Contributions of Smell and taste to overall intensity

The integration of olfaction and gustation in producing thesensation of flavor was studied using an almond extract stimuluspresented with the Two-Module Delivery System. Almond in themouth was found to influence the scaling of smell. The datasugest that this background effect is due to the diffusion ofairborne molecules from the oral cavity to the olfactory receptorsarea. At least under some stimulation conditons, almond vaporin the nose was shown to afect the scaling of taste. When scalingoverall intensity, subjects apparently added together the sensationsof smell and taste to produce the sensation of overall intensity.However, for all combinations of odorants and tastants, theestimate of overall intensity was less than the sum of the estimateof the smell and the estimate of the taste. The data furthersuggest that when estimating the overall intensity, the magnitudeassigned to the olfactory or gustatory sensations was less thanthat which have been assigned when estimating just smell ortaste.     

Taste Perception in Kallmann Syndrome,A Model of Congenital Anosmia

ObjectiveTo investigate taste, a component of flavor perception, using electrogustometry (EG) in patients with congenital anosmia associated with Kallmann syndrome (KS).MethodsFour patients with KS and 4 control subjects participated in this study. During the first phase of the investigation, the study subjects were administered the University of Pennsylvania Smell Identification Test. During the second phase of the study, EG testing of 2 regions on the anterior tongue tip was performed through an electrode.ResultsPatients with KS, as expected, scored in the anosmic range on the University of Pennsylvania Smell Identification Test, whereas the control group had a normal sense of smell. The difference in the olfaction scores was significant between the 2 study groups (P < 0.015). The result of taste assessment of patients with KS and control subjects with use of EG was not significantly different between the 2 study groups (P = 0.874).ConclusionThe current study demonstrates that patients with KS have a normal sense of taste, as determined by EG. This finding is consistent with the fact that the deficit in KS is purely olfactory. Because flavor perception is not a common complaint in patients with this condition, it may be postulated that persons with KS compensate for the absent sense of smell. Further studies need to be undertaken to explore how patients with KS compensate for the olfactory dysfunction, information that should contribute to the understanding of the interplay of the various components of flavor perception. (Endocr Pract. 2007;13:716-720)     

Aging Effects on Anatomy and Neurophysiology of Taste and Smell1

Taste and smell have a primary role in food ingestion. Therefore, to understand why eating habits alter in elderly people, age-related differences in the chemical senses should be investigated. In early anatomical studies, substantial decreases in numbers of taste buds in old human and mouse circumvallate papillae were observed. However, recent investigations in humans, monkeys, and rats indicate that there is not a significant loss of taste buds in old age. Neurophysiological recordings from the chorda tympani nerve, innervating taste buds in fungiform papillae, demonstrate significant but small differences in response magnitudes for some chemicals in old rats. Greater age-related differences have been observed in the olfactory sense. Numbers of receptor neurons in the rat olfactory epithelium initially increase in adults and then decline in old animals; this decline is reflected in subsequent changes in the olfactory bulb. However, numbers of synapses in the bulb per receptor neuron are increased in the oldest rats, suggesting some compensatory mechanism. Differences in degree of aging effects in taste and smell might relate to the nature of receptors: a modified epithelial cell in taste versus a neuron in smell. However, in both sensory systems, large numbers of receptors remain even in old age. Since taste bud cells and olfactory receptors turn over and are replaced throughout life, the peripheral taste and smell systems might be relatively resistant to aging effects.     

Cross-cultural comparison of data using the odor stick identification test for Japanese (OSIT-J)

A new olfactory test, the odor stick identification test for Japanese (OSIT-J), has been developed in Japan. To determine if the OSIT-J would be effective cross-culturally, we administered the test to 52 US and 50 Japanese subjects reporting normal olfactory function. The average composite OSIT-J test score for US subjects was significantly lower (77%) than that for Japanese subjects (94%, P < 0.0001). Both US and Japanese subjects correctly identified eight of the 13 odorants included in the OSIT-J with scores of 80% or higher. However, for five odorants, the US subjects' scores fell below 80% and were consistently lower than Japanese subjects, presumably reflecting cultural differences in odor experience. Most of the US subjects found the OSIT-J to be easy, interesting, pleasant, and short in duration. Although the 13-odorant OSIT-J was found to be suitable for testing US populations, elimination of five test odorants that were unfamiliar to US subjects significantly enhanced the test's effectiveness. Findings from this study emphasize the importance of identifying test odorants that may have a cultural bias, a crucial issue when comparing data obtained from different smell tests used at smell and taste centers around the world.     

Food perception with age and its relationship to pleasantness

Differences between elderly subjects (n = 46, 61-86 years) and young subjects (n = 36, 18-25 years) in food perception and food liking were investigated. Intensity and liking ratings were assessed for custard dessert, in which flavor enrichment, textural change, and irritant addition were incorporated as strategies to compensate for sensory losses with increasing age. The sensory acuity (taste, olfaction, irritation, chewing efficiency) of both young and elderly subjects was measured with the help of different sensitivity tests. The elderly perceived the custards differently from the young, mainly as less intense in flavor (cherry/vanilla) and less intense in creaminess/swallowing effort. Several of the observed interaction effects were different for the elderly and the young. The majority of these differences manifested as lower intensity slopes for the elderly. Losses in sensitivity to taste and to olfactory and trigeminal stimuli as well as a reduced chewing efficiency were observed on average for the elderly compared with the young. Furthermore, subgroups of the elderly were observed in which the compensatory strategies flavor enrichment, textural change, and irritant addition led to an increase in food liking. However, these subgroups did not differ in their sensory acuity. The present study does not support the assumption that age-associated changes in food perception-caused by losses in sensory acuity-inevitably reduce the food liking of the elderly.     

Modality-specific neural effects of selective attention to taste and odor

The insular cortex is implicated in general attention and in taste perception. The effect of selective attention to taste on insular responses may therefore reflect a general effect of attention or it may be (taste) modality specific. To distinguish between these 2 possibilities, we used functional magnetic resonance imaging to evaluate brain response to tastes and odors while subjects passively sampled the stimuli or performed a detection task. We found that trying to detect a taste (attention to taste) resulted in activation of the primary taste cortex (anterior and mid-dorsal insula) but not in the primary olfactory cortex (piriform). In contrast, trying to detect an odor (attention to odor) increased activity in primary olfactory but not primary gustatory cortex. However, we did identify a region of far anterior insular cortex that responded to both taste and odor "searches." These results demonstrate modality-specific activation of primary taste cortex by attention to taste and primary olfactory cortex by attention to odor and rule out the possibility that either response reflects a general effect of attentional deployment. The findings also support the existence of a multimodal region in far anterior insular cortex that is sensitive to directed attention to taste and smell.     

Odorant-specific patterns of sniffing during imagery distinguish 'bad' and 'good' olfactory imagers

There are large individual differences in the self-reported ability to form vivid olfactory mental imagery. Based on such self-reports, subjects have been classified as 'bad' or 'good' imagers. The present study examined whether a differential strategy in re-enacting the olfactomotor response during imagery may explain the dissociation between 'bad' and 'good' olfactory imagers. As previously reported, odor imagery was accompanied by sniffing. Although 'bad' and 'good' olfactory imagers did not differ in their overall sniffing volume, they used different strategies when re-enacting the motor component of olfaction during imagery. Particularly, as in real perception, 'good' but not 'bad' imagers generated bigger sniffs when imagining a pleasant smell compared with an unpleasant smell (P<0.02). Furthermore, preventing sniffing significantly hampered mental imagery of pleasant odors in 'good' but not 'bad' imagers (P<0.03). Taken together, these results suggest (i) the validity of the dissociation between 'bad' and 'good' olfactory imagers as revealed by self-report; (ii) that sniffing may be a causal factor in the creation of olfactory imagery; and (iii) that sniff measurements may serve as a reliable non-verbal tool in exploring individual differences in odor imagery.     

Ghrelin, leptin and adiponectin as possible predictors of the hedonic value of odors

Several lines of evidence point to a close relationship between the hormones of energy homeostasis and the olfactory system. Examples are the localization of leptin and adiponectin receptors in the olfactory system or increased activation of brain regions related to the palatability and the hedonic value of food in response to food pictures after application of ghrelin. In this preliminary study, we tested in 31 subjects (17 male and 14 female) if and to what extent the peripheral blood concentrations of "satiety" hormones, such as leptin, adiponectin, and ghrelin (acyl and total), are correlated with the self-ratings of odor pleasantness and with the objective olfactory and gustatory ability. The hedonic values of some odors were found to be differently rated between donors depending on gender and body weight. The concentrations of leptin, adiponectin and total ghrelin were significantly associated with the hedonic value of pepper black oil, but failed to show significant correlations for 5 other odors tested. Except for a significant association between leptin and odor identification, hormone concentrations were not linked to the abilities of smell and taste. Peripheral adipokines and gut hormones may alter the perception and pleasantness of specific odors, presumably either directly through their receptors in the olfactory system or indirectly through central interfaces between the regulation systems of olfaction, appetite control, memory and motivation.     

Direct Behavioral and Neurophysiological Evidence for Retronasal Olfaction in Mice

The neuroscience of flavor perception is hence becoming increasingly important to understand food flavor perception that guides food selection, ingestion and appreciation. We recently provided evidence that rats can use the retronasal mode of olfaction, an essential element of human flavor perception. We showed that in rats, like humans, odors can acquire a taste. We and others also defined how the input of the olfactory bulb (OB) -not functionally imageable in humans- codes retronasal smell in anesthetized rat. The powerful awake transgenic mouse, however, would be a valuable additional model in the study of flavor neuroscience. We used a go/no-go behavioral task to test the mouse''s ability to detect and discriminate the retronasal odor amyl acetate. In this paradigm a tasteless aqueous odor solution was licked by water-restricted head-fixed mice from a lick spout. Orthonasal contamination was avoided. The retronasal odor was successfully discriminated by mice against pure distilled water in a concentration-dependent manner. Bulbectomy removed the mice''s ability to discriminate the retronasal odor but not tastants. The OB showed robust optical calcium responses to retronasal odorants in these awake mice. These results suggest that mice, like rats, are capable of smelling retronasally. This direct neuro-behavioral evidence establishes the mouse as a useful additional animal model for flavor research.     

Changes in taste intensity perception following anterior temporal lobe removal in humans

To investigate the role of the anterior temporal lobe in taste perception, we compared taste intensity estimations made by patients who had removal from either the left or the right anterior temporal lobe for the treatment of intractable epilepsy with a group of healthy control subjects. Estimations were made for five concentrations of each of four different tastes, as well as for five cards of varying saturations of gray, which served as a control task. A cross-modal magnitude estimation procedure was employed in which subjects used distance on a measuring tape to reflect intensity estimation. Distances were then transformed into logs, and the slope and the correlation with stimulus concentration or saturation was calculated. Correlation was taken as a measure of accuracy of estimation and slope was taken as a measure of perceived intensity. As predicted, repeated measures analysis of variance (ANOVA) revealed a significant difference between the control group and both patient groups in taste intensity estimations, but not for grayness, reflecting the importance of the anterior temporal lobe in low-level gustatory but not visual perception. Additionally, repeated measures ANOVA for slopes indicated that subjects in the right temporal group rated the bitter taste as more intense than did subjects in other groups, possibly reflecting increased intensity perception of the unpleasant bitter taste.     

Do tastants have a smell?

The stimuli used in taste research are usually considered to be odourless. This was tested in two experiments with aqueous solutions of two representative compounds for each of the five taste qualities including umami. In the first experiment elderly and young subjects rated the intensity and pleasantness of three concentrations of the stimuli, while wearing or not wearing a noseclip. Saliva production was also measured. Blocking olfaction only influenced salivation for umami. It reduced taste intensity ratings, but as in an earlier experiment with the same compounds in food products, this effect was stronger in the young, who also liked the stimuli better wearing the noseclip. In the second experiment, another group of young people tried to detect the odours of the tastants dissolved in demineralized, double-distilled or Evian water. A considerable number of subjects could regularly detect seven of the ten tastants by olfaction and the extent to which they did correlated significantly with the reduction in taste intensity ratings for the different tastants found in the first experiment. We suggest that most tastants can be smelled and that this smell contributes to taste intensity ratings.     

Heterogeneity of molecular mechanisms the olfactor reception

In experiments on the frog isolated olfactory epithelium by using vital fluorescent microscope, odorants with fruit, rank, flower and camphor smell were shown to involve intracellular signaling systems in olfactory transduction. The odorants with different qualitative smells have different messenger and activity mechanisms. Intracellular messengers do not participate in reception of odorants with piquant and rotten smells. Thus the perception of different odour substances is maintained by physical and chemical processes. Hence, not only taste, carotid, medullar, but olfactory reception as well are characterised by heterogeneity of biophysical mechanisms.     

fMRI activation in response to odorants orally delivered in aqueous solutions

During food intake flavor perception results from simultaneous stimulation of the gustatory, olfactory and trigeminal systems. Olfactory stimulation occurs mainly through the retronasal pathway and the resulting perception is often interpreted as a taste perception, thus leading to the well-known sensory confusion between taste and olfaction. The present experiment was designed to study, with functional magnetic resonance imaging (fMRI), the cortical representation of olfactory perception in humans in response to retronasal stimulation by odorants delivered in aqueous solution. Psychophysical evaluation confirmed that the stimuli acted as pure olfactory stimuli through the retronasal pathway and did not present any taste component. Results showed activation in all brain regions previously described with neuroimaging techniques using olfactory stimulation with an odorized air flow. Piriform and orbitofrontal cortex were found activated as well as the hippocampal region, the amygdala, the insular lobe, the cingulate gyrus and the cerebellum. These results demonstrate the feasibility of efficiently stimulating the olfactory system in an fMRI scanner through the retronasal pathway with liquids delivered to the oral cavity. The presentation of olfactory stimuli in liquids to the mouth is a realistic model for the study of food-related flavor perception. This stimulation protocol furthermore allows presenting taste and olfactory stimuli separately or combined, thus allowing for direct comparisons between single modality representation, taste or olfaction, and representation of multi-modality mixtures.     

Comparisons of the estimates of smell, taste and overall intensity in young and elderly people

Absolute magnitude estimates of the smell, taste and overallintensity were examined in young (18–21 years) and elderly(61–94 years) people. The odorant/tastant pairs were eithersolutions of almond extract or lime/sucrose. For each age groupestimates of overall intensity were less than the sum of theestimates of each stimulus pair, these ratios were not affectedby the age of the subjects. This occurred even though, on average,when compared with young and elderly people when the odorantwas lime and the tastant sucrose. Thus, the data suggest thataging does not affect the process by which overall intensityis determined.     

Taste- and odor-reactivity in elderly demented patients

Previous studies demonstrated that hedonically different chemical(taste or smell) stimuli induceinnate, inherited, differentialand distinct fixed reflectory motion features in the oral andfacial area. In the present study 20 elderly demented patients,suffering from ‘probable’ or ‘possible’Alzheimer's disease, and 20 normally functioning elderly subjectswere tested. The facial expressive behavioral reactions triggeredby a set of common gustatory and olfactory stimuli were videotaped.Both psychophysical and stimulus-dependent behavioral responseswere obtained from the control group, while for the dementedpatients only behavioral reactions were recorded. Results revealedthat: (i) severely demented elderly subjects displayed differentialand distinct orofacial responses indicating ‘acceptance’and ‘aversion’. These were found to be analogousto but less intense than those displayed by control age mates;(ii) the duration of responses induced by aversive tastes islonger than that triggered by pleasant or indifferent ones,for both groups; (iii) all gustatory and olfactory stimuli triggera longer lasting behavioral response in demented than in normalsubjects; and (iv) psychophysical and behavioral responses ofthe control subjects gave similar results for taste- and odor-hedonicsas well as for their intensity. This finding clearly indicatesthe validity of the alternative use of psychophysical and behavioraltesting procedures.     

The addition of CO2 to traditional taste solutions alters taste quality

Previous studies of the effect of carbonation on taste perception have suggested that it may be negligible, manifesting primarily in increases in the perceived intensity of weak salt and sour stimuli. Assuming CO2 solutions in the mouth stimulate only trigeminal nerve endings, this result is not altogether surprising; however, there are neurophysiological data indicating that CO2 stimulates gustatory as well as trigeminal fibers. In that case, carbonation might alter the quality profile of a stimulus without producing substantial changes in overall taste intensity--much as occurs when qualitatively different taste stimuli are mixed. To address this possibility, subjects were asked to rate the total taste intensity of moderate concentrations of stimuli representing each of the basic tastes and their binary combinations, with an without added carbonation. They then subdivided total taste intensity into the proportions of sweetness, saltiness, sourness, bitterness and 'other taste qualities' they perceived. The addition of carbonation produced only small increases in ratings of total taste intensity. However, rather dramatic alterations in the quality profiles of stimuli were observed, particularly for sweet and salty tastes. The nature of the interaction is consistent with a direct effect of carbonation/CO2 on the gustatory system, although the possibility that at least some of the observed effects reflect trigeminal-gustatory interactions cannot be ruled out.      

Measurement of sensitivity to olfactory flavor: application in a study of aging and dentures

Olfaction involves a dual sensory process for perceiving odors orthonasally (through the nostrils) and retronasally (through the mouth). This investigation entailed developing a measure of sensitivity to an odor delivered in an orally sampled food (orange flavoring in a sucrose-sweetened gelatin) and examining sensitivity in the elderly. In experiment 1, olfactory flavor sensitivity was 49 times lower in elderly (n = 21) than in young (n = 28) subjects. In experiment 2, with 73 elderly women, higher olfactory flavor sensitivity correlated significantly with higher orthonasal perception (Connecticut Chemosensory Clinical Research Center test). Some women, however, exhibited low olfactory flavor sensitivity despite high orthonasal perception; none had high olfactory flavor sensitivity and low orthonasal perception. Those who wore complete or palatal covering dentures had lower olfactory flavor sensitivity than those who were dentate or wore dentures that did not cover the palate. Through multiple regression analysis, orthonasal perception and denture status were found to be independent contributors to predicting olfactory flavor sensitivity. In summary, elderly subjects showed depressed olfactory flavor sensitivity (i.e. retronasal sensitivity) that related to poor orthonasal olfactory perception and denture characteristic. Thus, while good orthonasal olfaction may be necessary for good olfactory flavor sensitivity, it is not sufficient. Other factors, some associated with oral conditions, may impede release and retronasal transport of odors from the mouth to the olfactory receptors.     

Olfaction and olfactory learning in Drosophila: recent progress

The olfactory system of Drosophila resembles that of vertebrates in its overall anatomical organization, but is considerably reduced in terms of cell number, making it an ideal model system to investigate odor processing in a brain [Vosshall LB, Stocker RF: Molecular architecture of smell and taste in Drosophila. Annu Rev Neurosci 2007, 30:505-533]. Recent studies have greatly increased our knowledge about odor representation at different levels of integration, from olfactory receptors to 'higher brain centers'. In addition, Drosophila represents a favourite model system to study the neuronal basis of olfactory learning and memory, and considerable progress during the last years has been made in localizing the structures mediating olfactory learning and memory [Davis RL: Olfactory memory formation in Drosophila: from molecular to systems neuroscience. Annu Rev Neurosci 2005, 28:275-302; Gerber B, Tanimoto H, Heisenberg M: An engram found? Evaluating the evidence from fruit flies. Curr Opin Neurobiol 2004, 14:737-744; Keene AC, Waddell S: Drosophila olfactory memory: single genes to complex neural circuits. Nat Rev Neurosci 2007, 8:341-354]. This review summarizes recent progress in analyzing olfactory processing and olfactory learning in Drosophila.