Olfactory influences on appetite and satiety in humans

Odor stimuli play a major role in perception of food flavor. Food-related odors have also been shown to increase rated appetite, and induce salivation and release of gastric acid and insulin. However, our ability to identify an odor as food-related, and our liking for food-related odors, are both learned responses. In conditioning studies, repeated experience of odors with sweet and sour tastes result in enhanced ratings of sensory quality of the paired taste for the odor on its own. More recent studies also report increased pleasantness ratings for odors paired with sucrose for participants who like sweet tastes, and conversely decreased liking and increased bitterness for quinine-paired odors. When odors were experienced in combination with sucrose when hungry, liking was not increased if tested sated, suggesting that expression of acquired liking for odors depends on current motivational state. Other studies report sensory-specific satiety is seen with food-related odors. Overall, these studies suggest that once an odor is experienced in a food-related context, that odor acquires the ability to modify both preparatory and satiety-related components of ingestion.     

Olfactory influences on appetite and satiety in humans

Odor stimuli play a major role in perception of food flavor. Food-related odors have also been shown to increase rated appetite, and induce salivation and release of gastric acid and insulin. However, our ability to identify an odor as food-related, and our liking for food-related odors, are both learned responses. In conditioning studies, repeated experience of odors with sweet and sour tastes result in enhanced ratings of sensory quality of the paired taste for the odor on its own. More recent studies also report increased pleasantness ratings for odors paired with sucrose for participants who like sweet tastes, and conversely decreased liking and increased bitterness for quinine-paired odors. When odors were experienced in combination with sucrose when hungry, liking was not increased if tested sated, suggesting that expression of acquired liking for odors depends on current motivational state. Other studies report sensory-specific satiety is seen with food-related odors. Overall, these studies suggest that once an odor is experienced in a food-related context, that odor acquires the ability to modify both preparatory and satiety-related components of ingestion.     

Taste and olfaction in human obesity

Earlier studies have shown differences between normal weight and obese humans in responsivity to external and internal stimuli. This study shows that normal weight and obese subjects do not differ in hedonic response to sucrose (taste) and benzaldehyde (odor). However, a perceptual typing of individuals based upon hedonic response is possible for both gustatory and olfactory processes. Ratings of pleasantness for the sweet taste of sucrose appear to generalize to the food-related odor of bitter almonds. The method of magnitude estimation as applied to the study of taste and olfaction in man may reveal relationship between changes in internal state and hedonic behavior.     

ERP correlates of associative learning

We examined changes of event-related potentials (ERPs) while participants learned stimulus-to-stimulus relations in an S1-S2 task. The design allowed for separating processes of associative learning from nonspecific effects. Participants had to respond to S2 by a left or right key-press dependent on S2 identity (letter W or M). Preparation for S2 could be improved by using the associative information given by S1. The S1 was an arrow pointing to the left or right. In combination with its color, arrow direction was informative about location and identity of S2, but participants were not informed about the relevance of color. Arrows in two of the colors were fully predictive for the S2 whereas the third color gave no valid information. This third stimulus controlled for habituation and procedural learning. Six blocks with 200 trials each and all three S1 colors in random order were presented. Behavioral and ERP differences in each block between "learning" and control trials were used to identify processes of associative learning. Several effects of associative learning were identified indicating the involvement of specific stages of information processing: a continuous increase of P3 amplitude evoked by S1 was accompanied by a decrease of P3 evoked by S2. These changes reflected the modifications of stimulus weights for response selection and the strengthened association between the two stimulus complexes in the time course of learning. The related motor preparation benefited from learning too, expressed in a decrease of CNV amplitude and an increase of LRP amplitude. Finally a decrease of N1 amplitude evoked by S2 indicated the reduced need to allocate spatial attention to the S2 location according to the learned meaning of S1.     

Olfactory and taste sensitivity in children with cystic fibrosis.

Psychophysical experiments measured both olfactory sensitivity to 1-butanol and sodium chloride and taste sensitivity to sodium chloride in normal children and in children with cystic fibrosis. The sensitivity of the children with cystic fibrosis fell within the normal range. These results stand in contrast to those of Henkin and Powell, who found children with cystic fibrosis to be hypersensitive to both olfactory and taste stimuli. Apparent hypersensitivity to taste stimuli could have resulted from the adapting effects of the relatively high level of sodium in the saliva of children with cystic fibrosis. If the contaminating effects of saliva are removed, children with cystic fibrosis yield, on the average, a taste threshold identical to that of normal children. There appears to be no simple explanation for why Henkin and Powell found hypersensitivity to odorants. The present results indicate that, in fact, children with cystic fibrosis display a slight hyposensitivity to odorants.     

Left frontal lobe in verbal associative learning: a slow potential study

Summary In the present experiment pairs of words had to be memorized. The words were either meaningful or meaningless. The experimental design compares conditions of preestablished learning (L-) with active learning (L+). The effects of these two factors, semantic content (S) and learning (L), on the slow potential shifts accompanying presentation and processing of the verbal material were tested. In the memorizing tasks, the two words were given in a fixed temporal sequence. A slow negative potential shift having a maximum in parietal leads emerged within the inter-stimulus-interval. Its amplitudes were larger in the learning tasks (L+) than in conditions of pre-established learning (L-). This difference of amplitudes may reflect different levels of attention: In L-, the second word could be anticipated, but not in the L+ tasks. After the presentation of the second item, learning tasks (L+) were characterized by a slow negative potential shift in the recordings of the left dorso-lateral frontal lobe. It is assumed that this potential shift may indicate an importance of the left frontal lobe in the elaborative encoding of verbal material.Dedicated to the memory of Richard Jung (1911–1986)     

Effects of distribution of the drug unconditioned stimulus on taste-aversion learning

Rats injected with lithium chloride after exposure to a taste or olfactory stimulus learn stronger aversions to these cues if the drug is administered in two small injections 35 min apart than if all of the drug is given in a single injection. This facilitation of conditioning produced by distribution of the drug unconditioned stimulus occurs with both low and high lithium doses (Experiments 1 and 2), is more evident in male than in female rats (Experiment 1), and is directly related to the amount of the flavored solution consumed prior to drug treatment (Experiment 4). Increasing the interval between two small drug injections beyond an optimal value results in a progressive loss of the facilitation of conditioning (Experiments 2 and 3), and the optimal drug distribution interval may be shorter for olfactory cues (Experiment 3) than for taste stimuli (Experiments 1 and 2). Control observations (Experiments 5A and 5B) showed that the drug distribution effect is not due to handling or other non-drug factors involved in giving two rather than only one injection. The phenomenon is consistent with recently-proposed models of conditioning and suggests that the differential effectiveness of various drugs in taste aversion conditioning may be related to differences in the time course of the unconditioned drug effects.     

Gustatory and olfactory dysfunction in dementia: Not specific to Alzheimer''s disease

Significant losses in the ability to detect the taste of glutamic acid and to recognize odorants were found in demented patients when compared with age-matched controls. These losses were not specific to patients with Alzheimer's disease (AD) but were also found in demented patients without AD. Detection thresholds for bitter-tasting quinine HCl were not higher in demented patients than in age-matched controls but were higher than thresholds in young subjects. These data suggest that losses in taste in demented patients may vary with the chemical structure of the tastant. The degree of loss in the ability to recognize odorants was greater in patients with a family history of senile dementia.     

Accumulating evidence supports a taste component for free fatty acids in humans

The requisite criteria for what constitutes a taste primary have not been established. Recent advances in understanding of the mechanisms and functions of taste have prompted suggestions for an expanded list of unique taste sensations, including fat, or more specifically, free fatty acids (FFA). A set of criteria are proposed here and the data related to FFA are reviewed on each point. It is concluded that the data are moderate to strong that there are: A) adaptive advantages to FFA detection in the oral cavity; B) adequate concentrations of FFA to serve as taste stimuli; C) multiple complimentary putative FFA receptors on taste cells; D) signals generated by FFA that are conveyed by gustatory nerves; E) sensations generated by FFA that can be detected and scaled by psychophysical methods in humans when non-gustatory cues are masked; and F) physiological responses to oral fat/FFA exposure. On no point is there strong evidence challenging these observations. The reviewed findings are suggestive, albeit not definitive, that there is a taste component for FFA.     

Effect of solution temperature on taste intensity in humans

The dependence of taste intensity upon both molar concentration and solution temperature was investigated by the method of magnitude estimation. For each of the four taste substances (glucose, NaCl, citric acid, quinine sulfate) 4–5 concentrations of solution were evaluated at each of six temperatures (25–50°C). Power functions of the form T = kCⁿ related subjective intensity to molarity at a fixed solution temperature. The exponent n for all tastes but citric acid was unaffected by temperature, suggesting that the growth rate of intensity with concentrations is unaffected within a 25° change. The intercept k varied with temperature for glucose, and was linearly related to temperature for NaCl.     

Gustatory stimulus control and reversal learning in rats

Behavioral indices of taste discrimination have been used to study gustatory physiology and psychophysics though they are often biased by taste and position preferences and offer limited stimulus control. In the present experiment five rats were trained to discriminate distilled water from 0.9% NaCl using a discretetrials go-no go procedure. When two rats were further trained on a series of 0.1% saccharin/NaCl discrimination reversals, correct responding occurred more rapidly following saccharin presentations independent of its stimulus property (S^D or S^Δ). The results are discussed in the context of experimental methods and the usefulness of the present technique for studies of gustatory physiology.     

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

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.     

Feeding preferences in lambs influenced by prenatal flavour exposure

The aim of the present study is to highlight and evaluate the role of flavour prenatal exposure in postnatal feeding preferences in sheep after weaning and until the early stages of puberty. 16 lambs were selected from two groups of ewes; the first group was fed with a control diet, consisted of concentrated feed and alfalfa hay and the second group with the same diet, with the only difference that the concentrate was supplemented with oregano essential oil (1 ml/kg), during the period of pregnancy (50th-130th day). Lambs were later individually subjected to feeding preference tests, at the age of 3, 4.5, 6, 7.5 months old. Each test lasted 25 min and it was a free choice situation between 3 different test feeds supplemented with eucalyptus or orange or oregano essential oil (1 ml/kg). Lambs born to oregano-treated ewes ate higher amounts of the oregano supplemented test feed during all feeding preference tests compared to lambs from the control group (P<0.01). Rates for occurrences and duration of eating were also greater in lambs born to oregano-treated ewes than the offspring of the control ewe group (P<0.01). On the other hand, animals without a flavour exposure precedent did not exhibit an evident strong preference for a specific test feed, although orange supplemented feed tended to be more preferable compared to the other offered feeds. Average total intake of lambs were not different between groups during all feeding preference tests at 3, 41/2, 6 and 71/2 months old. As it is concluded, prenatal exposure to oregano essential oil via maternal ingestion drastically influences lamb feeding preferences till adulthood.     

Age-related alterations in performance monitoring during and after learning

Changes of performance monitoring during and after learning of stimulus-response (S-R) associations were examined in younger and older adults using event-related brain potentials (ERPs). To determine whether age-related differences in performance monitoring are attenuated when learning performance is equalized between age groups, participants learned S-R associations until they reached a learning criterion. After completed learning, time pressure was introduced to assess whether time pressure pronounces age-related differences in performance monitoring. In the initial learning phase, performance monitoring was unspecific, indicated by similar-sized ERN/Ne (error-related/error negativity) and CRN (correct response-related negativity) amplitudes. With advanced learning, ERN/Ne and CRN amplitudes dissociated in younger participants, suggesting error-specific performance monitoring, whereas in older participants, performance monitoring remained unspecific. However, after completed learning and under time pressure, larger ERN/Ne than CRN amplitudes were observed in older participants, too. This finding suggests that error-specific performance monitoring develops with learning in younger, but not necessarily in older adults. The administration of time pressure seemed to force older adults to monitor error responses more specifically.     

Anatomophysiological relationships and clinical considerations of taste and smell loss in patients with COVID-19

BACKGROUNDThere are numerous conflicting discussions about the outbreak of the new coronavirus 2019 (COVID-19).AIMTo present some anatomical and physiological considerations about two of the symptoms reported by patients: The loss or reduction of smell and taste. METHODSThe loss or reduction of smell and taste is presented in a peculiar way, with some cases of persistence even after COVID-19. For this, it was searched in three databases, PubMed/MEDLINE, Web of Science, and Scopus, using the following keywords: Smell, Taste, Smell AND COVID-19, Taste AND COVID-19, with no publication time restriction, only in English with full text available, excluding also brief communications, letters to the editor, editorials, reviews, comments, and conference abstracts. RESULTSThe search found 776 articles in the PubMed/MEDLINE database, 1018 in the Web of Science database, and 552 in the Scopus database, from which duplicates were removed (104 articles). Finally, 17 studies were selected for detailed analysis within the eligibility criteria, with titles and abstracts related to central nervous system lesions responsible for smell and taste. This review suggests that viral mechanisms of action may be related to lesions both at the local level and at the level of the central nervous system, lasting up to 3 to 4 wk. It is considered persistent if it exceeds this period, as reported in one case in this review. There are still few studies about the treatment, and among those addressed in this review, only two studies reported possible treatments and emphasized the scarcity of data, with the best option being treatments that do not cause harm, such as gustatory and olfactory physiotherapyCONCLUSIONGiven the scarcity of data, this review emphasizes the importance of prevention, through the correct use of personal protective equipment by health professionals and respect for local behavioral indications. It is also emphasized, through five studies, that there is a predominance of such symptoms in patients with COVID-19, which can be a tool to control dissemination, through the early isolation of patients until the results are ready.     

Emotional qualities of odors and their influence on the startle reflex in humans

Recent human and animal research suggests that the startle reflex might serve as a psychophysiological indicator of the emotional valence of foreground stimulation. The present experiment was designed to evaluate the emotional effects of positive and negative odorant stimuli. We examined the effects of continuous hydrogen sulphide (H2S) and vanillin stimulation on the magnitude of the acoustic startle reflex (measured at the M. orbicularis oculi) and on ratings of subjective valence in 16 healthy subjects. In accordance with the view that odors have emotional qualities, we found that H2S, a presumed negative foreground stimulus, significantly enhanced the startle-reflex amplitude relative to neutral air stimulation, whereas vanillin, a positive foreground stimulus, tended to reduce the reflex amplitude compared with neutral air stimulation. Both odorant stimuli were rated as equally intense by the subjects, and heart rate and electrodermal activity were not affected differentially by the two odorants.     

Electrophysiology of appetitive taste and appetitive taste conditioning in humans

The present study examines two novel aspects of appetitive processing and conditioning: the electrophysiological response to (a) the appetitive taste of a sweet fluid and (b) appetitively/taste conditioned visual stimuli. Event-related potentials (ERPs) were recorded from 32 subjects while they performed a taste conditioning task, in which two symbols were paired with a sweet or a neutral fluid. The results show a clear P1/N1/P2 complex, and a late positive potential (P3) with maxima at right fronto-central electrode sites, in response to the taste stimuli. Of these components particularly the P3 showed robust differences between the sweet and neutral taste. In addition, the electrophysiological response to the taste conditioned stimuli (CS), also showed the expected differences for both P2 and P3 at frontocentral electrodes. The present data demonstrate that the employed paradigm is a useful methodology to study the electrophysiological responses to unconditioned appetitive stimuli. Since appetitive conditioning is assumed to be disturbed in many psychopathological conditions, such as substance dependence, depression and eating disorders, this paradigm could be employed to get more insight in these conditions in humans.     

Dietary nimodipine improves associative learning in aging rabbits

The effects of oral nimodipine on the acquisition of the conditioned eye-blink response in aging rabbits were examined. Three groups of rabbits were compared: young and aging controls fed NIH-09 rabbit chow and aging rabbits fed rabbit chow containing 860 ppm nimodipine. Aging control rabbits are dramatically impaired in the acquisition of this task compared to young controls. Aging rabbits receiving nimodipine reached a behavioral criterion of 4 conditioned responses in any block of 5 trials significantly faster than aging controls. Young controls reached this criterion faster than the aging controls. There were no significant group effects on either the amplitude or the latency of the conditioned response. This study extends earlier reports that IV nimodipine enhances the associative learning abilities of aging rabbits and examined the route of administration most likely to be used clinically in aging humans.     

Prefrontal unit activity during associative learning in the monkey

Summary Single unit activity was recorded from the dorsolateral prefrontal cortex of two monkeys which were trained on a stimulus-reward association task. The monkeys were trained on a reaction time task overlapped with a classical conditioning paradigm. The sequential events of the task were as follows: (1) lever pressing to start the trial; (2) presentation of a visual cue for 1 s; (3) delay period of 1 s; (4) imperative stimulus presentation; and (5) release of the lever by the animal. The visual cue signaled whether or not a drop of fruit juice would be given (its associative significance) for the animal's release response instead of signaling what response the animal should perform (its behavioral significance). In this task, the animal had to release the lever even on the trial where no juice was given in order to advance to the next trial. A total of 423 units showed activity changes in relation to one or more of the task events, such as the cue presentation, delay, release response and reward delivery. Among 313 units which showed cue-related activity changes, 179 units showed differential activity in relation to the different cues. A majority of them (Type M; n = 120) showed activity changes in relation to whether the cue indicated juice delivery or not, independent of its physical properties. The activity of 13 units (Type P) was related to the physical properties of the stimulus, and the activity of the remaining 46 units (Type MP) appeared to be related to both aspects of the stimulus. Sustained activity changes during the delay period were observed in 68 Type M, in 3 Type P and in 24 Type MP units. The results suggest that the prefrontal cortex plays important roles in the stimulus-reward association and that prefrontal units are involved in higher order information processing, extracting and retaining the associative significance of the stimulus independent of its physical properties.