Effects of acute tryptophan depletion on central processing of CT-targeted and discriminatory touch in humans

Existing and Future Strategies to Manipulate the Gut Microbiota With Diet as a Potential Adjuvant Treatment for Psychiatric Disorders

Nutrition and diet quality play key roles in preventing and slowing cognitive decline and have been linked to multiple brain disorders. This review compiles available evidence from preclinical studies and clinical trials on the impact of nutrition and interventions regarding major psychiatric conditions and some neurological disorders. We emphasize the potential role of diet-related microbiome alterations in these effects and highlight commonalities between various brain disorders related to the microbiome. Despite numerous studies shedding light on these findings, there are still gaps in our understanding due to the limited availability of definitive human trial data firmly establishing a causal link between a specific diet and microbially mediated brain functions and symptoms. The positive impact of certain diets on the microbiome and cognitive function is frequently ascribed with the anti-inflammatory effects of certain microbial metabolites or a reduction of proinflammatory microbial products. We also critically review recent research on pro- and prebiotics and nondietary interventions, particularly fecal microbiota transplantation. The recent focus on diet in relation to brain disorders could lead to improved treatment outcomes with combined dietary, pharmacological, and behavioral interventions.

Characterizing affiliative touch in humans and its role in advancing haptic design

An emerging view in cognitive neuroscience holds that the extraction of emotional relevance from sensory experience extends beyond the centralized appraisal of sensation in associative brain regions, including frontal and medial-temporal cortices. This view holds that sensory information can be emotionally valenced from the point of contact with the world. This view is supported by recent research characterizing the human affiliative touch system, which carries signals of soft, stroking touch to the central nervous system and is mediated by dedicated C-tactile afferent receptors. This basic scientific research on the human affiliative touch system is informed by, and informs, technology design for communicating and regulating emotion through touch. Here, we review recent research on the basic biology and cognitive neuroscience of affiliative touch, its regulatory effects across the lifespan, and the factors that modulate it. We further review recent work on the design of haptic technologies, devices that stimulate the affiliative touch system, such as wearable technologies that apply the sensation of soft stroking or other skin-to-skin contact, to promote physiological regulation. We then point to future directions in interdisciplinary research aimed at both furthering scientific understanding and application of haptic technology for health and wellbeing.

Depression severity is associated with reduced pleasantness of observed social touch and fewer current intimate touch experiences

Depression is associated with loss of pleasure in previously enjoyed activities and withdrawal from social interactions. Depression alters the perception of social cues, but it is currently unclear whether this extends to social touch. In the current cross-sectional study, we explored the association between depression severity, perceived pleasantness of observed social touch, and general longing for touch. For observed touch, we contrasted videos of slow touch (1-10cm/s), which optimally activates C tactile afferent nerve fibres and generally feels pleasant, with 'non-CT-optimal' touch (i.e., outside the 1-10cm/s range, commonly rated more neutral). We predicted that greater depression severity would be related to lower pleasantness ratings specifically for CT-optimal touch, and less longing for touch. N = 226 adults completed self-report measures of depression severity and longing for touch, and rated touch pleasantness for six videos depicting social touch at three velocities (3cm/s in the CT-optimal range, 0.5 and 30cm/s outside this range) and at two locations varying in CT innervation (palm vs. arm). We controlled for general anhedonia and individual differences in touch experiences and attitudes. Across touch locations, greater depression severity was associated with lower perceived pleasantness of touch, especially for the fastest non-CT-optimal (rather than the CT-optimal) velocity, contrary to our prediction. However, when grouping participants into probable vs. no/minimal depression, the probable depression group rated both the fastest non-CT-optimal and the CT-optimal velocity as less pleasant than did the no/minimal depression group. Overall, while depression was associated with perceived pleasantness of observed touch, this was not specific to CT-optimal touch. Furthermore, touch longing was not associated with depression severity. Instead, variance in depression symptoms was better explained by reduced levels of current intimate touch. Though the direction of causality is unclear, greater depression severity is related to lower pleasantness of observed social touch, and lower levels of current intimate touch.

Hold me or stroke me? Individual differences in static and dynamic affective touch

Low-threshold mechanosensory C-fibres, C-tactile afferents (CTs), respond optimally to sensations associated with a human caress. Additionally, CT-stimulation activates brain regions associated with processing affective states. This evidence has led to the social touch hypothesis, that CTs have a key role in encoding the affective properties of social touch. Thus, to date, the affective touch literature has focussed on gentle stroking touch. However, social touch interactions involve many touch types, including static, higher force touch such as hugging and holding. This study aimed to broaden our understanding of the social touch hypothesis by investigating relative preference for static vs dynamic touch and the influence of force on these preferences. Additionally, as recent literature has highlighted individual differences in CT-touch sensitivity, this study investigated the influence of affective touch experiences and attitudes, autistic traits, depressive symptomology and perceived stress on CT-touch sensitivity. Directly experienced, robotic touch responses were obtained through a lab-based study and vicarious touch responses through an online study where participants rated affective touch videos. Individual differences were determined by self-report questionnaire measures. In general, static touch was preferred over CT-non-optimal stroking touch, however, consistent with previous reports, CT-optimal stroking (velocity 1-10 cm/s) was rated most pleasant. However, static and CT-optimal vicarious touch were rated comparably for dorsal hand touch. For all velocities, 0.4N was preferred over 0.05N and 1.5N robotic touch. Participant dynamic touch quadratic terms were calculated for robotic and vicarious touch as a proxy CT-sensitivity measure. Attitudes to intimate touch significantly predict robotic and vicarious quadratic terms, as well as vicarious static dorsal hand touch ratings. Perceived stress negatively predicted robotic static touch ratings. This study has identified individual difference predictors of CT-touch sensitivity. Additionally, it has highlighted the context dependence of affective touch responses and the need to consider static, as well as dynamic affective touch.

Acute tryptophan depletion alters affective touch perception

RATIONALE

Affiliative tactile interactions help regulate physiological arousal and confer resilience to acute and chronic stress. C-tactile afferents (CTs) are a population of unmyelinated, low threshold mechanosensitive cutaneous nerve fibres which respond optimally to a low force stimulus, moving at between 1 and 10 cm/s. As CT firing frequencies correlate positively with subjective ratings of touch pleasantness, they are hypothesised to form the first stage of encoding affiliative tactile interactions. Serotonin is a key modulator of social responses with known effects on bonding.

OBJECTIVES

The aim of the present study was to determine the effect of acutely lowering central serotonin levels on perceptions of CT-targeted affective touch.

METHODS

In a double blind, placebo-controlled design, the effect of acute tryptophan depletion (ATD) on 25 female participants' ratings of directly and vicariously experienced touch was investigated. Psychophysical techniques were used to deliver dynamic tactile stimuli; some velocities were targeted to optimally activate CTs (1-10 cm/s), whereas other, faster and slower strokes fell outside the CT optimal range. Discriminative tactile function, cold pain threshold and tolerance were also measured.

RESULTS

ATD significantly increased pleasantness ratings of both directly and vicariously experienced affective touch, increasing discrimination of the specific hedonic value of CT targeted velocities. While ATD had no effect on either tactile or cold pain thresholds, there was a trend for reduced tolerance to cold pain.

CONCLUSIONS

These findings are consistent with previous reports that depletion of central serotonin levels modulates neural and behavioural responsiveness to appetitive sensory signals.

Strengths and weaknesses of affective touch studies over the lifetime: A systematic review

C-Tactile (CT) fibers are activated by slow, caress-like stimulations, and convey a specific tactile processing of hedonic and interpersonal components, defined as affective touch. Given the beneficial effects deriving from affective tactile experiences in social interactions at all ages, a systematic review of experimental studies on affective touch perception across the lifespan was performed with the aims of 1) examining whether and how affective touch has been studied in a systematic manner throughout the lifespan; 2) verifying whether the pleasantness associated to affective stimulations is found during the entire lifespan. Empirical human studies on affective touch were searched in two databases (PubMed, PsychINFO) and 112 articles were retrieved. Results indicated that most of the studies recruited participants with a mean age ranging from 18 to 40 years, whereas other age ranges came out as under-represented or not represented at all. Despite high heterogeneity across studies, affective touch was considered as a pleasant experience across the lifetime, and it was associated to specific psychophysiological patterns in infants and adults.

Social interoception and social allostasis through touch: Legacy of the Somatovisceral Afference Model of Emotion

John Cacioppo and colleagues' Somatovisceral Afference Model of Emotion (SAME) highlighted the importance of interoception in emotional experience. Here we compare how the SAME and the more recent theory of constructed emotion (TCE) view the role of interoceptive signals in creating emotional experiences. We describe the characteristics of touch sensations that are carried by thin, unmyelinated fibers called C-tactile afferents (CTs) to the posterior insula, and are thus deemed interoceptive despite their typically social (external) origin. We explore how this social interoceptive input might contribute to the emotion-related effects of social touch more generally, and speculate that all social touch, with or without CT afferent stimulation, can directly influence allostasis, or the predictive regulation of short- and long-term energy resources required by the body. Finally, we describe several features of CT-optimal touch that make it a potentially useful tool to help illuminate basic interoceptive mechanisms, emotion-related phenomena, and disorders involving atypical affect or somatosensation. These proposed ideas demonstrate the long intellectual reach of John Cacioppo and Gary Berntson's highly productive scientific collaboration, which was formative for the fields of social neuroscience, social psychophysiology, and affective neuroscience.

Effects of Acute Tryptophan Depletion on Human Taste Perception

Taste perception has been reported to vary with changes in affective state. Distortions of taste perception, including blunted recognition thresholds, intensity, and hedonic ratings have been identified in those suffering from depressive disorders. Serotonin is a key neurotransmitter implicated in the etiology of anxiety and depression; systemic and peripheral manipulations of serotonin signaling have previously been shown to modulate taste detection. However, the specific effects of central serotonin function on taste processing have not been widely investigated. Here, in a double-blind placebo-controlled study, acute tryptophan depletion was used to investigate the effect of reduced central serotonin function on taste perception. Twenty-five female participants aged 18-28 attended the laboratory on two occasions at least 1 week apart. On one visit, they received a tryptophan depleting drink and on the other, a control drink was administered. Approximately, 6 h after drink consumption, they completed a taste perception task which measured detection thresholds and supra-threshold perceptions of the intensity and pleasantness of four basic tastes (sweet, sour, bitter, and salt). While acutely reducing central levels of serotonin had no effect on the detection thresholds of sweet, bitter, or sour tastes, it significantly enhanced detection of salt. For supra-threshold stimuli, acutely reduced serotonin levels significantly enhanced the perceived intensity of both bitter and sour tastes and blunted pleasantness ratings of bitter quinine. These findings show manipulation of central serotonin levels can modulate taste perception and are consistent with previous reports that depletion of central serotonin levels enhances neural and behavioral responsiveness to aversive signals.

Dopaminergic and opioidergic regulation during anticipation and consumption of social and nonsocial rewards

The observation of animal orofacial and behavioral reactions has played a fundamental role in research on reward but is seldom assessed in humans. Healthy volunteers (N = 131) received 400 mg of the dopaminergic antagonist amisulpride, 50 mg of the opioidergic antagonist naltrexone, or placebo. Subjective ratings, physical effort, and facial reactions to matched primary social (affective touch) and nonsocial (food) rewards were assessed. Both drugs resulted in lower physical effort and greater negative facial reactions during reward anticipation, especially of food rewards. Only opioidergic manipulation through naltrexone led to a reduction in positive facial reactions to liked rewards during reward consumption. Subjective ratings of wanting and liking were not modulated by either drug. Results suggest that facial reactions during anticipated and experienced pleasure rely on partly different neurochemical systems, and also that the neurochemical bases for food and touch rewards are not identical.

Defining pleasant touch stimuli: a systematic review and meta-analysis

Pleasantness is generally overlooked when investigating tactile functions. Addition of a pleasant stimulus could allow for a more complete characterisation of somatosensory function. The aims of this review were to systematically assess the methodologies used to elicit a pleasant sensation, measured via psychophysical techniques, and to perform a meta-analysis to measure the effect of brush stroking velocity on touch pleasantness. Eighteen studies were included in the systematic review, with five studies included in the meta-analysis. The review found that factors such as texture, velocity, force, and the duration of continuous stroking influence tactile evoked pleasantness. Specifically, using a soft material and stroking at a velocity of 3 cm/s with light force is generally considered as particularly pleasant. The meta-analysis showed that a brush stroking velocity of 30 cm/s was rated as less pleasant than 3 cm/s, on the forearm. The present study collates the factors that are most likely to provide a stimulus to elicit a pleasant sensation. The results should be important for studies requiring a well-defined pleasant stimulus including neurosensory assessment protocols, allowing for a more complete multimodality assessment of somatosensory function.

Effects of MDMA on attention to positive social cues and pleasantness of affective touch

The psychostimulant drug ±3,4-methylenedioxymethamphetamine (MDMA) reportedly produces distinctive feelings of empathy and closeness with others. MDMA increases social behavior in animal models and has shown promise in psychiatric disorders, such as autism spectrum disorder (ASD) and post-traumatic stress disorder (PTSD). How it produces these prosocial effects is not known. This behavioral and psychophysiological study examined the effects of MDMA, compared with the prototypical stimulant methamphetamine (MA), on two measures of social behavior in healthy young adults: (i) responses to socially relevant, "affective" touch, and (ii) visual attention to emotional faces. Men and women (N = 36) attended four sessions in which they received MDMA (0.75 or 1.5 mg/kg), MA (20 mg), or a placebo in randomized order under double-blind conditions. Responses to experienced and observed affective touch (i.e., being touched or watching others being touched) were assessed using facial electromyography (EMG), a proxy of affective state. Responses to emotional faces were assessed using electrooculography (EOG) in a measure of attentional bias. Subjective ratings were also included. We hypothesized that MDMA, but not MA, would enhance the ratings of pleasantness and psychophysiological responses to affective touch and increase attentional bias toward positive facial expressions. Consistent with this, we found that MDMA, but not MA, selectively enhanced ratings of pleasantness of experienced affective touch. Neither drug altered the ratings of pleasantness of observed touch. On the EOG measure of attentional bias, MDMA, but not MA, increased attention toward happy faces. These results provide new evidence that MDMA can enhance the experience of positive social interactions; in this case, pleasantness of physical touch and attentional bias toward positive facial expressions. The findings are consistent with evidence that the prosocial effects are unique to MDMA relative to another stimulant. Understanding the behavioral and neurobiological processes underlying the distinctive social effects of MDMA is a key step to developing the drug for psychiatric disorders.

EEG captures affective touch: CT-optimal touch and neural oscillations

Tactile interactions are of developmental importance to social and emotional interactions across species. In beginning to understand the affective component of tactile stimulation, research has begun to elucidate the neural mechanisms that underscore slow, affective touch. Here, we extended this emerging body of work and examined whether affective touch (C tactile [CT]-optimal speed), as compared to nonaffective touch (non-CT-optimal speed) and no touch conditions, modulated EEG oscillations. We report an attenuation in alpha and beta activity to affective and nonaffective touch relative to the no touch condition. Further, we found an attenuation in theta activity specific to the affective, as compared to the nonaffective touch and no touch conditions. Similar to theta, we also observed an attenuation of beta oscillations during the affective touch condition, although only in parietal scalp sites. Decreased activity in theta and parietal-beta ranges may reflect attentional-emotional regulatory mechanisms; however, future work is needed to provide insight into the potential neural coupling between theta and beta and their specific role in encoding slow, tactile stimulation.

Modeling Group-Level Repeated Measurements of Neuroimaging Data Using the Univariate General Linear Model

Group-level repeated measurements are common in neuroimaging, yet their analysis remains complex. Although a variety of specialized tools now exist, it is surprising that to-date there has been no clear discussion of how repeated-measurements can be analyzed appropriately using the standard general linear model approach, as implemented in software such as SPM and FSL. This is particularly surprising given that these implementations necessitate the use of multiple models, even for seemingly conventional analyses, and that without care it is very easy to specify contrasts that do not correctly test the effects of interest. Despite this, interest in fitting these types of models using conventional tools has been growing in the neuroimaging community. As such it has become even more important to elucidate the correct means of doing so. To begin, this paper will discuss the key concept of the expected mean squares (EMS) for defining suitable F-ratios for testing hypotheses. Once this is understood, the logic of specifying correct repeated measurements models in the GLM should be clear. The ancillary issue of specifying suitable contrast weights in these designs will also be discussed, providing a complimentary perspective on the EMS. A set of steps will then be given alongside an example of specifying a 3-way repeated-measures ANOVA in SPM. Equivalency of the results compared to other statistical software will be demonstrated. Additional issues, such as the inclusion of continuous covariates and the assumption of sphericity, will also be discussed. The hope is that this paper will provide some clarity on this confusing topic, giving researchers the confidence to correctly specify these forms of models within traditional neuroimaging analysis tools.

Assessment of experimental orofacial pain, pleasantness and unpleasantness via standardized psychophysical testing

BACKGROUND

Somatosensory assessment within the orofacial region may be performed using highly standardized quantitative sensory testing (QST). However, the function of the C-tactile (CT) afferent, a nerve fibre linked to the perception of pleasant touch, is usually not evaluated. Furthermore, the perception of unpleasantness is also rarely assessed, a dimension not only limited to a painful experience. Therefore, the primary aim was to apply standardized QST stimuli as well as standardized pleasant stimuli and evaluate their potential capacity for evocation of perceived pain, pleasant and unpleasant sensations in the facial region.

METHODS

Twenty-one female participants underwent QST as per the protocol derived from the German Research Network on Neuropathic Pain. For the first time, two modified protocols were used to investigate stimuli for perceived pleasantness and unpleasantness.

RESULTS

Thermal stimuli provided separate thresholds for each sensation. From certain mechanical stimuli (e.g., vibration), overlap between the perceived sensations of pleasantness and unpleasantness was identified. It was not possible to evoke only an unpleasant sensation without a painful contribution, and both these sensations increased significantly when utilizing an increasing pinprick force (p < 0.011). Between dynamic stimuli, the brush was rated as significantly more pleasant than the cotton wool tip (p = 0.015). A quadratic model provided the best fit for velocity against mean pleasantness ratings (R²  = 0.62 ± 0.08), supporting previous CT afferent literature to some extent.

CONCLUSION

Stimuli were generally not isolated to one sensation, highlighting the multidimensional construct of stimulus perception and the need for scales to capture this.

SIGNIFICANCE

The battery of QST tests from the DFNS protocol has been modified to investigate pleasant and unpleasant sensations. This allows the evaluation of psychophysical properties across standardized dimensions to provide a thorough view of somatosensory function and to better understand the affective spectrum of somatosensory function.

Extreme enhancement or depletion of serotonin transporter function and serotonin availability in autism spectrum disorder

A variety of human and animal studies support the hypothesis that serotonin (5-hydroxytryptamine or 5-HT) system dysfunction is a contributing factor to the development of autism in some patients. However, many questions remain about how developmental manipulation of various components that influence 5-HT signaling (5-HT synthesis, transport, metabolism) persistently impair social behaviors. This review will summarize key aspects of central 5-HT function important for normal brain development, and review evidence implicating perinatal disruptions in 5-HT signaling in the pathophysiology of autism spectrum disorder. We discuss the importance, and relative dearth, of studies that explore the possible correlation to autism in the interactions between important intrinsic and extrinsic factors that may disrupt 5-HT homeostasis during development. In particular, we focus on exposure to 5-HT transport altering mechanisms such as selective serotonin-reuptake inhibitors or genetic polymorphisms in primary or auxiliary transporters of 5-HT, and how they relate to neurological stores of serotonin and its precursors. A deeper understanding of the many mechanisms by which 5-HT signaling can be disrupted, alone and in concert, may contribute to an improved understanding of the etiologies and heterogeneous nature of this disorder. We postulate that extreme bidirectional perturbations of these factors during development likely compound or synergize to facilitate enduring neurochemical changes resulting in insufficient or excessive 5-HT signaling, that could underlie the persistent behavioral characteristics of autism spectrum disorder.

Construction and Validation of the Touch Experiences and Attitudes Questionnaire (TEAQ): A Self-report Measure to Determine Attitudes Toward and Experiences of Positive Touch

Despite growing interest in the beneficial effects of positive touch experiences throughout our lives, and individual differences in how these experiences are perceived, there is not yet available a contemporary self-report measure of touch experiences and attitudes, for which the factor structure has been validated. This article describes four studies carried out during the construction and validation of the Touch Experiences and Attitudes Questionnaire (TEAQ). The original TEAQ, containing 117 items relating to positive touch experiences was systematically constructed. Principal component analysis reduced this measure to 57 items and identified six components relating to touch experiences during childhood and adult experiences relating to current intimate touch and touch with friends and family. Three attitudinal components were identified relating to attitude to intimate touch, touch with unfamiliar people, and self-care. The structure of this questionnaire was confirmed through confirmatory factor analysis carried out on data obtained from a second sample. Good concurrent and predictive validity of the TEAQ compared to other physical touch measures currently available was identified. Known-group validity in terms of gender, marital status and age was determined, with expected group differences identified. This study demonstrates the TEAQ to have good face validity, internal consistency, construct validity in terms of discriminant validity, known-group validity and convergent validity, and criterion-related validity in terms of predictive validity and concurrent validity. We anticipate this questionnaire will be a valuable tool for the field of physical touch research.

Intrainsular connectivity and somatosensory responsiveness in young children with ASD

BACKGROUND

The human somatosensory system comprises dissociable paths for discriminative and affective touch, reflected in separate peripheral afferent populations and distinct cortical targets. Differences in behavioral and neural responses to affective touch may have an important developmental role in early social experiences, which are relevant for autism spectrum disorder (ASD).

METHODS

Using probabilistic tractography, we compared the structural integrity of white matter pathways for discriminative and affective touch in young children with ASD and their typically developing (TD) peers. We examined two tracts: (1) a tract linking the thalamus with the primary somatosensory cortex, which carries discriminative tactile information, and (2) a tract linking the posterior insula-the cortical projection target of unmyelinated tactile afferents mediating affective touch-with the anterior insula, which integrates sensory and visceral inputs to interpret emotional salience of sensory stimuli. We investigated associations between tract integrity and performance on a standardized observational assessment measuring tactile discrimination and affective responses to touch.

RESULTS

Both the thalamocortical and intrainsular tracts showed reduced integrity (higher mean diffusivity) in the ASD group compared to those in the TD group. Consistent with the previous findings, the ASD group exhibited impaired tactile discriminative ability, more tactile defensiveness, and more sensory seeking (e.g., enthusiastic play or repetitive engagement with a specific tactile stimulus). There was a significant relation between intrainsular tract integrity and tactile seeking. The direction of this relation differed between groups: higher intrainsular mean diffusivity (MD) (reflecting decreased tract integrity) was associated with increased tactile seeking in the TD group but with decreased tactile seeking in the ASD group. In the TD group, decreased tactile defensiveness was also associated with higher intrainsular MD, but there was no relation in the ASD group. Discriminative touch was not significantly associated with integrity of either tract in either group.

CONCLUSIONS

These results support previous findings suggesting a central role for the insula in affective response to touch. While both discriminative and affective touch and both somatosensory tracts are affected in ASD, the restriction of brain-behavior associations to the intrainsular tract and tactile seeking suggests more complex and perhaps higher-order influence on differences in tactile defensiveness and discrimination.