Taste, olfactory, and food reward value processing in the brain

Orbitofrontal Cortex Encodes Preference for Alcohol

Orbitofrontal cortex (OFC) plays a key role in representation and regulation of reward value, preference, and seeking. OFC function is disrupted in drug use and dependence, but its specific role in alcohol use disorders has not been thoroughly studied. In alcohol-dependent humans OFC activity is increased by alcohol cue presentation. Ethanol (EtOH) also alters OFC neuron excitability in vitro, and OFC manipulation influences EtOH seeking and drinking in rodents.

Orbitofrontal cortex (OFC) plays a key role in representation and regulation of reward value, preference, and seeking. OFC function is disrupted in drug use and dependence, but its specific role in alcohol use disorders has not been thoroughly studied. In alcohol-dependent humans OFC activity is increased by alcohol cue presentation. Ethanol (EtOH) also alters OFC neuron excitability in vitro, and OFC manipulation influences EtOH seeking and drinking in rodents. To understand the relationship between OFC function and individual alcohol use, we recorded OFC neuron activity in rats during EtOH self-administration, characterizing the neural correlates of individual preference for alcohol. After one month of intermittent access to 20% EtOH, male Long–Evans rats were trained to self-administer 20% EtOH, 10% EtOH, and 15% sucrose. OFC neuronal activity was recorded and associated with task performance and EtOH preference. Rats segregated into high and low EtOH drinkers based on homecage consumption and operant seeking of 20% EtOH. Motivation for 10% EtOH and sucrose was equally high in both groups. OFC neuronal activity was robustly increased or decreased during sucrose and EtOH seeking and consumption, and strength of changes in OFC activity was directly associated with individual preference for 20% EtOH. EtOH-associated OFC activity was more similar to sucrose-associated activity in high versus low EtOH drinkers. The results show that OFC neurons are activated during alcohol seeking based on individual preference, supporting this brain region as a potential substrate for alcohol motivation that may be dysregulated in alcohol misuse.

Single-nucleotide polymorphism rs1761667 in the CD36 gene is associated with orosensory perception of a fatty acid in obese and normal-weight Moroccan subjects

Obese subjects have shown a preference for dietary lipids. A recent collection of evidence has proposed that a variant in the CD36 gene plays a significant role in this pathway. We assessed the association between the orosensory detection of a long-chain fatty acid, i.e. oleic acid (OA), and genetic polymorphism of the lipid taste sensor CD36 in obese and normal-weight subjects. Adult participants were recruited in the fasting condition. They were invited to fat taste perception sessions, using emulsions containing OA and according to the three-alternative forced-choice (3-AFC) method. Genomic DNA was used to determine the polymorphism (SNP rs 1761667) of the CD36 gene. Obese (n 50; BMI 34⋅97 (sd 4⋅02) kg/m²) exhibited a significantly higher oral detection threshold for OA (3⋅056 (sd 3⋅53) mmol/l) than did the normal-weight (n 50; BMI 22⋅16 (sd 1⋅81) kg/m²) participants (1⋅20 (sd 3⋅23) mmol/l; P = 0⋅007). There was a positive correlation between OA detection thresholds and BMI in all subjects; evenly with body fat percentage (BF%). AA genotype was more frequent in the obese group than normal-weight group. OA detection thresholds were much higher for AA and AG genotypes in obese subjects compared with normal-weight participants. Higher oral detection thresholds for fatty acid taste are related to BMI, BF% and not always to CD36 genotype.

Hunger and BMI modulate neural responses to sweet stimuli: fMRI meta-analysis

OBJECTIVE

Consuming sweet foods, even when sated, can lead to unwanted weight gain. Contextual factors, such as longer time fasting, subjective hunger, and body mass index (BMI), may increase the likelihood of overeating. Nevertheless, the neural mechanisms underlying these moderating influences on energy intake are poorly understood.

METHODS

We conducted both categorical meta-analysis and meta-regression of factors modulating neural responses to sweet stimuli, using data from 30 functional magnetic resonance imaging (fMRI) articles incorporating 39 experiments (N = 995) carried out between 2006 and 2019.

RESULTS

Responses to sweet stimuli were associated with increased activity in regions associated with taste, sensory integration, and reward processing. These taste-evoked responses were modulated by context. Longer fasts were associated with higher posterior cerebellar, thalamic, and striatal activity. Greater self-reported hunger was associated with higher medial orbitofrontal cortex (OFC), dorsal striatum, and amygdala activity and lower posterior cerebellar activity. Higher BMI was associated with higher posterior cerebellar and insular activity.

CONCLUSIONS

Variations in fasting time, self-reported hunger, and BMI are contexts associated with differential sweet stimulus responses in regions associated with reward processing and homeostatic regulation. These results are broadly consistent with a hierarchical model of taste processing. Hunger, but not fasting or BMI, was associated with sweet stimulus-related OFC activity. Our findings extend existing models of taste processing to include posterior cerebellar regions that are associated with moderating effects of both state (fast length and self-reported hunger) and trait (BMI) variables.

A Systematic Review and Activation Likelihood Estimation Meta-Analysis of fMRI Studies on Sweet Taste in Humans

BACKGROUND

The reward value of palatable foods is often cited as an important influence on eating behaviors, including intake of sugars. However, human neuroimaging studies have generated conflicting evidence on the basic neural representation of taste and reward responses to caloric sweeteners (sucrose and glucose), and most relevant studies have used small subject numbers.

OBJECTIVE

We conducted a systematic review and a coordinate-based meta-analysis of studies reporting brain responses to oral sugar solutions.

METHODS

A systematic search of MEDLINE, Scopus, and PsycINFO through October 2019 identified fMRI studies (in healthy human adults, including those with overweight or obesity) assessing differences in responses to purified sweet and nonsweet taste stimuli. Data were extracted with the primary objective of quantifying evidence for the activation of brain regions associated with caloric sweet taste sensation. We used activation likelihood estimation meta-analysis methods. We also performed multiple sensitivity analyses to assess the generality of effects.

RESULTS

Of 455 unique articles, 15 met the criteria for inclusion. These contributed to 2 primary meta-analyses: 1) sucrose (13 experiments, 179 coordinates, n = 241) and 2) sucrose + glucose (16 experiments, 209 coordinates, n = 262). Consistent activation was apparent in primary taste areas: insula (69.2% of studies) and opercular cortex (76.9% of studies), precentral gyri (53.9% of studies), and globus pallidus and postcentral gyrus (30.8% of studies for each). Evidence of reward activity (caudate) was seen in the primary analyses (30.8% of studies) but not in sensitivity analysis.

CONCLUSIONS

We confirm the importance of primary taste areas for gustatory processing in human adults. We also provide tentative evidence for reward-related caudate activity in relation to the sweet taste of caloric sugars. A number of factors affect the observation and interpretation of brain responses, including reward-related activity. Firm conclusions require confirmation with large data set studies.

Selective impact of lateral orbitofrontal cortex inactivation on reinstatement of alcohol seeking in male Long-Evans rats

The orbitofrontal cortex (OFC) plays a fundamental role in motivated behavior and decision-making. In humans, OFC structure and function is significantly disrupted in drug using and dependent individuals, including those exhibiting chronic alcohol use and alcoholism. In animal models, the OFC has been shown to significantly influence the seeking of non-alcohol drugs of abuse. However direct investigations of the OFC during alcohol seeking and use have been more limited. In the studies reported here, we inactivated lateral (lOFC) or medial OFC (mOFC) subregions in rats during multiple stages of alcohol seeking. After one month of intermittent access to homecage 20% ethanol (EtOH), rats were trained to self-administer EtOH under an FR3 schedule and implanted with cannulae directed to lOFC or mOFC. We inactivated OFC subregions with baclofen/muscimol during EtOH self-administration, extinction, cue-induced reinstatement, and progressive ratio testing to broadly characterize the influence of these subregions on alcohol seeking. There were no significant effects of mOFC or lOFC inactivation during FR3 self-administration, extinction, or progressive ratio self-administration. However, lOFC, and not mOFC, inactivation significantly decreased cue-induced reinstatement of EtOH seeking. These findings contribute new information to the specific impact of OFC manipulation on operant alcohol seeking, support previous studies investigating the role of OFC in seeking and consumption of alcohol and other drugs of abuse, and indicate a specific role for lOFC vs. mOFC in reinstatement.

Laboratory-based interventions targeting food craving: A systematic review and meta-analysis

This systematic review and meta-analysis aimed to quantify the effects of laboratory-based interventions targeting specific mechanisms of food craving, to identify moderators of effects, and to qualitatively summarize findings. The study was conducted and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Sixty-nine studies were included in the quantitative synthesis, and separate meta-analyses were conducted for the outcomes self-reported craving and objective food intake. Results show small to medium positive effects across specific craving interventions on both outcomes. Effect sizes were partly moderated by intervention type. The most effective intervention regarding food intake was in sensu cue exposure. For subjective craving, the most robust evidence was found for beneficial effects of cognitive regulation strategies (ie, reappraisal, suppression, and distraction). Results further indicate that training inhibitory control through behavioral inhibition might be more effective than approach-avoidance training when considering its effect on subjective craving and food intake. People with external eating habits, overeating, or loss-of-control eating might benefit from these types of specific craving interventions. Future research should focus on long-term effects, transferability, and effectiveness in clinical samples.

Metabotropic glutamate receptor 5 in bulimia nervosa

Bulimia nervosa (BN) shares central features with substance-related and addictive disorders. The metabotropic glutamate receptor subtype 5 (mGlu5) plays an important role in addiction. Based on similarities between binge eating and substance-related and addictive disorders, we investigated mGlu5 in vivo in 15 female subjects with BN and 15 matched controls. We measured mGlu5 distribution volume ratio (DVR) with positron emission tomography (PET) using [11 C]ABP688. In BN mGlu5 DVR was higher in the anterior cingulate cortex (ACC), subgenual prefrontal cortex, and straight gyrus (p < 0.05). In BN, higher mGlu5 DVR in various brain regions, including ACC, pallidum, putamen, and caudate, positively correlated with "maturity fears" as assessed using the Eating Disorder Inventory-2 (p < 0.05). In BN and controls, smokers had globally decreased mGlu5 DVR. We present the first evidence for increased mGlu5 DVR in BN. Our findings suggest that pharmacological agents inhibiting mGlu5 might have a therapeutic potential in BN.

Disturbance of taste reactivity and other behavioral alterations after bilateral interleukin-1β microinjection into the cingulate cortex of the rat

The anterior cingulate cortex (ACC), is known to be intimately involved in food-related motivational processes and their behavioral organization, primarily by evaluating hedonic properties of the relevant stimuli. In the present study, the involvement of cingulate cortical interleukin-1β (IL-1β) mediated mechanisms in a) gustation associated facial and somato-motor behavioral patterns of Wistar rats were examined in taste reactivity test (TR). In addition, b) conditioned taste aversion (CTA) paradigm was performed to investigate the role of these cytokine mechanisms in taste sensation associated learning processes, c) the general locomotor activity of the animals was observed in open field test (OPF), and d) the potentially negative reinforcing effect of IL-1β was examined in conditioned place preference test (CPP). During the TR test, species specific behavioral patterns in response to the five basic tastes were analyzed. Response rates of ingestive and aversive patterns of the cytokine treated and the control groups differed significantly in case of the weaker bitter (QHCl, 0.03 mM), and the stronger umami (MSG, 0.5 M) tastes. IL-1β itself did not elicit CTA, it did not interfere with the acquisition of LiCl induced CTA, and it also failed to cause place preference or aversion in the CPP test. In the OPF paradigm, however, significant differences were found between the cytokine treated and the control groups in the rearing and grooming, the number of crossings, and in the distance moved. Our results indicate the involvement of cingulate cortical IL-1β mechanisms in the control of taste perception and other relevant behavioral processes.

First step of odorant detection in the olfactory epithelium and olfactory preferences differ according to the microbiota profile in mice

We have previously provided the first evidence that the microbiota modulates the physiology of the olfactory epithelium using germfree mice. The extent to which changes to the olfactory system depend on the microbiota is still unknown. In the present work, we explored if different microbiota would differentially impact olfaction. We therefore studied the olfactory function of three groups of mice of the same genetic background, whose parents had been conventionalized before mating with microbiota from three different mouse strains. Caecal short chain fatty acids profiles and 16S rRNA gene sequencing ascertained that gut microbiota differed between the three groups. We then used a behavioural test to measure the attractiveness of various odorants and observed that the three groups of mice differed in their attraction towards odorants. Their olfactory epithelium properties, including electrophysiological responses recorded by electro-olfactograms and expression of genes related to the olfactory transduction pathway, also showed several differences. Overall, our data demonstrate that differences in gut microbiota profiles are associated with differences in olfactory preferences and in olfactory epithelium functioning.

Differential Cerebral Gustatory Responses to Sucrose, Aspartame, and Stevia Using Gustatory Evoked Potentials in Humans

Aspartame and Stevia are widely substituted for sugar. Little is known about cerebral activation in response to low-caloric sweeteners in comparison with high-caloric sugar, whereas these molecules lead to different metabolic effects. We aimed to compare gustatory evoked potentials (GEPs) obtained in response to sucrose solution in young, healthy subjects, with GEPs obtained in response to aspartame and Stevia. Twenty healthy volunteers were randomly stimulated with three solutions of similar intensities of sweetness: Sucrose 10 g/100 mL of water, aspartame 0.05 g/100 mL, and Stevia 0.03 g/100 mL. GEPs were recorded with EEG (Electroencephalogram) electrodes. Hedonic values of each solution were evaluated using the visual analog scale (VAS). The main result was that P1 latencies of GEPs were significantly shorter when subjects were stimulated by the sucrose solution than when they were stimulated by either the aspartame or the Stevia one. P1 latencies were also significantly shorter when subjects were stimulated by the aspartame solution than the Stevia one. No significant correlation was noted between GEP parameters and hedonic values marked by VAS. Although sucrose, aspartame, and Stevia lead to the same taste perception, cerebral activation by these three sweet solutions are different according to GEPs recording. Besides differences of taste receptors and cerebral areas activated by these substances, neural plasticity, and change in synaptic connections related to sweet innate preference and sweet conditioning, could be the best hypothesis to explain the differences in cerebral gustatory processing after sucrose and sweeteners activation.

The Neuronal Encoding of Oral Fat by the Coefficient of Sliding Friction in the Cerebral Cortex and Amygdala

Fat in the diet contributes to the pleasant mouthfeel of many foods, but overconsumption may contribute to obesity. Here we analyze what properties of fat in the mouth are sensed, by analyzing the responses of neurons in the macaque insular taste cortex, and two areas to which it projects the orbitofrontal cortex where the pleasantness of fat is represented, and the amygdala. We discovered that the firing rate responses of these fat-responsive neurons are correlated with the coefficient of sliding friction (CSF) and not with viscosity which reflects food thickness. Other, not fat-sensitive, neurons encoded viscosity and not the CSF. Neuronal population analyses confirmed that fat-responsive neurons conveyed information about the CSF but not about viscosity. Conversely the viscosity-sensitive neuronal population conveyed information about viscosity but not about the CSF. This new understanding of the representation of oral fat in the cerebral cortex and amygdala opens the way for the systematic development of foods with the pleasant mouthfeel of fat, together with ideal nutritional content and has great potential to contribute to healthy eating and a healthy body weight.

The texture and taste of food in the brain

Oral texture is represented in the brain areas that represent taste, including the primary taste cortex, the orbitofrontal cortex, and the amygdala. Some neurons represent viscosity, and their responses correlate with the subjective thickness of a food. Other neurons represent fat in the mouth, and represent it by its texture not by its chemical composition, in that they also respond to paraffin oil and silicone in the mouth. The discovery has been made that these fat-responsive neurons encode the coefficient of sliding friction and not viscosity, and this opens the way for the development of new foods with the pleasant mouth feel of fat and with health-promoting designed nutritional properties. A few other neurons respond to free fatty acids (such as linoleic acid), do not respond to fat in the mouth, and may contribute to some "off" tastes in the mouth. Some other neurons code for astringency. Others neurons respond to other aspects of texture such as the crisp fresh texture of a slice of apple versus the same apple after blending. Different neurons respond to different combinations of these texture properties, oral temperature, taste, and in the orbitofrontal cortex to olfactory and visual properties of food. In the orbitofrontal cortex, the pleasantness and reward value of the food is represented, but the primary taste cortex represents taste and texture independently of value. These discoveries were made in macaques that have similar cortical brain areas for taste and texture processing as humans, and complementary human functional neuroimaging studies are described.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Leptin modulates olfactory discrimination and neural activity in the olfactory bulb

AIM

Leptin is an important peptide hormone that regulates food intake and plays a crucial role in modulating olfactory function. Although a few previous studies have investigated the effect of leptin on odor perception and discrimination in rodents, research on the neural basis underlying the behavioral changes is lacking. Here we study how leptin affects behavioral performance during a go/no-go task and how it modulates neural activity of mitral/tufted cells in the olfactory bulb, which plays an important role in odor information processing and representation.

METHODS

A go/no-go odor discrimination task was used in the behavioral test. For in vivo studies, single unit recordings, local field potential recordings and fiber photometry recordings were used. For in vitro studies, we performed patch clamp recordings in the slice of the olfactory bulb.

RESULTS

Behaviorally, leptin affects performance and reaction time in a difficult odor-discrimination task. Leptin decreases the spontaneous firing of single mitral/tufted cells, decreases the odor-evoked beta and high gamma local field potential response, and has bidirectional effects on the odor-evoked responses of single mitral/tufted cells. Leptin also inhibits the population calcium activity in genetically identified mitral/tufted cells and granule cells. Furthermore, in vitro slice recordings reveal that leptin inhibits mitral cell activity through direct modulation of the voltage-sensitive potassium channel.

CONCLUSIONS

The behavioral reduction in odor discrimination observed after leptin administration is likely due to decreased neural activity in mitral/tufted cells, caused by modulation of potassium channels in these cells.

Cross-modal interactions as a strategy to enhance salty taste and to maintain liking of low-salt food: a review

Salt reduction in foods is becoming an important challenge to protect population health from severe diseases as recommended by different health agencies worldwide. Among the reduction strategies already evaluated in order to lower sodium salt content in foods, the use of cross-modal interactions between taste and odour, regardless of saltiness, was revealed to be a very promising method to improve saltiness perception. Cross-modal odour-taste interactions, as means to enhance salty taste in foods, is reviewed. Salt-related odours can enhance salty taste in water solutions containing a low level of sodium chloride through odour-induced changes in taste perception. Odour-induced saltiness perception enhancement (OISE) depends on salt concentration (intensity). OISE was also found to be effective in low salt content solid model cheese but was texture- and composition-dependent. A significant enhancement in saltiness perception induced by Comté cheese and sardine odours was observed only in model foods with soft textures. In ternary odour-sour-salty solutions, sourness additively enhanced saltiness perception with salt-related odours. Finally, in cream-based food systems, a strategy combining OISE and heterogeneous distribution of stimuli was found to compensate for a greater than 35% decrease in salt content without significant loss of acceptability. However, variation in the composition of the food matrix influenced aroma and salt release and consequently the overall saltiness perception. A better knowledge of the mechanisms involved in cross-modal perceptual interactions at the central level should allow for the optimization of their use as salt reduction strategies for healthier food design.

Colouring perception: Package colour cues affect neural responses to sweet dairy drinks in reward and inhibition related regions

Extrinsic product cues such as package colour may change product perception and perceived reward value during product evaluation. Healthier foods (i.e., 'light', sugar- or fat-reduced) often have different packages than regular products, e.g., they may be less vibrantly coloured. People vary in their degree of health-interest and self-control ability and may be affected differently by package colour. This study assesses the extent to which package colour and participant characteristics interact and influence product perception and brain responses. Thirty-four healthy females performed a functional MRI task in which they viewed four differently coloured packages (regular vs. healthier; differing in brightness and saturation levels) with or without simultaneously tasting a either a regular or a healthier calorie-reduced drink. Results indicate main effects of package and taste and a package*taste interaction effect. Compared to healthier packages viewing regular packages enhanced activation in region implicated in inhibitory control (inferior frontal gyrus) and a reward-related region (striatum), the latter even more so as participants' health interest increased (r = 0.43, p = 0.01). Incongruent package-taste combinations decreased activation in the orbitofrontal cortex (OFC, a region implicated in reward representation) compared to congruent combinations. Tasting the healthier compared to regular product enhanced activation in the middle and superior frontal gyrus, which are implicated in inhibitory control, as well as the striatum and OFC, suggesting a cognitively driven preference for the healthier product. In conclusion, this paper provides evidence for the conditions under which package colour and taste properties modulate neural correlates related to reward and inhibition. Individual differences in health-interest and impulsivity influence package- and taste-related neural correlates and thus underscore the importance of taking participant characteristics into account in food research.

Distinct brain representations of processed and unprocessed foods

Among all of the stimuli surrounding us, food is arguably the most rewarding for the essential role it plays in our survival. In previous visual recognition research, it has already been demonstrated that the brain not only differentiates edible stimuli from non-edible stimuli but also is endowed with the ability to detect foods' idiosyncratic properties such as energy content. Given the contribution of the cooked diet to human evolution, in the present study we investigated whether the brain is sensitive to the level of processing food underwent, based solely on its visual appearance. We thus recorded visual evoked potentials (VEPs) from normal-weight healthy volunteers who viewed color images of unprocessed and processed foods equated in caloric content. Results showed that VEPs and underlying neural sources differed as early as 130 ms post-image onset when participants viewed unprocessed versus processed foods, suggesting a within-category early discrimination of food stimuli. Responses to unprocessed foods engaged the inferior frontal and temporal regions and the premotor cortices. In contrast, viewing processed foods led to the recruitment of occipito-temporal cortices bilaterally, consistently with other motivationally relevant stimuli. This is the first evidence of diverging brain responses to food as a function of the transformation undergone during its preparation that provides insights on the spatiotemporal dynamics of food recognition.

Neural correlates of taste reward value across eating disorders

Individuals with eating disorders (ED) make extreme food choices, raising the possibility of altered food-value computation. We utilized an associative taste reward learning paradigm to test whether value signaling differs between participants with EDs vs. healthy controls (HC). We followed up on previous work examining prediction error (PE) signaling, which is a brain response to violation of a learned reward contingency. Expected value (EV) signal is a trial-by-trial assessment of reward significance accounting for error signaling, reward-likelihood, and learning rate. Adult female participants (N = 111) performed a temporal difference (TD) fMRI taste task, which is a specific type of associative reward learning paradigm, to determine EV signal: Anorexia Nervosa-ill (N = 28), Anorexia Nervosa-recovered (N = 20), Bulimia Nervosa (BN) (N = 20), and HC (N= 43). Anatomical region-of-interest (ROI) analyses were performed utilizing EV regressors derived via algorithm, with ROIs based on prior EV analyses: orbitofrontal cortex, anterior cingulate (ACC), amygdala, and striatum. EV signal was elevated in the bilateral ACC in AN-ill vs. HC and BN. Intolerance of uncertainty negatively correlated with EV in AN-ill. BMI and EV were negatively-correlated across groups. Altered ACC EV computation in response to food stimuli could contribute to food restriction in AN-ill.

Assessing the Overlap Between Three Measures of Food Reward

Food reward is an important concept for research in eating behaviors. Many food reward tasks have been developed and are in active use. However, little is known how much these tasks overlap. Here, we sought to compare three promising food reward tasks: (1) the Leeds Food Preference Questionnaire (LFPQ; a procedure combining explicit ratings of wanting and liking and an implicit wanting task based on forced choice), (2) a hand grip force task, and (3) an emotional attentional blink (EAB) task. Specifically, we assessed whether the tasks are sensitive to changes in hunger, correlate with each other, and correlate with trait binge eating and snack food calorie intake. Thirty-nine women aged 25.51 ± 5.99 years, with a BMI of 22.51 ± 3.58 kg/m² completed the three tasks twice: after a 6-h fast and following a breakfast meal. In the fasted condition, participants were also given ad libitum access to snack foods to assess calorie intake. Prior to the two laboratory sessions, participants completed a trait binge eating questionnaire. Results revealed that the LFPQ’s explicit wanting and explicit liking subscales, as well as grip force reflected higher food reward scores in the fasted condition. The three metrics also correlated positively with each other. Explicit wanting and liking correlated with snack food intake, while grip force did not. None of the tasks were related to trait binge eating. Reaction times in the forced choice procedure did not reflect changes in hunger, but the task was nevertheless able to differentiate between foods varying in taste and fat content. The EAB was not sensitive to the hunger manipulation; neither did the task correlate with binge eating or energy intake. Collectively, our findings suggest that the explicit wanting and liking scales and the grip force task measure the same construct, whereas EAB results may be obscured by a variety of potential confounding factors. Future research could include additional food reward tasks in comparisons, measure covariates that may moderate the variables’ associations, and compare hunger-dependent changes in food reward in different subgroups.

Associations between olfactory pathway gene methylation marks, obesity features and dietary intakes

Background

Olfaction is an important sense influencing food preferences, appetite, and eating behaviors. This hypothesis-driven study aimed to assess associations between olfactory pathway gene methylation signatures, obesity features, and dietary intakes.

Methods

A nutriepigenomic analysis was conducted in 474 adults from the Methyl Epigenome Network Association (MENA) project. Anthropometric measurements, clinical data, and serum metabolic profiles of the study population were obtained from structured databases of the MENA cohorts. Habitual dietary intake was assessed using a validated semiquantitative food frequency questionnaire. DNA methylation was measured in circulating white blood cells by microarray (Infinium Human Methylation 450 K BeadChips). FDR values (p < 0.0001) were used to select those CpGs that showed the best correlation with body mass index (BMI) and waist circumference (WC). Pathway analyses involving the characterization of genes involved in the olfactory transduction system were performed using KEGG and pathDIP reference databases.

Results

Overall, 15 CpG sites at olfactory pathway genes were associated with BMI (p < 0.0001) and WC (p < 0.0001) after adjustments for potential confounding factors. Together, methylation levels at the15 CpG sites accounted for 22% and 20% of the variability in BMI and WC (r² = 0.219, p < 0.001, and r² = 0.204, p < 0.001, respectively). These genes encompassed olfactory receptors (OR4D2, OR51A7, OR2T34, and OR2Y1) and several downstream signaling molecules (SLC8A1, ANO2, PDE2A, CALML3, GNG7, CALML6, PRKG1, and CAMK2D), which significantly regulated odor detection and signal transduction processes within the complete olfactory cascade, as revealed by pathway enrichment analyses (p = 1.94 × 10^–10). Moreover, OR4D2 and OR2Y1 gene methylation patterns strongly correlated with daily intakes of total energy (p < 0.0001), carbohydrates (p < 0.0001), protein (p < 0.0001), and fat (p < 0.0001).

Conclusions

The results of this study suggest novel relationships between olfactory pathway gene methylation signatures, obesity indices, and dietary intakes.

Unique infant neurobiology produces distinctive trauma processing

Trauma experienced in early life has unique neurobehavioral outcomes related to later life psychiatric sequelae. Recent evidence has further highlighted the context of infant trauma as critical, with trauma experienced within species-atypical aberrations in caregiving quality as particularly detrimental. Using data from primarily rodent models, we review the literature on the interaction between trauma and attachment in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. Together these data suggest that infant trauma processing and its enduring effects are impacted by both the immaturity of brain areas for processing trauma and the unique functioning of the early-life brain, which is biased towards forming robust attachments regardless of the quality of care. Understanding the critical role of the caregiver in further altering early life brain processing of trauma is important for developing age-relevant treatment and interventions.

Reduced sensitivity to devaluation for instrumental but not consummatory behaviors in binge eating prone rats

Binge eating is characterized by the consumption of a large amount of palatable food in a short period of time and is a core feature of many eating disorders. Patients with eating disorders are also known to display impairments in inhibitory control, cognition and decision-making, which may promote and maintain binge eating symptomology. In the current study, we examined whether rats that were subsequently characterized as displaying a higher propensity to binge eat would show pre-existing deficits in reinforcer devaluation-a paradigm used to examine decision-making following reductions in the value of a food reinforcer. Female rats were first trained to respond on two levers for the delivery of two food reinforcers (sucrose and maltodextrin solutions). At the test stage, rats were provided 1 h access to one of the two reinforcers to allow for devaluation via sensory specific satiety, immediately followed by an extinction test with both levers. Normal rats typically show reductions in responding on the lever associated with the devalued reinforcer (i.e., intact goal-directed responding). Subsequently, we used intermittent access to palatable food to identify high (BE prone [BEP]; n = 14), intermediate (BE neutral [BEN]; n = 48), and low (BE resistant [BER]; n = 13) phenotypes of binge eating. Prior reinforcer devaluation performance showed BEN and BER rats suppressed responding on the lever associated with the devalued reinforcer while BEP rats did not. This insensitivity to instrumental reinforcer devaluation in BEP rats did not reflect impaired sensory-specific satiety as during a food choice test, BEP rats showed a more robust alteration in food preferences following devaluation. Additionally, across all rats sensory specific satiety was correlated with subsequent intake of palatable food. Collectively, these findings suggest dissociable effects of devaluation procedures on instrumental actions and consummatory behaviors in BEP rats, and may indicate that pre-existing differences in goal-directed behavior and sensory-specific satiety contribute to the propensity to overeat palatable food.

Functional specialization of macaque premotor F5 subfields with respect to hand and mouth movements: A comparison of task and resting-state fMRI

Based on architectonic, tract-tracing or functional criteria, the rostral portion of ventral premotor cortex in the macaque monkey, also termed area F5, has been divided into several subfields. Cytoarchitectonical investigations suggest the existence of three subfields, F5c (convexity), F5p (posterior) and F5a (anterior). Electrophysiological investigations have suggested a gradual dorso-ventral transition from hand- to mouth-dominated motor fields, with F5p and ventral F5c strictly related to hand movements and mouth movements, respectively. The involvement of F5a in this respect, however, has received much less attention. Recently, data-driven resting-state fMRI approaches have also been used to examine the presence of distinct functional fields in macaque ventral premotor cortex. Although these studies have suggested several functional clusters in/near macaque F5, so far the parcellation schemes derived from these clustering methods do not completely retrieve the same level of F5 specialization as suggested by aforementioned invasive techniques. Here, using seed-based resting-state fMRI analyses, we examined the functional connectivity of different F5 seeds with key regions of the hand and face/mouth parieto-frontal-insular motor networks. In addition, we trained monkeys to perform either hand grasping or ingestive mouth movements in the scanner in order to compare resting-state with task-derived functional hand and mouth motor networks. In line with previous single-cell investigations, task-fMRI suggests involvement of F5p, dorsal F5c and F5a in the execution of hand grasping movements, while non-communicative mouth movements yielded particularly pronounced responses in ventral F5c. Corroborating with anatomical tracing data of macaque F5 subfields, seed-based resting-state fMRI suggests a transition from predominant functional correlations with the hand-motor network in F5p to mostly mouth-motor network functional correlations in ventral F5c. Dorsal F5c yielded robust functional correlations with both hand- and mouth-motor networks. In addition, the deepest part of the fundus of the inferior arcuate, corresponding to area 44, displayed a strikingly different functional connectivity profile compared to neighboring F5a, suggesting a different functional specialization for these two neighboring regions.

Taste and smell processing in the brain

Taste pathways in humans and other primates project from the nucleus of the solitary tract directly to the taste thalamus, and then to the taste insula. The taste cortex in the anterior insula provides separate and combined representations of the taste, temperature, and texture of food in the mouth independently of hunger and thus of reward value and pleasantness. One synapse on, in the orbitofrontal cortex, these sensory inputs are for some neurons combined by associative learning with olfactory inputs received from the pyriform cortex, and visual inputs from the temporal lobe, and these neurons encode food reward value in that they only respond to food when hungry, and in that activations correlate linearly with subjective pleasantness. Cognitive factors, including word-level descriptions, and selective attention to affective value, modulate the representation of the reward value of taste, olfactory and flavor stimuli in the orbitofrontal cortex and a region to which it projects, the anterior cingulate cortex. These food reward representations are important in the control of appetite, and the liking of food. Individual differences in these reward representations may contribute to obesity, and there are age-related differences in these reward representations.

Cerebral gustatory activation in response to free fatty acids using gustatory evoked potentials in humans

There is some evidence of specific oro-detection of FFAs in rodents and humans. The aim of this study was to record gustatory evoked potentials (GEPs) in response to FFA solutions and to compare GEPs in response to linoleic acid solution with GEPs obtained after stimulation with sweet and salty tastants. Eighteen healthy men were randomly stimulated with fatty (linoleic acid), sweet (sucrose), and salty (NaCl) solutions at two concentrations in the first experiment. Control recordings (n = 14) were obtained during stimulation by a paraffin oil mixture without FFA or by water. In the second experiment, 28 men were randomly stimulated with five FFA solutions and a paraffin emulsion. GEPs were recorded with electroencephalographic electrodes at Cz, Fz, and Pz. GEPs were observed in response to FFA in all participants. GEP characteristics did not differ according to the quality and the concentration of the solutions in the first experiment and according to the FFA in the second experiment. This study describes for the first time GEPs in response to FFA and demonstrates that the presence of FFA in the mouth triggers an activation of the gustatory cortex. These data reinforce the concept that fat taste could be the sixth primary taste.

The role of the orbitofrontal cortex in alcohol use, abuse, and dependence

One of the major functions of the orbitofrontal cortex (OFC) is to promote flexible motivated behavior. It is no surprise, therefore, that recent work has demonstrated a prominent impact of chronic drug use on the OFC and a potential role for OFC disruption in drug abuse and addiction. Among drugs of abuse, the use of alcohol is particularly salient with respect to OFC function. Although a number of studies in humans have implicated OFC dysregulation in alcohol use disorders, animal models investigating the association between OFC and alcohol use are only beginning to be developed, and there is still a great deal to be revealed. The goal of this review is to consider what is currently known regarding the role of the OFC in alcohol use and dependence. I will first provide a brief, general overview of current views of OFC function and its contributions to drug seeking and addiction. I will then discuss research to date related to the OFC and alcohol use, both in human clinical populations and in non-human models. Finally I will consider issues and strategies to guide future study that may identify this brain region as a key player in the transition from moderated to problematic alcohol use and dependence.

Activity of frontal pole cortex reflecting hedonic tone of food and drink: fNIRS study in humans

Cognitive and hedonic aspects of taste have been studied using different neuroimaging techniques in humans. However, the methods used are unsuitable for easy monitoring of hedonics induced by intake of foods and beverages. Here we have tried to monitor changes in oxygenated hemoglobin (oxyHb) levels in the anterior prefrontal cortex (aPFC, frontopolar cortex, Brodmann area 10) in response to intake of hedonically different edibles in healthy adults. When subjects tasted sweet and bitter solutions freely without any particular instruction, cortical activation varied greatly among subjects and between the two stimuli, and no consistent results were obtained. Subjects then ate or drank preferred (hedonically positive) and disliked (hedonically negative) edibles. Although these stimuli differed among subjects, hedonically positive stimuli decreased oxyHb, whereas hedonically negative stimuli increased oxyHb, particularly in the ventral aPFC. When subjects tasted 4 kinds of jellies with different flavors and evaluated the degree of pleasantness, oxyHb level in the ventral region correlated negatively with pleasantness score. These results revealed that pleasant and unpleasant edibles tended to elicit decreased and increased oxyHb levels, respectively, within the ventral aPFC, suggesting that monitoring of oxyHb in this region may prove useful for objective evaluation of pleasantness of food and drink.

The essence of appetite: does olfactory receptor variation play a role?

Olfactory receptors are G-protein-coupled chemoreceptors expressed on millions of olfactory sensory neurons within the nasal cavity. These receptors detect environmental odorants and signal the brain regarding the location of feed, potential mates, and the presence of possible threats (e.g., predators or chemical toxins). Olfactory receptors also are present in organs outside of the nasal cavity where they bind to molecules such as nutrients and metabolites from the animal's internal environment to elicit physiological responses, including changes in gut motility, ventilation rate, and cellular migration. Recent evidence supports an additional role of olfactory receptors in the regulation of appetite in humans and rodents. In particular, genetic variation among individuals in specific odorant receptor genes has been linked to differences in their feeding behaviors, food choices, and the regulation of energy balance. This review provides a general overview of the olfactory receptors of vertebrates and their genetic variability and provides supporting evidence for a physiological role of olfactory receptors in appetite regulation of livestock. Basic research on olfactory receptors of livestock and their ligands should facilitate the development of novel odorant receptor agonists and identification of specific olfactory receptor variants that may be developed to enhance animal production efficiency.

Optimizing Texture Modified Foods for Oro-pharyngeal Dysphagia: A Difficult but Possible Target?

Dysphagia is a swallowing disorder characterized by the difficulty in transferring solid foods and/or liquids from the oral cavity to the stomach, imparing autonomous, and safe oral feeding. The main problems deriving from dysphagia are tracheo-bronchial aspiration, aspiration pneumonia, malnutrition and dehydration. In order to overcome dysphagia-induced problems, over the years water and food thickening has been used, focusing specifically on viscosity increase, but limited results have been obtained. Elastic components and their effects on the cohesiveness on the bolus should be taken into account in the first place. We provide an analysis of dysphagia and suggest possible corrections to the protocols which are being used at present, taking into account rheological properties of food and the effect of saliva on the bolus. We reckon that considering such aspects in the dysphagia management market and healthcare catering would result in significant clinical risk reduction.

Taste sensitivity in anorexia nervosa: A systematic review

OBJECTIVE

There is evidence for altered processing of taste in anorexia nervosa, particularly in the areas of reward processing and hedonic sensitivity. However, research on whether people with anorexia nervosa identify taste stimuli accurately, known as taste sensitivity, has yielded mixed findings. The objective of this study was to synthesize the literature on taste sensitivity in this disorder to provide a basis for future discussion on whether altered taste sensitivity may be also implicated in wider atypical taste processing in anorexia.

METHOD

Electronic databases were searched systematically to identify published research examining taste sensitivity in anorexia. Search terms were "anorexia nervosa", or "eating disorder", combined with "taste". 18 studies met inclusion criteria.

RESULTS

The review of the findings suggest that individuals with AN may experience reduced taste sensitivity that may improve following recovery. However, there was a significant variability in results across studies, potentially reflecting methodological problems including low sample sizes, experimental designs, and uncontrolled confounding variables.

DISCUSSION

This review suggests that altered taste sensitivity could represent a component in the wider altered taste processing observed in anorexia nervosa. However, the heterogeneity of findings highlight the need for future research to consider methodological issues raised by this review.

Endocannabinoids in Body Weight Control

Maintenance of body weight is fundamental to maintain one's health and to promote longevity. Nevertheless, it appears that the global obesity epidemic is still constantly increasing. Endocannabinoids (eCBs) are lipid messengers that are involved in overall body weight control by interfering with manifold central and peripheral regulatory circuits that orchestrate energy homeostasis. Initially, blocking of eCB signaling by first generation cannabinoid type 1 receptor (CB1) inverse agonists such as rimonabant revealed body weight-reducing effects in laboratory animals and men. Unfortunately, rimonabant also induced severe psychiatric side effects. At this point, it became clear that future cannabinoid research has to decipher more precisely the underlying central and peripheral mechanisms behind eCB-driven control of feeding behavior and whole body energy metabolism. Here, we will summarize the most recent advances in understanding how central eCBs interfere with circuits in the brain that control food intake and energy expenditure. Next, we will focus on how peripheral eCBs affect food digestion, nutrient transformation and energy expenditure by interfering with signaling cascades in the gastrointestinal tract, liver, pancreas, fat depots and endocrine glands. To finally outline the safe future potential of cannabinoids as medicines, our overall goal is to address the molecular, cellular and pharmacological logic behind central and peripheral eCB-mediated body weight control, and to figure out how these precise mechanistic insights are currently transferred into the development of next generation cannabinoid medicines displaying clearly improved safety profiles, such as significantly reduced side effects.

Spike-Timing of Orbitofrontal Neurons Is Synchronized With Breathing

The orbitofrontal cortex (OFC) has been implicated in a multiplicity of complex brain functions, including representations of expected outcome properties, post-decision confidence, momentary food-reward values, complex flavors and odors. As breathing rhythm has an influence on odor processing at primary olfactory areas, we tested the hypothesis that it may also influence neuronal activity in the OFC, a prefrontal area involved also in higher order processing of odors. We recorded spike timing of orbitofrontal neurons as well as local field potentials (LFPs) in awake, head-fixed mice, together with the breathing rhythm. We observed that a large majority of orbitofrontal neurons showed robust phase-coupling to breathing during immobility and running. The phase coupling of action potentials to breathing was significantly stronger in orbitofrontal neurons compared to cells in the medial prefrontal cortex. The characteristic synchronization of orbitofrontal neurons with breathing might provide a temporal framework for multi-variable processing of olfactory, gustatory and reward-value relationships.

How Does Food Taste in Anorexia and Bulimia Nervosa? A Protocol for a Quasi-Experimental, Cross-Sectional Design to Investigate Taste Aversion or Increased Hedonic Valence of Food in Eating Disorders

Background: Despite on-going efforts to better understand dysregulated eating, the olfactory-gustatory deficits and food preferences in eating disorders (ED), and the mechanisms underlying the perception of and responses to food properties in anorexia nervosa (AN) and bulimia nervosa (BN) remain largely unknown; both during the course of the illness and compared to healthy populations. It is, therefore, necessary to systematically investigate the gustatory perception and hedonics of taste in patients with AN and BN. To this end, we will examine whether aversions to the taste of high-calorie food is related to the suppression of energy intake in restricting-type AN, and whether an increased hedonic valence of sweet, caloric-dense foods may be part of the mechanisms triggering binge-eating episodes in BN. In addition, the role of cognitions influencing these mechanisms will be examined.

Method: In study 1, four mixtures of sweet-fat stimuli will be presented in a sensory two-alternative forced-choice test involving signal detection analysis. In study 2, a full-scale taste reactivity test will be carried out, including psychophysiological and behavioral measures to assess subtle and covert hedonic changes. We will compare the responses of currently-ill AN and BN patients to those who have recovered from AN and BN, and also to those of healthy normal-weight and underweight individuals without any eating disorder pathology.

Discussion: If taste response profiles are differentially linked to ED types, then future studies should investigate whether taste responsiveness represents a useful diagnostic measure in the prevention, assessment and treatment of EDs. The expected results on cognitive mechanisms in the top-down processes of food hedonics will complement current models and contribute to the refinement of interventions to change cognitive aspects of taste aversions, to establish functional food preferences and to better manage food cravings associated with binge-eating episodes. No trial registration was required for this protocol, which was approved by the Swiss ethics committee (CER-VD, n° 2016-02150) and the Ethics Review Panel of the University of Luxembourg.

Body weight and its association with impulsivity in middle and old age individuals

Impulsivity, conceptualized as impulsive personality trait, poor inhibitory control and enhanced reward sensitivity, has been strongly linked to obesity. In particular, a disequilibrium between cognitive control and reward sensitivity has been observed in obese individuals in both behavioural and imaging studies. While this issue has been widely investigated in children and adults, it has received little attention in older adults. Here, obese and non-obese participants aged between 40 and 70 years completed the Barratt Impulsiveness scale (assessing motor, non-planning and attentional impulsiveness), a Go/no-go task with foods and non-foods (assessing inhibitory control) and a reward sensitivity battery with high and low caloric foods (assessing liking, wanting, tastiness and frequency of consumption). We observed that participants with higher BMI reported increased wanting for high calorie foods, but did not show poorer inhibitory control. Interestingly, participants who scored lower on the MMSE reported to consume high calorie more than low calorie foods. Finally, those who presented low scores on non-planning and motor impulsiveness subscales reported higher tastiness ratings for low calorie foods. These results show that increased reward sensitivity but not reduced inhibitory control may characterize higher BMI during aging. Importantly, they also highlight new findings concerning food preferences among older adults.

Neural Network Alterations Across Eating Disorders: A Narrative Review of fMRI Studies

BACKGROUND

Functional magnetic resonance imaging (fMRI) has provided insight on how neural abnormalities are related to the symptomatology of the eating disorders (EDs): anorexia nervosa (AN), bulimia nervosa (BN), and binge eating disorder (BED). More specifically, an increasingly growing number of brain imaging studies has shed light on how functionally connected brain networks contribute not only to disturbed eating behavior, but also to transdiagnostic alterations in body/interoceptive perception, reward processing and executive functioning.

METHODS

This narrative review aims to summarize recent advances in fMRI studies of patients with EDs by highlighting studies investigating network alterations that are shared across EDs.

RESULTS AND CONCLUSION

Findings on reward processing in both AN and BN patients point to the presence of altered sensitivity to salient food stimuli in striatal regions and to the possibility of hypothalamic inputs being overridden by top-down emotional-cognitive control regions. Additionally, innovative new lines of research suggest that increased activations in fronto-striatal circuits are strongly associated with the maintenance of restrictive eating habits in AN patients. Although significantly fewer studies have been carried out in patients with BN and BED, aberrant neural responses to both food cues and anticipated food receipt appear to occur in these populations. These altered responses, coupled with diminished recruitment of prefrontal cognitive control circuitry, are believed to contribute to the binge eating of palatable foods. Results from functional network connectivity studies are diverse, but findings tend to converge on indicating disrupted resting-state connectivity in executive networks, the default-mode network and the salience network across EDs.

Walnut consumption increases activation of the insula to highly desirable food cues: A randomized, double-blind, placebo-controlled, cross-over fMRI study

AIMS

The use of walnuts is recommended for obesity and type 2 diabetes, although the mechanisms through which walnuts may improve appetite control and/or glycaemic control remain largely unknown.

MATERIALS AND METHODS

To determine whether short-term walnut consumption could alter the neural control of appetite using functional magnetic resonance imaging, we performed a randomized, placebo-controlled, double-blind, cross-over trial of 10 patients who received, while living in the controlled environment of a clinical research center, either walnuts or placebo (using a validated smoothie delivery system) for 5 days each, separated by a wash-out period of 1 month.

RESULTS

Walnut consumption decreased feelings of hunger and appetite, assessed using visual analog scales, and increased activation of the right insula to highly desirable food cues.

CONCLUSIONS

These findings suggest that walnut consumption may increase salience and cognitive control processing of highly desirable food cues, leading to the beneficial metabolic effects observed.

The bioelectronic nose and tongue using olfactory and taste receptors: Analytical tools for food quality and safety assessment

Food intake is the primary method for obtaining energy and component materials in the human being. Humans evaluate the quality of food by combining various facets of information, such as an item of food's appearance, smell, taste, and texture in the mouth. Recently, bioelectronic noses and tongues have been reported that use human olfactory and taste receptors as primary recognition elements, and nanoelectronics as secondary signal transducers. Bioelectronic sensors that mimic human olfaction and gustation have sensitively and selectively detected odor and taste molecules from various food samples, and have been applied to food quality assessment. The portable and multiplexed bioelectronic nose and tongue are expected to be used as next-generation analytical tools for rapid on-site monitoring of food quality. In this review, we summarize recent progress in the bioelectronic nose and tongue using olfactory and taste receptors, and discuss the potential applications and future perspectives in the food industry.

Food-Related Odors Activate Dopaminergic Brain Areas

Food-associated cues of different sensory categories have often been shown to be a potent elicitor of cerebral activity in brain reward circuits. Smells influence and modify the hedonic qualities of eating experience, and in contrast to smells not associated with food, perception of food-associated odors may activate dopaminergic brain areas. In this study, we aimed to verify previous findings related to the rewarding value of food-associated odors by means of an fMRI design involving carefully preselected odors of edible and non-edible substances. We compared activations generated by three food and three non-food odorants matching in terms of intensity, pleasantness and trigeminal qualities. We observed that for our mixed sample of 30 hungry and satiated participants, food odors generated significantly higher activation in the anterior cingulate cortex (right and left), insula (right), and putamen (right) than non-food odors. Among hungry subjects, regardless of the odor type, we found significant activation in the ventral tegmental area in response to olfactory stimulation. As our stimuli were matched in terms of various perceptual qualities, this result suggests that edibility of an odor source indeed generates specific activation in dopaminergic brain areas.

Detection thresholds for four different fatty stimuli are associated with increased dietary intake of processed high-caloric food

BMI-specific differences in food choice and energy intake have been suggested to modulate taste perception. However, associations between body composition and fat taste sensitivity are controversial. The objective of this study was to examine the association between body composition, dietary intake and detection thresholds of four fatty stimuli (oleic acid, paraffin oil, canola oil, and canola oil spiked with oleic acid) that could be perceived via gustatory and/or textural cues. In 30 participants, fat detection thresholds were determined in a repeated measurements design over twelve days. Weight status was examined by measuring the participants' BMI, waist circumference and waist-to-hip ratio. The habitual food intake was assessed via several questionnaires and twelve, non-consecutive 24-hour food diaries. In this study, a negative correlation was found between fat detection thresholds and the intake of food rich in vitamins and fibre. Moreover, a positive correlation was identified between the intake of high-fat food and fat detection thresholds. No differences in fat detection thresholds were observed due to variations in BMI or waist-to-hip ratio. These findings indicate that a regular intake of fatty foods might decrease an individuals' perceptual response to fats which might lead to excess fat intake on the long term.

Insulin Modulates Neural Activity of Pyramidal Neurons in the Anterior Piriform Cortex

Insulin is an important peptide hormone that regulates food intake and olfactory function. While a multitude of studies investigated the effect of insulin in the olfactory bulb and olfactory epithelium, research on how it modulates higher olfactory centers is lacking. Here we investigate how insulin modulates neural activity of pyramidal neurons in the anterior piriform cortex, a key olfactory signal processing center that plays important roles in odor perception, preference learning, and odor pattern separation. In vitro we find from brain slice recordings that insulin increases the excitation of pyramidal neurons, and excitatory synaptic transmission while it decreases inhibitory synaptic transmission. In vivo local field potential (LFP) recordings indicate that insulin decreases both ongoing gamma oscillations and odor evoked beta responses. Moreover, recordings of calcium activity from pyramidal neurons reveal that insulin modulates the odor-evoked responses by an inhibitory effect. These results indicate that insulin alters olfactory signal processing in the anterior piriform cortex.

Obesity is associated with altered mid-insula functional connectivity to limbic regions underlying appetitive responses to foods

Obesity is fundamentally a disorder of energy balance. In obese individuals, more energy is consumed than is expended, leading to excessive weight gain through the accumulation of adipose tissue. Complications arising from obesity, including cardiovascular disease, elevated peripheral inflammation, and the development of Type II diabetes, make obesity one of the leading preventable causes of morbidity and mortality. Thus, it is of paramount importance to both individual and public health that we understand the neural circuitry underlying the behavioral regulation of energy balance. To this end, we sought to examine obesity-related differences in the resting state functional connectivity of the dorsal mid-insula, a region of gustatory and interoceptive cortex associated with homeostatically sensitive responses to food stimuli. Within the present study, obese and healthy weight individuals completed resting fMRI scans during varying interoceptive states, both while fasting and after a standardized meal. We examined group differences in the pre- versus post-meal functional connectivity of the mid-insula, and how those differences were related to differences in self-reported hunger ratings and ratings of meal pleasantness. Obese and healthy weight individuals exhibited opposing patterns of eating-related functional connectivity between the dorsal mid-insula and multiple brain regions involved in reward, valuation, and satiety, including the medial orbitofrontal cortex, the dorsal striatum, and the ventral striatum. In particular, healthy weight participants exhibited a significant positive relationship between changes in hunger and changes in medial orbitofrontal functional connectivity, while obese participants exhibited a complementary negative relationship between hunger and ventral striatum connectivity to the mid-insula. These obesity-related alterations in dorsal mid-insula functional connectivity patterns may signify a fundamental difference in the experience of food motivation in obese individuals, wherein approach behavior toward food is guided more by reward-seeking than by homeostatically relevant interoceptive information from the body.

The neurobiology of safety and threat learning in infancy

What an animal needs to learn to survive is altered dramatically as they change from dependence on the parent for protection to independence and reliance on self-defense. This transition occurs in most altricial animals, but our understanding of the behavioral neurobiology has mostly relied on the infant rat. The transformation from dependence to independence occurs over three weeks in pups and is accompanied by complex changes in responses to both natural and learned threats and the supporting neural circuitry. Overall, in early life, the threat system is quiescent and learning is biased towards acquiring attachment related behaviors to support attachment to the caregiver and proximity seeking. Caregiver-associated cues learned in infancy have the ability to provide a sense of safety throughout lifetime. This attachment/safety system is activated by learning involving presumably pleasurable stimuli (food, warmth) but also painful stimuli (tailpinch, moderate shock). At about the midway point to independence, pups begin to have access to the adult-like amygdala-dependent threat system and amygdala-dependent responses to natural dangers such as predator odors. However, pups have the ability to switch between the infant and adult-like system, which is controlled by maternal presence and modification of stress hormones. Specifically, if the pup is alone, it will learn fear but if with the mother it will learn attachment (10-15days of age). As pups begin to approach weaning, pups lose access to the attachment system and rely only on the amygdala-dependent threat system. However, pups learning system is complex and exhibits flexibility that enables the mother to override the control of the attachment circuit, since newborn pups may acquire threat responses from the mother expressing fear in their presence. Together, these data suggest that the development of pups' threat learning system is not only dependent upon maturation of the amygdala, but it is also exquisitely controlled by the environment. Most notably the mother can switch pup learning between attachment to threat learning in a moment's notice. This enables the mother to navigate pup's learning about the world and what is threatening and what is safe.