Responses of primate taste cortex neurons to the astringent tastant tannic acid

Astringency and its sub-qualities: a review of astringency mechanisms and methods for measuring saliva lubrication

Astringency is an important mouthfeel attribute that influences the sensory experiences of many food and beverage products. While salivary lubricity loss and increased oral friction were previously believed to be the only astringency mechanisms, recent research has demonstrated that nontactile oral receptors can trigger astringency by responding to astringents without mechanical stimulation. Various human factors have also been identified that affect individual responses to astringents. This article presents a critical review of the key research milestones contributing to the current understanding of astringency mechanisms and the instrumental approaches used to quantify perceived astringency intensity. Although various chemical assays or physical measures mimic in-mouth processes involved in astringent mouthfeel, this review highlights how one chemical or physical approach can only provide a single measure of astringency determined by a specific mechanism. Subsequently, using a single measurement to predict astringency perception is overly idealistic. Astringency has not been quantified beyond the loss of saliva lubrication; therefore, nontactile receptor-based responses must also be explored. An important question remains about whether astringency is a single perception or involves distinct sub-qualities such as pucker, drying, and roughness. Although these sub-quality lexicons have been frequently cited, most studies currently view astringency as a single perception rather than dividing it into sub-qualities and investigating the potentially independent mechanisms of each. Addressing these knowledge gaps should be an important priority for future research.

Processing of Sweet, Astringent and Pungent Oral Stimuli in the Human Brain

Taste and oral somatosensation are intimately related to each other from peripheral receptors to the central nervous system. Oral astringent sensation is thought to contain both gustatory and somatosensory components. In the present study, we compared the cerebral response to an astringent stimulus (tannin), with the response to one typical taste stimulus (sweet - sucrose) and one typical somatosensory stimulus (pungent - capsaicin) using functional magnetic resonance imaging (fMRI) of 24 healthy subjects. Three distributed brain sub-regions responded significantly different to the three types of oral stimulations: lobule IX of the cerebellar hemisphere, right dorsolateral superior frontal gyrus, and left middle temporal gyrus. This suggests that these regions play a major role in the discrimination of astringency, taste, and pungency.

The orbitofrontal cortex, food reward, body weight and obesity

In primates including humans, the orbitofrontal cortex is the key brain region representing the reward value and subjective pleasantness of the sight, smell, taste and texture of food. At stages of processing before this, in the insular taste cortex and inferior temporal visual cortex, the identity of the food is represented, but not its affective value. In rodents, the whole organisation of reward systems appears to be different, with reward value reflected earlier in processing systems. In primates and humans, the amygdala is overshadowed by the great development of the orbitofrontal cortex. Social and cognitive factors exert a top-down influence on the orbitofrontal cortex, to modulate the reward value of food that is represented in the orbitofrontal cortex. Recent evidence shows that even in the resting state, with no food present as a stimulus, the liking for food, and probably as a consequence of that body mass index, is correlated with the functional connectivity of the orbitofrontal cortex and ventromedial prefrontal cortex. This suggests that individual differences in these orbitofrontal cortex reward systems contribute to individual differences in food pleasantness and obesity. Implications of how these reward systems in the brain operate for understanding, preventing and treating obesity are described.

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.

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.

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.

Seasonal changes in leaf chemistry and leaf selection of the Japanese giant flying squirrel upon two tree species

Tree leaves are important food sources for arboreal herbivores, such as primates, rodents, and marsupials. These animals do not eat leaves randomly in habitats with many tree species but rather choose based on the chemical components of leaves, such as sugars, fibers, proteins, and toxins. However, the effects of the microscale distribution of these chemicals within each leaf have not been examined for these animals. The giant flying squirrels Petaurista leucogenys are entirely arboreal, nocturnal herbivores, usually feeding on leaves and dropping leaf debris on the ground after partially consuming them. Therefore, we could easily assess which species of trees and which parts of the individual leaves they preferred to eat. We also examined microscale distributions of phenolics, sugar, and water within individual leaves. Of the two dominant food tree species, the deciduous Quercus acutissima was preferred over the evergreen Q. sessilifolia. The latter tree is only used during winter to early spring when the former had no leaves. Our chemical analyses revealed that Q. acutissima contained much more glucose than Q. sessilifolia in all seasons. Three types of leaf debris, eaten apically, basally, or centrally with a hole, were found. In Q. sessilifolia, which had low phenolic concentrations, apical eating was most common, whereas central eating was rare. In Q. acutissima, which had high phenolics, basal or central eating was common. Central feeding may be caused by avoiding the periphery because of a higher phenolic concentration in the leaf margin. Thus, microscale distributions of phenolics within individual leaves affect which parts P. leucogenys eats, whereas leaf sugar concentration is an important factor affecting which species of leaves they eat.

Controversies in fat perception

Nutritional fat is one of the most controversial topics in nutritional research, particularly against the background of obesity. Studies investigating fat taste perception have revealed several associations with sensory, genetic, and personal factors (e.g. BMI). However, neuronal activation patterns, which are known to be highly sensitive to different tastes as well as to BMI differences, have not yet been included in the scheme of fat taste perception. We will therefore provide a comprehensive survey of the sensory, genetic, and personal factors associated with fat taste perception and highlight the benefits of applying neuroimaging research. We will also give a critical overview of studies investigating sensory fat perception and the challenges resulting from multifaceted methodological approaches. In conclusion, we will discuss a multifactorial approach to fat perception to gain a better understanding of the underlying mechanisms that cause varying fat sensitivity which could be responsible for overeating. Such knowledge might be beneficial in new treatment strategies for obesity and overweight.

Limbic systems for emotion and for memory, but no single limbic system

The concept of a (single) limbic system is shown to be outmoded. Instead, anatomical, neurophysiological, functional neuroimaging, and neuropsychological evidence is described that anterior limbic and related structures including the orbitofrontal cortex and amygdala are involved in emotion, reward valuation, and reward-related decision-making (but not memory), with the value representations transmitted to the anterior cingulate cortex for action-outcome learning. In this 'emotion limbic system' a computational principle is that feedforward pattern association networks learn associations from visual, olfactory and auditory stimuli, to primary reinforcers such as taste, touch, and pain. In primates including humans this learning can be very rapid and rule-based, with the orbitofrontal cortex overshadowing the amygdala in this learning important for social and emotional behaviour. Complementary evidence is described showing that the hippocampus and limbic structures to which it is connected including the posterior cingulate cortex and the fornix-mammillary body-anterior thalamus-posterior cingulate circuit are involved in episodic or event memory, but not emotion. This 'hippocampal system' receives information from neocortical areas about spatial location, and objects, and can rapidly associate this information together by the different computational principle of autoassociation in the CA3 region of the hippocampus involving feedback. The system can later recall the whole of this information in the CA3 region from any component, a feedback process, and can recall the information back to neocortical areas, again a feedback (to neocortex) recall process. Emotion can enter this memory system from the orbitofrontal cortex etc., and be recalled back to the orbitofrontal cortex etc. during memory recall, but the emotional and hippocampal networks or 'limbic systems' operate by different computational principles, and operate independently of each other except insofar as an emotional state or reward value attribute may be part of an episodic memory.

Compositional, nutritional, and functional characteristics of instant teas produced from low- and high-quality black teas

Two types of instant teas produced from low- and high-quality black teas were examined for their proximate composition, dietary fiber, minerals, water-soluble vitamins, total phenolic content, various antioxidant assays, phenolics (flavanols, condensed phenolics, and phenolic acids), alkaloids, and carotenoids as well as taste-active compounds (sugars, organic acids, and free amino acids). Some variations, albeit to different extents, were observed (p < 0.05) among these parameters between instant teas produced from low- and high-quality black teas. With respect to proximate composition, carbohydrate was the predominant component (56.68-59.84 g/100 g), followed by protein (19.31-19.86 g/100 g). Ash, moisture, and, to a lesser extent, dietary fiber and fat were also present in both instant teas. Thirteen minerals, four water-soluble vitamins, six flavanols, two alkaloids, three condensed phenolics, one phenolic acid, and one carotenoid were identified. Total phenolic content varied between 17.35 and 17.82 g of gallic acid equivalents (GAE)/100 g instant tea. With regard to antioxidant activities, three different assays such as oxygen radical absorbance capacity (ORAC), trolox equivalent antioxidant capacity (TEAC), and cupric ion reducing antioxidant capacity (CUPRAC) were measured. No significant differences (p > 0.05) in total phenolic, ORAC, TEAC, and CUPRAC contents between low- and high-quality instant teas were observed. With regard to taste-active compounds, 3 sugars, 5 organic acids, and 18 free amino acids were positively identified, of which fructose, tannic acid, and theanine predominated, respectively. The present work suggests that despite some differences, instant teas produced from low- and high-quality black teas should not be distinguished on the basis of their compositional, nutritional, and functional characteristics as well as taste-active compounds.

Aggregation of the salivary proline-rich protein IB5 in the presence of the tannin EgCG

In the mouth, proline-rich proteins (PRP), which are major components of stimulated saliva, interact with tannins contained in food. We report in vitro interactions of the tannin epigallocatechin gallate (EgCG), with a basic salivary PRP, IB5, studied through electrospray ionization mass spectrometry (ESI-MS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). In dilute protein (IB5) solutions of low ionic strength (1 mM), the proteins repel each other, and the tannins bind to nonaggregated proteins. ESI-MS experiments determine the populations of nonaggregated proteins that have bound various numbers of tannin molecules. These populations match approximately the Poisson distribution for binding to n = 8 sites on the protein. MS/MS experiments confirm that complexes containing n = 1 to 8 EgCG molecules are dissociated with the same energy. Assuming that the 8 sites are equivalent, we calculate a binding isotherm, with a binding free energy Δμ = 7.26RT(a) (K(d) = 706 μM). In protein solutions that are more concentrated (0.21 mM) and at higher ionic strength (50 mM, pH 5.5), the tannins can bridge the proteins together. DLS experiments measure the number of proteins per aggregate. This number rises rapidly when the EgCG concentration exceeds a threshold (0.2 mM EgCG for 0.21 mM of IB5). SAXS experiments indicate that the aggregates have a core-corona structure. The core contains proteins that have bound at least 3 tannins and the corona has proteins with fewer bound tannins. These aggregates coexist with nonaggregated proteins. Increasing the tannin concentration beyond the threshold causes the transfer of proteins to the aggregates and a fast rise of the number of proteins per aggregate. A poisoned growth model explains this fast rise. Very large cationic aggregates, containing up to 10,000 proteins, are formed at tannin concentrations (2 mM) slightly above the aggregation threshold (0.2 mM).

Taste, olfactory and food texture reward processing in the brain and the control of appetite

Complementary neuronal recordings and functional neuroimaging in human subjects show that the primary taste cortex in the anterior insula provides separate and combined representations of the taste, temperature and texture (including fat texture) of food in the mouth independently of hunger and thus of reward value and pleasantness. One synapse on, in the orbitofrontal cortex (OFC), these sensory inputs are for some neurons combined by learning with olfactory and visual inputs, and these neurons encode food reward in that they only respond to food when hungry, and in that activations correlate with subjective pleasantness. Cognitive factors, including word-level descriptions, and attention modulate the representation of the reward value of food in the OFC and a region to which it projects, the anterior cingulate cortex. Further, there are individual differences in the representation of the reward value of food in the OFC. It is argued that over-eating and obesity are related in many cases to an increased reward value of the sensory inputs produced by foods, and their modulation by cognition and attention that over-ride existing satiety signals. It is proposed that control of all rather than one or several of these factors that influence food reward and eating may be important in the prevention and treatment of overeating and obesity.

Flavor characteristics of seven grades of black tea produced in Turkey

Seven grades of black tea [high-quality black tea (grades 1-3) and low-quality black tea (grades 4-7)], processed by ÇAYKUR Tea Processing Plant (Rize, Turkey), were compared for their differences in descriptive sensory analysis (DSA), aroma-active compounds (volatile compounds), and taste-active compounds (sugar, organic acid, and free amino acid compositions). Ten flavor attributes such as 'after taste', 'astringency', 'bitter', 'caramel-like', 'floral/sweet', 'green/grassy', 'hay-like', 'malty', 'roasty', and 'seaweed' were identified. Intensities for a number of flavor attributes ('after taste', 'caramel-like', 'malty', and 'seaweed') were not significantly different (p > 0.05) among seven grades of black tea. A total of 57 compounds in seven grades of black tea (14 aldehydes, eight alcohols, eight ketones, two esters, four aromatic hydrocarbons, five aliphatic hydrocarbons, nine terpenes, two pyrazines, one furan, two acids, and two miscellaneous compounds) were tentatively identified. Of these, aldeyhdes comprised more than 50% to the total volatile compounds identified. In general, high-grade quality tea had more volatiles than low-grade quality tea. With respect to taste-active compounds, five sugars, six organic acids, and 18 free amino acids were positively identified in seven grades of black tea, of which fructose, tannic acid, and theanine predominated, respectively. Some variations (p < 0.05), albeit to different extents, were observed among volatile compounds, sugars, organic acids, and free amino acids in seven grades of black tea. The present study suggests that a certain flavor attributes correlate well with taste- and aroma-active compounds. High- and low-quality black teas should not be distinguished solely on the basis of their DSA and taste- and aroma-active compounds. The combination of taste-active compounds together with aroma-active compounds renders combination effects that provide the characteristic flavor of each grade of black tea.

Tannic acid down-regulates the angiotensin type 1 receptor through a MAPK-dependent mechanism

In the present study, we investigated the effects of tannic acid (TA), a hydrolysable polyphenol, on angiotensin type 1 receptor (AT1R) expression in continuously passaged rat liver epithelial cells. Under normal conditions, exposure of cells to TA resulted in the down-regulation of AT1R-specific binding in concentrations ranging from 12.5-100 μg/ml (7.34-58.78 μm) over a time period of 2-24 h with no change in receptor affinity to angiotensin II (AngII). The inhibitory effect of TA on AT1R was specific and reversible. In TA-treated cells, we observed a significant reduction in AngII-mediated intracellular calcium signaling, a finding consistent with receptor down-regulation. Under similar conditions, TA down-regulated AT1R mRNA expression without changing the rate of mRNA degradation, suggesting that TA's effect is mediated through transcriptional inhibition. Cells expressing recombinant AT1R without the native promoter show no change in receptor expression, whereas a pCAT reporter construct possessing the rat AT1R promoter was significantly reduced in activity. Furthermore, TA induced the phosphorylation of MAPK p42/p44. Pretreatment of the cells with a MAPK kinase (MEK)-specific inhibitor PD98059 prevented TA-induced MAPK phosphorylation and down-regulation of the AT1R. Moreover, there was no reduction in AngII-mediated intracellular calcium release upon MEK inhibition, suggesting that TA's observed inhibitory effect is mediated through MEK/MAPK signaling. Our findings demonstrate, for the first time, that TA inhibits AT1R gene expression and cellular response, suggesting the observed protective effects of dietary polyphenols on cardiovascular conditions may be, in part, through inhibition of AT1R expression.

Chemosensory learning in the cortex

Taste is a primary reinforcer. Olfactory–taste and visual–taste association learning takes place in the primate including human orbitofrontal cortex to build representations of flavor. Rapid reversal of this learning can occur using a rule-based learning system that can be reset when an expected taste or flavor reward is not obtained, that is by negative reward prediction error, to which a population of neurons in the orbitofrontal cortex responds. The representation in the orbitofrontal cortex but not the primary taste or olfactory cortex is of the reward value of the visual/olfactory/taste input as shown by devaluation experiments in which food is fed to satiety, and by correlations of the activations with subjective pleasantness ratings in humans. Sensory-specific satiety for taste, olfactory, visual, and oral somatosensory inputs produced by feeding a particular food to satiety is implemented it is proposed by medium-term synaptic adaptation in the orbitofrontal cortex. Cognitive factors, including word-level descriptions, modulate the representation of the reward value of food in the orbitofrontal cortex, and this effect is learned it is proposed by associative modification of top-down synapses onto neurons activated by bottom-up taste and olfactory inputs when both are active in the orbitofrontal cortex. A similar associative synaptic learning process is proposed to be part of the mechanism for the top-down attentional control to the reward value vs. the sensory properties such as intensity of taste and olfactory inputs in the orbitofrontal cortex, as part of a biased activation theory of selective attention.

Prediction of subjective affective state from brain activations

Decoding and information theoretic techniques were used to analyze the predictions that can be made from functional magnetic resonance neuroimaging data on individual trials. The subjective pleasantness produced by warm and cold applied to the hand could be predicted on single trials with typically in the range 60-80% correct from the activations of groups of voxels in the orbitofrontal and medial prefrontal cortex and pregenual cingulate cortex, and the information available was typically in the range 0.1-0.2 (with a maximum of 0.6) bits. The prediction was typically a little better with multiple voxels than with one voxel, and the information increased sublinearly with the number of voxels up to typically seven voxels. Thus the information from different voxels was not independent, and there was considerable redundancy across voxels. This redundancy was present even when the voxels were from different brain areas. The pairwise stimulus-dependent correlations between voxels, reflecting higher-order interactions, did not encode significant information. For comparison, the activity of a single neuron in the orbitofrontal cortex can predict with 90% correct and encode 0.5 bits of information about whether an affectively positive or negative visual stimulus has been shown, and the information encoded by small numbers of neurons is typically independent. In contrast, the activation of a 3 x 3 x 3-mm voxel reflects the activity of approximately 0.8 million neurons or their synaptic inputs and is not part of the information encoding used by the brain, thus providing a relatively poor readout of information compared with that available from small populations of neurons.

Astringency: mechanisms and perception

Astringency plays an important role in the sensory experience of many foods and beverages, ranging from wine to nuts. Given the recent trend toward fortifying consumables with astringent compounds and the evidence regarding the health benefits of some astringents, the mechanisms and perceptual characteristics of astringency warrant further discussion and investigation. This paper reviews the current state of the literature, including consideration of new methods for describing and measuring astringency, and provides an overview of research concerned with elucidating the physical, physiological, and psychological factors that underlie and mediate perception of this sensation.

The orbitofrontal cortex and beyond: from affect to decision-making

The orbitofrontal cortex represents the reward or affective value of primary reinforcers including taste, touch, texture, and face expression. It learns to associate other stimuli with these to produce representations of the expected reward value for visual, auditory, and abstract stimuli including monetary reward value. The orbitofrontal cortex thus plays a key role in emotion, by representing the goals for action. The learning process is stimulus-reinforcer association learning. Negative reward prediction error neurons are related to this affective learning. Activations in the orbitofrontal cortex correlate with the subjective emotional experience of affective stimuli, and damage to the orbitofrontal cortex impairs emotion-related learning, emotional behaviour, and subjective affective state. With an origin from beyond the orbitofrontal cortex, top-down attention to affect modulates orbitofrontal cortex representations, and attention to intensity modulates representations in earlier cortical areas of the physical properties of stimuli. Top-down word-level cognitive inputs can bias affective representations in the orbitofrontal cortex, providing a mechanism for cognition to influence emotion. Whereas the orbitofrontal cortex provides a representation of reward or affective value on a continuous scale, areas beyond the orbitofrontal cortex such as the medial prefrontal cortex area 10 are involved in binary decision-making when a choice must be made. For this decision-making, the orbitofrontal cortex provides a representation of each specific reward in a common currency.

Functions of the orbitofrontal and pregenual cingulate cortex in taste, olfaction, appetite and emotion

Complementary neurophysiological recordings in macaques and functional neuroimaging in humans show that the primary taste cortex in the rostral insula and adjoining frontal operculum provides separate and combined representations of the taste, temperature, and texture (including viscosity and fat 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 learning with olfactory and visual inputs. Different neurons respond to different combinations, providing a rich representation of the sensory properties of food. The representation of taste and other food-related stimuli in the orbitofrontal cortex of macaques is found from its lateral border throughout area 13 to within 7 mm of the midline, and in humans the representation of food-related and other pleasant stimuli is found particularly in the medial orbitofrontal cortex. In the orbitofrontal cortex, feeding to satiety with one food decreases the responses of these neurons to that food, but not to other foods, showing that sensory-specific satiety is computed in the primate (including human) orbitofrontal cortex. Consistently, activation of parts of the human orbitofrontal cortex correlates with subjective ratings of the pleasantness of the taste and smell of food. Cognitive factors, such as a word label presented with an odour, influence the pleasantness of the odour, and the activation produced by the odour in the orbitofrontal cortex. Food intake is thus controlled by building a multimodal representation of the sensory properties of food in the orbitofrontal cortex, and gating this representation by satiety signals to produce a representation of the pleasantness or reward value of food which drives food intake. A neuronal representation of taste is also found in the pregenual cingulate cortex, which receives inputs from the orbitofrontal cortex, and in humans many pleasant stimuli activate the pregenual cingulate cortex, pointing towards this as an important area in motivation and emotion.

Sensory processing in the brain related to the control of food intake

Complementary neurophysiological recordings in rhesus macaques (Macaca mulatta) and functional neuroimaging in human subjects show that the primary taste cortex in the rostral insula and adjoining frontal operculum provides separate and combined representations of the taste, temperature and texture (including viscosity and fat 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 learning with olfactory and visual inputs. Different neurons respond to different combinations, providing a rich representation of the sensory properties of food. In the orbitofrontal cortex feeding to satiety with one food decreases the responses of these neurons to that food, but not to other foods, showing that sensory-specific satiety is computed in the primate (including the human) orbitofrontal cortex. Consistently, activation of parts of the human orbitofrontal cortex correlates with subjective ratings of the pleasantness of the taste and smell of food. Cognitive factors, such as a word label presented with an odour, influence the pleasantness of the odour, and the activation produced by the odour in the orbitofrontal cortex. Food intake is thus controlled by building a multimodal representation of the sensory properties of food in the orbitofrontal cortex and gating this representation by satiety signals to produce a representation of the pleasantness or reward value of food that drives food intake. Factors that lead this system to become unbalanced and contribute to overeating and obesity are described.

Brain mechanisms underlying flavour and appetite

Complementary neurophysiological recordings in macaques and functional neuroimaging in humans show that the primary taste cortex in the rostral insula and adjoining frontal operculum provides separate and combined representations of the taste, temperature and texture (including viscosity and fat 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 learning with olfactory and visual inputs. Different neurons respond to different combinations, providing a rich representation of the sensory properties of food. In the orbitofrontal cortex, feeding to satiety with one food decreases the responses of these neurons to that food, but not to other foods, showing that sensory-specific satiety is computed in the primate (including human) orbitofrontal cortex. Consistently, activation of parts of the human orbitofrontal cortex correlates with subjective ratings of the pleasantness of the taste and smell of food. Cognitive factors, such as a word label presented with an odour, influence the pleasantness of the odour and the activation produced by the odour in the orbitofrontal cortex. These findings provide a basis for understanding how what is in the mouth is represented by independent information channels in the brain; how the information from these channels is combined; and how and where the reward and subjective affective value of food is represented and is influenced by satiety signals. Activation of these representations in the orbitofrontal cortex may provide the goal for eating, and understanding them helps to provide a basis for understanding appetite and its disorders.

The representation of taste quality in the mammalian nervous system

The process by which the mammalian nervous system represents the features of a sapid stimulus that lead to a perception of taste quality has long been controversial. The labeled-line (sparse coding) view differs from the across-neuron pattern (ensemble) counterpoint in proposing that activity in a given class of neurons is necessary and sufficient to generate a specific taste perception. This article critically reviews molecular, electro-physiological, and behavioral findings that bear on the issue. In the peripheral gustatory system, the authors conclude that most qualities appear to be signaled by labeled lines; however, elements of both types of coding characterize signaling of sodium salts. Given the heterogeneity of neuronal tuning functions in the brain, the central coding mechanism is less clear. Both sparse coding and neuronal ensemble models remain viable possibilities. Furthermore, temporal patterns of discharge could contribute additional information. Ultimately, until specific classes of neurons can be selectively manipulated and perceptual consequences assessed, it will be difficult to go beyond mere correlation and conclusively discern the validity of these coding models.

Face-selective and auditory neurons in the primate orbitofrontal cortex

Neurons with responses selective for faces are described in the macaque orbitofrontal cortex. The neurons typically respond 2-13 times more to the best face than to the best non-face stimulus, and have response latencies which are typically in the range of 130-220 ms. Some of these face-selective neurons respond to identity, and others to facial expression. Some of the neurons do not have different responses to different views of a face, which is a useful property of neurons responding to face identity. Other neurons have view-dependent responses, and some respond to moving but not still heads. The neurons with face expression, face movement, or face view-dependent responses would all be useful as part of a system decoding and representing signals important in social interactions. The representation of face identity is also important in social interactions, for it provides some of the information needed in order to make different responses to different individuals. In addition, some orbitofrontal cortex neurons were shown to be tuned to auditory stimuli, including for some neurons, the sound of vocalizations. The findings are relevant to understanding the functions of the primate including human orbitofrontal cortex in normal behaviour, and to understanding the effects of damage to this region in humans.

Convergence of sensory systems in the orbitofrontal cortex in primates and brain design for emotion

In primates, stimuli to sensory systems influence motivational and emotional behavior via neural relays to the orbitofrontal cortex. This article reviews studies on the effects of stimuli from multiple sensory modalities on the brain of humans and some other higher primates. The primate orbitofrontal cortex contains the secondary taste cortex, in which the reward value of taste is represented. It also contains the secondary and tertiary olfactory cortical areas, in which information about the identity and also about the reward value of odors is represented. A somatosensory input is revealed by neurons that respond to the viscosity of food in the mouth, to the texture (mouth feel) of fat in the mouth, and to the temperature of liquids placed into the mouth. The orbitofrontal cortex also receives information about the sight of objects from the temporal lobe cortical visual areas. Information about each of these modalities is represented separately by different neurons, but in addition, other neurons show convergence between different types of sensory input. This convergence occurs by associative learning between the visual or olfactory input and the taste. In that emotions can be defined as states elicited by reinforcers, the neurons that respond to primary reinforcers (such as taste and touch), as well as learn associations to visual and olfactory stimuli that become secondary reinforcers, provide a basis for understanding the functions of the orbitofrontal cortex in emotion. In complementary neuroimaging studies in humans, it is being found that areas of the orbitofrontal cortex are activated by pleasant touch, by painful touch, by taste, by smell, and by more abstract reinforcers such as winning or losing money. Damage to the orbitofrontal cortex in humans can impair the learning and reversal of stimulus-reinforcement associations and thus the correction of behavioral responses when these are no longer appropriate because previous reinforcement contingencies change. It is striking that humans and other catarrhines, being visual specialists like other anthropoids, interface the visual system to other sensory systems (e.g., taste and smell) in the orbitofrontal cortex.

The functions of the orbitofrontal cortex

The orbitofrontal cortex contains the secondary taste cortex, in which the reward value of taste is represented. It also contains the secondary and tertiary olfactory cortical areas, in which information about the identity and also about the reward value of odours is represented. The orbitofrontal cortex also receives information about the sight of objects from the temporal lobe cortical visual areas, and neurons in it learn and reverse the visual stimulus to which they respond when the association of the visual stimulus with a primary reinforcing stimulus (such as taste) is reversed. This is an example of stimulus-reinforcement association learning, and is a type of stimulus-stimulus association learning. More generally, the stimulus might be a visual or olfactory stimulus, and the primary (unlearned) positive or negative reinforcer a taste or touch. A somatosensory input is revealed by neurons that respond to the texture of food in the mouth, including a population that responds to the mouth feel of fat. In complementary neuroimaging studies in humans, it is being found that areas of the orbitofrontal cortex are activated by pleasant touch, by painful touch, by taste, by smell, and by more abstract reinforcers such as winning or losing money. Damage to the orbitofrontal cortex can impair the learning and reversal of stimulus-reinforcement associations, and thus the correction of behavioural responses when there are no longer appropriate because previous reinforcement contingencies change. The information which reaches the orbitofrontal cortex for these functions includes information about faces, and damage to the orbitofrontal cortex can impair face (and voice) expression identification. This evidence thus shows that the orbitofrontal cortex is involved in decoding and representing some primary reinforcers such as taste and touch; in learning and reversing associations of visual and other stimuli to these primary reinforcers; and in controlling and correcting reward-related and punishment-related behavior, and thus in emotion. The approach described here is aimed at providing a fundamental understanding of how the orbitofrontal cortex actually functions, and thus in how it is involved in motivational behavior such as feeding and drinking, in emotional behavior, and in social behavior.

Orbitofrontal cortex: neuronal representation of oral temperature and capsaicin in addition to taste and texture

The primate orbitofrontal cortex is a site of convergence of information from primary taste, olfactory and somatosensory cortical areas. We describe the discovery of a population of single neurons in the macaque orbitofrontal cortex that responds to the temperature of a liquid in the mouth. The temperature stimuli consisted of water at 10 degrees C, 23 degrees C, 37 degrees C and 42 degrees C. Twenty-six of the 1149 neurons analyzed (2.3%) responded to oral temperature. The tuning profiles of the neurons to temperature showed that some of the neurons had graded responses to increasing temperature (27%), others responded to cold (10 degrees C) stimuli (27%), and others were tuned to temperature (46%). The neuronal responses were also measured to taste stimuli, viscosity stimuli (carboxymethyl-cellulose in the range 1-10,000 cP), and capsaicin (10 microM). Of 70 neurons with responses to any of these stimuli, 7.1% were unimodal temperature; 11.3% were temperature and taste-sensitive; 7.1% were temperature and viscosity-sensitive; and 11.3% were temperature, taste and viscosity sensitive. Capsaicin activated 15.7% of the population of responsive neurons tested. These results provide the first evidence of how the temperature of what is in the mouth is represented at the neuronal level in the orbitofrontal cortex and the first evidence for any primate cortical area that in some cases this information converges onto single neurons with inputs produced by other sensory properties of food, including taste and texture. The results provide a basis for understanding how particular combinations of oral temperature, taste, and texture can influence the palatability of foods.

Representations of the texture of food in the primate orbitofrontal cortex: neurons responding to viscosity, grittiness, and capsaicin

The primate orbitofrontal cortex (OFC) is a site of convergence from taste, olfactory, and somatosensory cortical areas. We describe a population of single neurons in the macaque OFC that responds to the texture of food in the mouth. Use of oral viscosity stimuli consisting of carboxymethylcellulose (CMC) in the range 1-10,000 centipoise showed that the responses of one subset of these neurons were related to stimulus viscosity. Some of the neurons had increasing responses to increasing viscosity, some had decreasing responses, and some neurons were tuned to a range of viscosities. These neurons are a different population to oral fat-sensitive neurons, in that their responses to fats (e.g., safflower oil), to silicone oil [(Si(CH3)2O)n], and to mineral oil (hydrocarbon) depended on the viscosity of these oils. Thus there is a dissociation between texture channels used to sense viscosity and fat. Some of these viscosity-sensitive single neurons were unimodal (somatosensory; 25%) and some received convergent taste inputs (75%). A second subpopulation of neurons responded to gritty texture (produced by microspheres suspended in CMC). A third subpopulation of neurons responded to capsaicin. These results provide evidence about the information channels used to represent the texture and flavor of food in a part of the brain important in appetitive responses to food and are relevant to understanding the physiological and pathophysiological processes related to food intake, food selection, and the effects of variety of food texture in combination with taste and other inputs that affect food intake.

Neurons in the primate orbitofrontal cortex respond to fat texture independently of viscosity

The primate orbitofrontal cortex (OFC) is a site of convergence from primary taste, olfactory, and somatosensory cortical areas. We describe the responses of a population of single neurons in the OFC that respond to orally applied fat (e.g., safflower oil) and to substances with a similar texture but different chemical composition, such as mineral oil (hydrocarbon) and silicone oil [(Si(CH3)2O)n]. These findings provide evidence that the neurons respond to the oral texture of fat, sensed by the somatosensory system. Use of an oral viscosity stimulus consisting of carboxymethyl-cellulose in the range 1-10,000 centipoise (cP) showed that the responses of these fat-sensitive neurons are not related to stimulus viscosity. Thus a textural component independent of viscosity and related to the slick or oily property is being used to activate these oral fat-sensitive neurons. Moreover, a separate population of neurons responds to viscosity (produced, e.g., by the carboxymethyl-cellulose series), but not to fat with the same viscosity. Thus there is a dissociation between texture channels used to sense fat viscosity and non-fat-produced viscosity. Further, free fatty acids such as linoleic acid do not activate these neurons, providing further evidence that the oral fat-sensing mechanism through which these OFC neurons are activated is not gustatory but textural. Most of this population of fat-sensitive neurons receive convergent taste inputs. These results provide evidence about how oral fat is sensed and are relevant to understanding the physiological and pathophysiological processes related to fat intake.

Memory systems in the brain

The operation of different brain systems involved in different types of memory is described. One is a system in the primate orbitofrontal cortex and amygdala involved in representing rewards and punishers, and in learning stimulus-reinforcer associations. This system is involved in emotion and motivation. A second system in the temporal cortical visual areas is involved in learning invariant representations of objects. A third system in the hippocampus is implicated in episodic memory and in spatial function. Fourth, brain systems in the frontal and temporal cortices involved in short term memory are described. The approach taken provides insight into the neuronal operations that take place in each of these brain systems, and has the aim of leading to quantitative biologically plausible neuronal network models of how each of these memory systems actually operates.

Responses to the sensory properties of fat of neurons in the primate orbitofrontal cortex

The primate orbitofrontal cortex is a site of convergence of information from primary taste, olfactory, and somatosensory cortical areas. We describe the responses of a population of single neurons in the orbitofrontal cortex that responds to fat in the mouth. The neurons respond, when fatty foods are being eaten, to pure fat such as glyceryl trioleate and also to substances with a similar texture but different chemical composition such as paraffin oil (hydrocarbon) and silicone oil [Si(CH3)2O)n]. This is evidence that the neurons respond to the oral texture of fat, sensed by the somatosensory system. Some of the population of neurons respond unimodally to the texture of fat. Other single neurons show convergence of taste inputs, and others of olfactory inputs, onto single neurons that respond to fat. For example, neurons were found that responded to the mouth feel of fat and the taste of monosodium glutamate (both found in milk), or to the mouth feel of fat and to odor. Feeding to satiety reduces the responses of these neurons to the fatty food eaten, but the neurons still respond to some other foods that have not been fed to satiety. Thus sensory-specific satiety for fat is represented in the responses of single neurons in the primate orbitofrontal cortex. Fat is an important constituent of food that affects its palatability and nutritional effects. The findings described provide evidence that the reward value (or pleasantness) of the mouth feel of fat is represented in the primate orbitofrontal cortex and that the representation is relevant to appetite.

The orbitofrontal cortex

The orbitofrontal cortex contains the secondary taste cortex, in which the reward value of taste is represented. It also contains the secondary and tertiary olfactory cortical areas, in which information about the identity and also about the reward value of odours is represented. The orbitofrontal cortex also receives information about the sight of objects from the temporal lobe cortical visual areas, and is involved in learning and in reversing stimulus-reinforcement associations. The stimulus might be a visual or olfactory stimulus, and the primary (unlearned) reinforcer a taste or touch. Damage to the orbitofrontal cortex impairs the learning and reversal of stimulus-reinforcement associations, and thus the correction of behavioural responses when these are no longer appropriate because previous reinforcement contingencies change. The information which reaches the orbitofrontal cortex for these functions includes information about faces, and damage to the orbitofrontal cortex can impair face expression identification. This evidence thus shows that the orbitofrontal cortex is involved in decoding some primary reinforcers such as taste; in learning and reversing associations of visual and other stimuli to these primary reinforcers; and plays an executive function in controlling and correcting reward-related and punishment-related behaviour, and thus in emotion.