorders-can be influenced by neuromodulation techniques (specifically, transcranial direct current stimulation and transcranial magnetic stimulation). Much of this work has been conducted in healthy populations, however, making it unclear whether these methods can be used effectively to modulate cognitive functioning in individuals with substance use and addictive disorders. The purpose of the current review is to shed light on the potential effectiveness and feasibility of neuromodulation as a means to improve cognitive deficits in substance use disorders. METHODS: The review will identify and evaluate studies that have examined the effects of transcranial direct current stimulation (tDCS) or transcranial magnetic stimulation (TMS) on cognitive task performance in individuals with chronic substance use or dependence. Relevant studies will be identified through searches in PubMed, PsycINFO, Scopus, and Embase, and narrative review will be used to explore evidence that these techniques can be used successfully to modulate cognitive performance in populations exhibiting addictive behaviour. Assessing individual cognitive domains in turn (e.g. risk-taking, impulsivity, attention), we will critically evaluate the validity and reliability of relevant studies and draw conclusions about the strength of evidence for effective use of neuromodulation in that domain. This protocol is not yet registered with PROSPERO. DISCUSSION: To determine whether neuromodulation holds promise as an effective treatment for neurocognitive deficits in substance use and addictive disorders, it is essential to look carefully at previous studies using this approach in addiction samples. This review will provide an objective and informative description of what is currently known about the efficacy of these techniques, shed light on the feasibility and potential challenges of using neuromodulation in individuals who exhibit addictive behaviour, and identify the most valuable next steps for future research.

Stimulation over primary motor cortex during action observation impairs effector recognition.

Recent work suggests that motor cortical processing during action observation plays a role in later recognition of the object involved in the action. Here, we investigated whether recognition of the effector making an action is also impaired when transcranial magnetic stimulation (TMS) - thought to interfere with normal cortical activity - is applied over the primary motor cortex (M1) during action observation. In two experiments, single-pulse TMS was delivered over the hand area of M1 while participants watched short clips of hand actions. Participants were then asked whether an image (experiment 1) or a video (experiment 2) of a hand presented later in the trial was the same or different to the hand in the preceding video. In Experiment 1, we found that participants' ability to recognise static images of hands was significantly impaired when TMS was delivered over M1 during action observation, compared to when no TMS was delivered, or when stimulation was applied over the vertex. Conversely, stimulation over M1 did not affect recognition of dot configurations, or recognition of hands that were previously presented as static images (rather than action movie clips) with no object. In Experiment 2, we found that effector recognition was impaired when stimulation was applied part way through (300ms) and at the end (500ms) of the action observation period, indicating that 200ms of action-viewing following stimulation was not long enough to form a new representation that could be used for later recognition. The findings of both experiments suggest that interfering with cortical motor activity during action observation impairs subsequent recognition of the effector involved in the action, which complements previous findings of motor system involvement in object memory. This work provides some of the first evidence that motor processing during action observation is involved in forming representations of the effector that are useful beyond the action observation period.

Effects of action observation on corticospinal excitability: Muscle specificity, direction, and timing of the mirror response.

Many human behaviours and pathologies have been attributed to the putative mirror neuron system, a neural system that is active during both the observation and execution of actions. While there are now a very large number of papers on the mirror neuron system, variations in the methods and analyses employed by researchers mean that the basic characteristics of the mirror response are not clear. This review focuses on three important aspects of the mirror response, as measured by modulations in corticospinal excitability: (1) muscle specificity; (2) direction; and (3) timing of modulation. We focus mainly on electromyographic (EMG) data gathered following single-pulse transcranial magnetic stimulation (TMS), because this method provides precise information regarding these three aspects of the response. Data from paired-pulse TMS paradigms and peripheral nerve stimulation (PNS) are also considered when we discuss the possible mechanisms underlying the mirror response. In this systematic review of the literature, we examine the findings of 85 TMS and PNS studies of the human mirror response, and consider the limitations and advantages of the different methodological approaches these have adopted in relation to discrepancies between their findings. We conclude by proposing a testable model of how action observation modulates corticospinal excitability in humans. Specifically, we propose that action observation elicits an early, non-specific facilitation of corticospinal excitability (at around 90ms from action onset), followed by a later modulation of activity specific to the muscles involved in the observed action (from around 200ms). Testing this model will greatly advance our understanding of the mirror mechanism and provide a more stable grounding on which to base inferences about its role in human behaviour.

Food intake is influenced by sensory sensitivity.

Wide availability of highly palatable foods is often blamed for the rising incidence of obesity. As palatability is largely determined by the sensory properties of food, this study investigated how sensitivity to these properties affects how much we eat. Forty females were classified as either high or low in sensory sensitivity based on their scores on a self-report measure of sensory processing (the Adult Sensory Profile), and their intake of chocolate during the experiment was measured. Food intake was significantly higher for high-sensitivity compared to low-sensitivity individuals. Furthermore, individual scores of sensory sensitivity were positively correlated with self-reported emotional eating. These data could indicate that individuals who are more sensitive to the sensory properties of food have a heightened perception of palatability, which, in turn, leads to a greater food intake.