Hedonic Processing Research Articles

Exploring the Effects of GDPR on the User Experience

With the implementation of the General Data Protection Regulation (GDPR) in the European Union, companies must be able to ensure that applications requesting the user to record personal data inform the user of the purpose for which the data collected is being collected. Furthermore, it is also necessary to ensure the data owner has given his/her explicit consent to the collection of these data and that such consent is recorded. This imperative has required a reworking of the interfaces of the applications. In this sense, this study seeks to explore the policies and practices that are being followed by organizations in designing interfaces. Four case studies were considered in the development of this study to explore this phenomenon in the dimensions of pragmatic hedonic processes of interface design, approaches to collect consent, and how personal data is managed.

Increased functional coupling of the mu opioid receptor in the anterior insula of depressed individuals.

The mu opioid receptor (MOR) is a G protein-coupled receptor that plays an essential role in reward and hedonic processes, and that has been implicated in disorders such as depression and addiction. Over the last decade, several brain imaging studies in depressed patients have consistently found that dysregulation of MOR function occurs in particular in the anterior insular cortex, an important brain site for the perception of internal states and emotional regulation. To investigate molecular mechanisms that may underlie these effects, here we assessed genetic polymorphisms, expression, and functional G-protein coupling of MOR in a large post-mortem cohort (N = 95) composed of depressed individuals who died by suicide, and healthy controls. Results indicated that depression, but not comorbid substance use disorder or acute opiate consumption, was associated with increased MOR activity. This effect was partly explained by a specific increase in expression of the inhibitory alpha G-protein subunit GNAI2. Consistent with previous neuroimaging studies, our findings support the notion that enhanced endogenous opioidergic tone in the anterior insula may buffer negative affective states in depressed individuals, a mechanism that could potentially contribute to the antidepressant efficacy of emerging opioid-based medications.

A reward and incentive-sensitization perspective on compulsive use of social networking sites – Wanting but not liking predicts checking frequency and problematic use behavior

The use of Social Networking Sites (SNSs) has exponentially increased over the past decade, leading to warnings about the addictive potential of this technology. Yet, the idea of SNS addiction remains controversial and more theory-driven research is required to understand the mechanisms of excessive and compulsive SNS use and to facilitate the development of targeted interventions helping affected users. In the present article we propose to utilize a reward-based approach to further our understanding of these behaviors. In particular, we suggest that concepts borrowed from the drug addiction literature that focus on incentive processes (incentive-sensitization and cue reactivity) can explain some SNS behaviors, such as compulsive checking. One elemental finding of the neurobiological drug addiction literature is that repeated exposure to a rewarding substance can render the brain’s reward system oversensitive to cues related to the drug. We report preliminary findings from 358 participants showing that cue-elicited urges to use SNSs characterized both excessive and problematic use behaviors. Moreover, desires and urges to use SNSs (wanting responses) could be reliably dissociated from the enjoyment and pleasure (liking responses) associated with SNSs, with the latter being less predictive of the intensity and problematicity of behaviors than the former. Such divergence between motivational and hedonic processes is another hallmark finding in the literature on drug and food rewards. Together our initial findings thus suggest that examining alterations of reward processes holds promise to explain the compulsive use of SNSs and to identify potential avenues to help affected individuals.

A Dual Pathway Model of Daily Proactive Work Behavior on Hedonic and Eudaimonic Well-Being

Recent research on proactive work behavior suggest that it entails negative consequences for actor’s hedonic well-being. This raises the question why people persevere acting proactively. We propose that in addition to its negative consequences for hedonic well-being proactivity exerts a positive effect on eudaimonic well-being, which may counterbalance the negative effects. Drawing on resources theory and models of eudaimonic well-being the present paper develops and tests a dual pathway model which explores the effect of daily proactivity on daily well-being. The model holds that on one hand proactivity has negative effects on hedonic well-being by increasing daily workload, finally negatively affecting general well-being experienced in the evening. On the other hand, proactivity has a positive effect on eudaimonic well-being because it allows authentic expression. This path results in a positive effect on evening general well-being. We conducted a daily diary study with four daily measurement points (early morning, morning, afternoon, evening) on five consecutive working days. Our sample consisted of n = 121 (k = 604) employees in finances and administration. Data was analyzed with a multilevel structural equation modelling approach. Findings support the dual pathway model. Proactivity exerts a negative effect on evening general well-being through the hedonic pathway and a positive effect through the eudaimonic pathway. The overall effect of proactive work behavior on evening general well-being is positive. Thus, findings suggest that in order to gain a full understanding of the well-being implications of proactivity, eudaimonic processes need to be taken into account in addition to hedonic processes.

Activation of Infralimbic to Nucleus Accumbens Shell Pathway Suppresses Conditioned Aversion in Male But Not Female Rats.

Hedonic processing plays an integral role in directing appropriate behavior, but disrupted hedonic processing is associated with psychiatric disorders such as depression. The infralimbic cortex (IL) is a key structure in affective processing in rodents and activation of its human homolog, the ventromedial prefrontal cortex, has been implicated in suppressing aversive states. Here, we tested whether optogenetic activation of glutamatergic projections from the IL to the nucleus accumbens shell (NAcSh) suppresses the aversive impact of sucrose devalued using the conditioned taste aversion paradigm in males and female rats. In naive rats, no significant differences in appetitive or aversive taste reactivity (TR) to sucrose was observed indicating that initial sucrose palatability was equivalent across sex. However, we found that optical activation of the IL-NAcSh pathway during intraoral infusion of devalued sucrose inhibited aversive TR in male but not female rats. Interestingly, when allowed to freely ingest water and sucrose in a two-bottle test both males and females with a history of IL-NAcSh stimulation exhibited greater preference for sucrose. Optical pathway activation failed to alter TR to innately bitter quinine in either sex. Finally, both sexes lever pressed to self-stimulate the IL-NAcSh pathway. These results indicate that the IL-NAcSh pathway plays an important role in suppressing learned aversive states selectively in males but spares hedonic processing of innately aversive tastants. Further, pathway activation is reinforcing in both sexes, indicating that suppression of conditioned aversive TR can be dissociable from the effects of unconditioned rewarding properties of IL-NAcSh pathway activation.SIGNIFICANCE STATEMENT Negative emotional states contribute to psychiatric disorders including depression and substance use disorders. In this study, we examined whether brain circuitry previously implicated in suppressing negative emotional states in humans can inhibit learned aversion in male and female rats. We found that optical activation of the infralimbic to nucleus accumbens shell pathway attenuates learned aversive responses in male but not female rats, indicating an important sex difference in the function of this brain pathway. Furthermore, we found that pathway stimulation was reinforcing in both sexes. Collectively, these findings support the role of the infralimbic cortex and its projection to the nucleus accumbens shell in suppressing learned negative emotional states and highlight an important sex-specific function of this pathway.

Kamikihito Enhances Cognitive Functions and Reward-Related Behaviors of Aged C57BL/6J Mice in an Automated Behavioral Assay System.

The cognitive and psychological domains of frailty in the elderly have drawn increasing attention given the aging of society. However, therapeutics to treat minor deficits in cognition and mental state in the elderly remain an unmet need. Kamikihito (KKT), a traditional Japanese Kampo medicine indicated for neuroses, anxiety, and insomnia, is effective for treating cognitive dysfunction and depressive-like behaviors in animal models, suggesting that it may have therapeutic potential for treating cognitive and/or mental frailty. In this study, we first validated the known anxiolytic effects of KKT in a conventional maze test. We then introduced an automated behavioral assay system, IntelliCage, to evaluate the therapeutic potential of KKT for age-related and diverse central functions by performing sequential behavioral tasks in young and aged mice to assess basal activities, cognitive functions, perseveration, and hedonic-related behaviors. Although young mice treated with KKT did not exhibit changes in diurnal variation, KKT-administered aged mice exhibited an accelerated decline in voluntary activity during the early part of the light period, implying that KKT may promote sleep onset in aged mice. Neither place learning acquisition for gaining rewards nor subsequent behavioral flexibility performance was altered by KKT in the young group, whereas the aged KKT group exhibited significantly enhanced performance in both phases of learning relative to age-matched controls. Conversely, perseverative nose-pokes (NPs) to gain rewards observed during place learning, indicative of compulsivity, were attenuated by KKT in both age groups. Regarding hedonic processing, aged mice exhibited a decreased preference for sweet solutions compared to young mice, which was effectively reversed by KKT treatment. Furthermore, KKT elevated high-effort choices for high-value reward in an effort-based decision-making paradigm in both age groups, implying augmentation of motivational behaviors by KKT. Collectively, KKT exerted various beneficial effects in cognitive and emotional domains, several of which were more evident in aged mice than in young mice, suggesting the potential of KKT for treating cognitive and mental frailty.

Examination of nicotine and saccharin reward in the Goto-Kakizaki diabetic rat model

Morbidity and mortality attributed to type 2 diabetes have exponentially increased in the US. At exceptionally high risk is a subpopulation of persons with type 2 diabetes who smoke, which are shown to have decreased success rates of smoking cessation than euglycemic smokers. Preclinical research in our laboratory has shown that the rewarding effects of nicotine are enhanced in the streptozotocin and high-fat diet rodent model of diabetes. It is presently unclear whether this enhancement of nicotine reward can be demonstrated in other insulin resistant rat models. This study aimed to determine if a similar increase in nicotine reward is found in Goto-Kakizaki (GK) rats, a model of the spontaneous formation of insulin resistance in an inbred sub-strain of Wistar rat. Nicotine conditioned place preference (CPP) was examined in Sprague-Dawley (SD), Wistar, and GK rats. A robust nicotine CPP was found in SD and Wistar rats, but nicotine CPP was not detected in GK rats. Locomotor activity was also evaluated in all three strains, and GK rats demonstrated significantly less activity as compared to SD and Wistar rats. To further assess reward behavior in GK rats, consumption of saccharin solution was measured over a 48 -h period. GK rats showed a significant increase in saccharin intake compared to SD rats. These findings suggest that GK rats experience an enhanced hedonic processing as compared to SD rats. The lack of nicotine CPP in GK rats may be due to deficits in learning and memory, thus hindering their ability to acquire or express a place preference.

Revisiting anticipatory hedonic processing in patients with schizophrenia: An examination between representation activation and maintenance

BackgroundAnticipatory anhedonia is one of the key deficits found in patients with schizophrenia (SCZ). However, the underlying mechanism of this deficit remains unclear. The present study examined whether representation activation and maintenance capacity influenced anticipatory experiences in SCZ patients. MethodsWe recruited 46 SCZ patients (26 males) and 45 matched healthy controls (24 males). The Reward Representation Activation and Maintenance (RRAM) Task was administrated to assess anticipatory experience and representation activation and maintenance capacity. ResultsSCZ patients exhibited lower subjective arousal than controls in anticipation of rewards with high probability when representation activation and maintenance were difficult to accomplish. SCZ patients also tended to reduce their button presses more than HC when they were required to maintain reward representation. ConclusionsOur findings suggest that representation activation and maintenance may partially account for anticipatory anhedonia observed in SCZ patients.

Reduced mu opioid receptor availability in schizophrenia revealed with [11C]-carfentanil positron emission tomographic Imaging

Negative symptoms, such as amotivation and anhedonia, are a major cause of functional impairment in schizophrenia. There are currently no licensed treatments for negative symptoms, highlighting the need to understand the molecular mechanisms underlying them. Mu-opioid receptors (MOR) in the striatum play a key role in hedonic processing and reward function and are reduced post-mortem in schizophrenia. However, it is unknown if mu-opioid receptor availability is altered in-vivo or related to negative symptoms in schizophrenia. Using [11 C]-carfentanil positron emission tomography (PET) scans in 19 schizophrenia patients and 20 age-matched healthy controls, here we show a significantly lower MOR availability in patients with schizophrenia in the striatum (Cohen’s d = 0.7), and the hedonic network. In addition, we report a marked global increase in inter-regional covariance of MOR availability in schizophrenia, largely due to increased cortical-subcortical covariance.

Striatal responsiveness to reward under threat-of-shock and working memory load: A preliminary study.

IntroductionReward and stress are important determinants of motivated behaviors. Striatal regions play a crucial role in both motivation and hedonic processes. So far, little is known on how cognitive effort interacts with stress to modulate reward processes. This study examines how cognitive effort (load) interacts with an unpredictable acute stressor (threat‐of‐shock) to modulate motivational and hedonic processes in healthy adults.Materials and MethodsA reward task, involving stress with unpredictable mild electric shocks, was conducted in 23 healthy adults aged 20–37 (mean age: 24.7 ± 0.9; 14 females) during functional magnetic resonance imaging (fMRI). Manipulation included the use of (a) monetary reward for reinforcement, (b) threat‐of‐shock as the stressor, and (c) a spatial working memory task with two levels of difficulty (low and high load) for cognitive load. Reward‐related activation was investigated in a priori three regions of interest, the nucleus accumbens (NAcc), caudate nucleus, and putamen.ResultsDuring anticipation, threat‐of‐shock or cognitive load did not affect striatal responsiveness to reward. Anticipated reward increased activation in the ventral and dorsal striatum. During feedback delivery, both threat‐of‐shock and cognitive effort modulated striatal activation. Higher working memory load blunted NAcc responsiveness to reward delivery, while stress strengthened caudate nucleus reactivity regardless reinforcement or load.ConclusionsThese findings provide initial evidence that both stress and cognitive load modulate striatal responsiveness during feedback delivery but not during anticipation in healthy adults. Of clinical importance, sustained stress exposure might go along with dysregulated arousal, increasing therefore the risk for the development of maladaptive incentive‐triggered motivation. This study brings new insight that might help to build a framework to understand common stress‐related disorders, given that these psychiatric disorders involve disturbances of the reward system, cognitive deficits, and abnormal stress reactivity.

Diminished Anticipatory and Consummatory Pleasure in Dysphoria: Evidence From an Experience Sampling Study.

Anhedonia, the experience of diminished pleasure, is a core feature of major depressive disorder and is often present long before the diagnosis of depression. Most previous studies have investigated anhedonia with self-report measures of trait anhedonia or with behavioral paradigms using laboratory stimuli, and the real-time characteristics of hedonic processing in subclinical depression remain under-investigated. We used the experience sampling method to evaluate momentary experience of hedonic feelings in the context of daily life. Dysphoric (n = 49) and non-dysphoric (n = 51) college students completed assessments of their current positive affect (PA), as well as state anticipatory and consummatory pleasure, 3 or 4 times a day every day for 2 weeks. The results showed that dysphoric individuals reported less state anticipatory and consummatory pleasure compared with non-dysphoric individuals. Moreover, significant time-lagged associations between anticipatory pleasure and follow-up consummatory pleasure were found in the whole sample, after adjustment for current PA. The current findings thus hold considerable promise in advancing our understanding of anhedonia as well as the important role of state anticipatory pleasure in relation to depression.

Hippocampal Input to the Nucleus Accumbens Shell Enhances Food Palatability

BackgroundInsight into the neural basis of hedonic processing has come from studies of food palatability in rodents. Pharmacological manipulations of the nucleus accumbens shell (NAcSh) have repeatedly been demonstrated to increase hedonic taste reactivity, yet the contribution of specific NAcSh circuit components is unknown. MethodsBidirectional optogenetic manipulations were targeted to the principal NAcSh projection neurons and afferent pathways in mice during free feeding assays. Number of licks per bout of consumption was used as a measure of food palatability as it was confirmed to track sucrose concentration and subjective flavor preferences. ResultsPhotoinhibition of NAcSh neurons, whether general or cell-type specific, was found to alter consumption without affecting its hedonic impact. Among the principal excitatory afferent pathways, we showed that ventral hippocampal (vHipp) input alone enhances palatability upon low-frequency photostimulation time-locked to consumption. This enhancement in palatability was independent of opioid signaling and not recapitulated by NAcSh or dopamine neuron photostimulation. We further demonstrated that vHipp input photostimulation is sufficient to condition a flavor preference, while its inhibition impedes sucrose-driven flavor preference conditioning. ConclusionsThese results demonstrate a novel contribution of vHipp–NAcSh pathway activity to palatability that may relate to its innervation of a particular region or neuronal ensemble in the NAcSh. These findings are consistent with the evidence that vHipp–NAcSh activity is relevant to the pathophysiology of anhedonia and depression as well as the increasing appreciation of hippocampal involvement in people’s food pleasantness ratings, hunger, and weight.

Facial responses of adult humans during the anticipation and consumption of touch and food rewards

Whether cognitive, motivational and hedonic aspects of reward anticipation and consumption can be reliably assessed with explicit and implicit measures, and if different motivational (decision utility) and hedonic (experienced utility) processes get recruited by distinct reward types, remain partly unsolved questions that are relevant for theories of social and non-social decision-making. We investigated these topics using a novel experimental paradigm, including carefully matched social and nonsocial rewards, and by focusing on facial responses. Facial expressions are indeed an often-cited implicit measure of rewards’ hedonic impact. For example, food rewards elicit powerful facial responses – characterized by lip smacking, tongue protrusion, and relaxation of the middle face – in human newborns, juvenile monkeys, and adult rats. The same stimuli elicit more nuanced facial reactions in adult humans, which can be best captured with facial electromyography (fEMG). However, little is known about facial expressions preceding reward consumption, reflecting the motivation to obtain and possibly the expected pleasantness of a reward, and whether similar facial expressions are elicited by different types of rewards. To investigate these questions, a novel within-subject experimental paradigm was developed. During the anticipation and consumption of social (affective touch) and nonsocial (food) rewards, explicit (ratings of wanting and liking, physical effort) and implicit (fEMG) measures of wanting and liking were taken in 43 healthy adult participants. Reduced activation of the Corrugator Supercilii (CS) muscle (reflecting less frowning and indicating greater positive response) was found in trials with higher wanting and effort during the anticipation of food rewards, as well as in trials with higher liking and effort during the consumption of food rewards. The CS muscle is thus a sensitive measure of wanting and liking of food rewards both during their anticipation and consumption. Crucially, thanks to careful reward matching, these results cannot be explained by differences in subjective wanting, liking, or effort produced to obtain the two types of rewards. No significant modulation of the Zygomaticus Major (ZM) muscle was found for social or food rewards. Explorative analyses however indicated that the ZM may activate during the delivery of the most wanted touch, but not for the most wanted food. The absence of significant effects of social rewards on the activation of CS and ZM muscles are discussed in relation to the specifics of this innovative task comparing two types of matched rewards in the same participants. The present findings contribute to the understanding of the processes underlying motivational and hedonic aspects of rewards, and may therefore inform models of social and non-social decision-making.

Yetişkin Profesyonel Erkek Futbolcuların Hedonik Açlık ve Beslenme Durumlarının Belirlenmesi

Amaç: Bu çalışma; yüksek şiddetli uzun süreli egzersiz yapan profesyonel futbolcular ile sedanter kontrol grubunun hedonik açlık ve beslenme durumlarını belirleyerek fiziksel aktivitenin hedonik açlığa etkisini araştırmak amacıyla yapılmıştır.
 Bireyler ve Yöntem: Çalışmanın örneklem grubunu Aralık 2018- Mart 2019 tarihleri arasında Türkiye Futbol Federasyonu 1. Lig takımlarından birinde oynayan profesyonel 25 futbolcu ve aynı takımda çalışan 25 sedanter erkek olmak üzere 18-35 yaş arası toplam 50 sağlıklı birey oluşturmuştur. Ayrıca sporcu grubu kendi içerisinde fiziksel olarak aktif ve inaktif oldukları (devre arası tatil dönemi) dönem olarak ikiye ayrılmış ve sonuçlar araştırma kapsamında değerlendirilmiştir. Bireylerin hedonik açlık durumları Besin Gücü Ölçeği (BGÖ) ile saptanmıştır. Beş maddelik bir Likert ölçeği ile cevaplandırılan BGÖ’nün değerlendirme aşamasında toplam ve alt grup puanları 1-5 puan arasında değerlendirilmiştir. Beş puan üzerinden yapılan değerlendirme sonucunda ölçek puanlarının ortalama 2.5’un üzerine çıkması hedonik açlığın varlığını ve besinden etkilenildiğini ifade etmektedir.
 Bulgular: Futbolcu (aktif dönem-inaktif dönem) ve sedanter grubun BGÖ ortalama toplam puanları sırasıyla 3.0±0.62, 2.5±0.55, 2.4±0.56 olarak bulunmuştur. BGÖ toplam puanı ile “besin bulunabilirliği” ve “besinlerin tadına bakılması” alt grup ortalama puanlarının futbolcu aktif grubunda sedanter gruba göre daha yüksek olduğu belirlenmiştir (p<0.05). Futbolcu grubunun aktif döneminde BGÖ toplam puanı ve tüm alt grup puanlarının inaktif oldukları döneme göre daha yüksek olduğu bulunmuştur (p<0.05). Futbolcularda hedonik açlığın ve besinden etkilenmenin sedanter bireylere göre daha yüksek olduğu saptanmıştır (p<0.05). Bireylerin vücut kompozisyonu ile BGÖ toplam puanı arasındaki ilişki futbolcuların aktif döneminde ve sedanter grupta negatif ilişkili bulunurken; futbolcuların inaktif döneminde pozitif ilişkili saptanmıştır (p>0.05).
 Sonuç: Bu çalışma, uzun süreli ve yoğun egzersizin hedonik süreçler üzerinde etkili olabileceğinin bir göstergesi olabilir. Ancak bu konuya ilişkin literatürdeki çelişkili bulgular nedeni ile olası etkileşimi anlamak için daha fazla araştırma yapılması gerekmektedir.

Could the link between drug addiction in adulthood and substance use in adolescence result from a blurring of the boundaries between incentive and hedonic processes?

There is a broad consensus that the development of drug addiction in adulthood is closely linked to the onset of drug use in adolescence. However, the relationship between drug exposure during adolescence and subsequent vulnerability to addiction is yet to be fully understood. This review will first use evidence from adult studies on reward and addiction to give an up-to-date reference point of normal reward-circuitry and the maladaptive changes that later occur in addiction. This will then be compared with current evidence from adolescent studies on reward-circuitry. Similarities between the reward processes governing characteristic behavioral traits in adolescence and the reward profile in adult addiction could help to explain why the risk of later developing addiction is increased when substance use is initiated in adolescence. We argue that the age of onset is a major risk factor in the development of substance use disorder due to a blurring of the boundaries between incentive and hedonic processes, which occurs during adolescence. A deeper understanding of the processes that mediate this blurring could open new avenues for the prevention and treatment of adult drug addiction.

The effects of increasing dietary protein during energy balance and energy restriction on homeostatic and hedonic processes

The effects of increasing dietary protein during energy balance and energy restriction on homeostatic and hedonic processes

Decrease in Operant Responding Under Obesogenic Diet Exposure is not Related to Deficits in Incentive or Hedonic Processes.

A growing body of evidence suggests that obesity could result from alterations in reward processing. In rodent models, chronic exposure to an obesogenic diet leads to blunted dopamine signaling and related incentive responding. This study aimed to determine which reward-related behavioral dimensions are actually impacted by obesogenic diet exposure. Mice were chronically exposed to an obesogenic diet. Incentive and hedonic processes were tested through operant conditioning and licking microstructures, respectively. In parallel, mesolimbic dopamine transmission was assessed using microdialysis. Prolonged high-fat (HF) diet exposure led to blunted mesolimbic dopamine release, paralleled by a decrease in operant responding in all schedules tested. HF-fed and control animals similarly decreased their operant responding in an effort-based choice task, and HF-fed animals displayed an overall lower calorie intake in this task. Analysis of the licking microstructures during consumption of a freely accessible reward suggested a decrease in basal hunger and a potentiation of gastrointestinal inhibition in HF-fed animals, without changes in hedonic reactivity. These results suggest that the decrease in operant responding under prolonged HF diet exposure is mainly driven by decrease in hunger as well as stronger postingestive negative feedback mechanisms, rather than by a decrease in incentive or hedonic responses.

Sex differences in hedonic and homeostatic aspects of palatable food motivation

Feeding behaviors can be modified via homeostatic and hedonic mechanisms. Homeostasis, while primarily concerned with maintaining energy balance via food consumption and energy expenditure, can alter food reward and motivation in response to food deprivation. Alternatively, reward and motivation of food is also driven by its palatability or hedonic nature, and this process can be augmented by opioid receptor activation. The present study examined sex differences in the motivational properties of sucrose pellets through manipulation of homeostatic and hedonic processes via acute food deprivation and acute systemic administration of morphine, respectively. The results showed that regardless of sex, systemic injections of morphine did not alter the motivation to obtain a sucrose pellet on a progressive ratio schedule of reinforcement but does significantly increase consumption of sucrose pellets when freely available. Male and female rats demonstrated similar increased consumption of sucrose pellets under free feeding conditions following acute (24-hours) food deprivation, compared to the non-deprived conditions. Overall, the findings from these experiments indicate that female rats work harder in order to obtain a sucrose pellet (under a Progressive Ratio (PR) schedule of reinforcement) and consume more sucrose pellets than males. However, while acute morphine administration causes similar increases on feeding in males and females, it does not alter motivation as measured by breakpoint on a PR schedule of reinforcement.

Hedonic processing in humans is mediated by an opioidergic mechanism in a mesocorticolimbic system.

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Touch-induced pupil size reflects stimulus intensity, not subjective pleasantness

Interpersonal touch is known to influence human communication and emotion. An important system for interpersonal touch is the C-tactile (CT) system, which is activated by a soft stroke on hairy skin with a velocity of 1–10 cms−1. This system been proposed to play a unique role in hedonic valence and emotion of touch. For other sensory modalities, hedonic processing has been associated with pupil dilation. However, it is unclear whether pupil dilation can be modulated by hedonic touch. The current study investigated in two experiments how pupil size reacts to both affective and non-affective stroking. Pupil-size data were obtained to investigate differences between stroking conditions. In addition, an adjusted version of the Touch Perception Task (TPT) was used to assess subjective touch pleasantness ratings. In Experiment 1, affective (3 cms−1) and non-affective (0.3 and 30 cms−1) stroking was applied to the dorsal side of the right hand. Results revealed that stroking velocity had a significant effect on TPT-item scores, showing higher that affective touch was rated as more pleasant compared to non-affective touch, thereby replicating the previous studies. Results, however, revealed no specific pupil dilation for the 3 cms−1 condition; instead, a logarithmic relation was found between pupil-size dilation and stroking velocity. This relation was confirmed in a second experiment. Furthermore, the palm of the hand was used as a control site for tactile stimulation, for which similar findings were obtained as for the dorsal side of the hand. In addition, skin conductance recordings showed a pattern of response to different stroking velocities similar to pupil dilation. These results suggest that pupil-size dilation does respond to tactile input, but that this response is related to arousal caused by changes in stimulus intensity (e.g., stroking velocity) rather than specific C-tactile stimulation.