Intranasal trigeminal detection of chemical vapors by humans.

The majorityof 14 human observers lacking olfactory nerve function detected, by nasal inhalation, over one quarter of 31 chemicals commonly used in olfactory research. No major differences were noted between anosmic observers with and without documented anatomic, genetic, or physiologic etiology. Detected compounds differed significantly from nondetected ones on the basis of a number of chemical properties, including water solubility and molecular weight. Detection thresholds of the anosmics, established by a single staircase procedure, were approximately two log volume concentration steps above those of normals for two compounds examined. Exponents of power functions fitted to magnitude estimates of the anosmics did not differ significantly from those fitted to the estimates of matched normal controls. These data indicate the trigeminal nerve may play an important role in human nasal chemoreception and that effective trigeminal stimulants may be predictable on physicochemical grounds.     

Assessment of intranasal trigeminal function.

Intranasal trigeminal function is more and more understood as an integral part of human chemosensory perception. Sensations like burning, stinging, warmth, coolness, or itching are produced by almost all odorants so that they can be perceived by anosmics. Electrophysiological responses to trigeminal stimuli allow the specific assessment of trigeminally mediated information at different levels of processing including the periphery or the cortex. Information regarding the localization of these processes can be derived from magnetoencephalographic recordings or functional imaging data. When using these techniques in combination with psychophysical measures, it seems to be possible to specifically describe how and where the processing of irritation takes place, how it may interact with olfactory mediated sensations, and how it is modulated, e.g. by environmental influences or analgesic drugs.     

Emotional reactivity to odors: Olfactory sensitivity and the span of emotional evaluation separate the genders

Numerous studies report gender differences in emotional reactivity in health and disease and the perception of odors is closely linked to the limbic system. In order to investigate gender differences in the emotional perception of odors we extended the Sniffin’ Stick Test with analogue rating scales for hedonic (pleasantness/unpleasantness) and intensity estimates. We matched 172 healthy subjects (86 females and 86 males) on age in order to balance the study population for three age strata (A: 19–39 years, B: 40–59, C: 60 years and above). Overall odors in our statistical analysis demonstrated significant gender differences for the absolute hedonic estimates but not for the relative hedonic and not for the intensity estimates. These findings demonstrate that women evaluate the pleasantness of perceived odors in a more extreme manner than men without significant differences in hedonic polarity (pleasantness/unpleasantness). Thus, we report a singular significant effect of gender on the dimension valence (hedonic estimation) independent from the dimension intensity. Our findings are in accordance with gender differences in facial reactivity to auditory stimuli and differences in the evaluation of emotional pictures. Investigating olfactory sensitivity females detected n-butanol and discriminated the 16 odors of the test significantly better than males. These results indicate that females possess higher olfactory sensitivity.     

Cross-modal integration between odors and abstract symbols

This study aimed to investigate the cross-modal association of an “abstract symbol,” designed for representation of an odor, with its corresponding odor. First, to explore the associations of abstract symbols with odors, participants were asked to match 8 odors with 19 different abstract symbols (Experiment 1). Next, we determined whether congruent symbols could modulate olfactory perception and olfactory event-related potentials (ERPs) (Experiment 2). One of two odors (phenylethanol (PEA) or 1-butanol) was presented with one of three conditions (congruent or incongruent symbol, no-symbol), and participants were asked to rate odor intensity and pleasantness during olfactory ERP recordings. Experiment 1 demonstrated that certain abstract symbols could be paired with specific odors. In Experiment 2 congruent symbol enhanced the intensity of PEA compared to no-symbol presentation. In addition, the respective congruent symbol increased the pleasantness of PEA and the unpleasantness of 1-butanol. Finally, compared to the incongruent symbol, the congruent symbol produced significantly higher amplitudes and shorter latencies in the N1 peak of olfactory ERPs. In conclusion, our findings demonstrated that abstract symbols may be associated with specific odors.     

Thresholds for odor and nasal pungency

Detection thresholds were measured repeatedly for 11 chemicals in normosmic and anosmic subjects. The stimuli comprised the first eight members of the series of n-aliphatic alcohols, phenyl ethyl alcohol, pyridine, and menthol. Results showed that anosmics could detect, via pungency, all but phenyl ethyl alcohol reliably. In the aliphatic series, both odor and pungency thresholds declined with chain length in a way that implied dependence of both in part on phase distribution in the mucosa. Odor thresholds, however, declined more rapidly than pungency thresholds: the ratio of anosmics threshold/normosmics threshold increased from 23 for methanol to 10,000 for 1-octanol. The outcome of a scaling experiment employing normosmic subjects indicated that, with the exception of methanol and ethanol, pungency arose when perceived intensity reached a narrowly tuned criterion level. When thresholds were expressed as percentages of saturated vapor, an index of thermodynamic activity, thereby accounting for differences in solubility and in phase distribution in the mucosa among the various stimuli, both odor and pungency thresholds depicted a striking constancy across stimuli.     

Odor perception in patients with mood disorders

BACKGROUNDS: Olfaction has obvious correlates with emotional processes but little is known about the several aspects of olfaction in psychiatric disorders characterized by mood disturbance. This research aims at pointing out the specificities of olfactory perception in patients in order to identify the specific cerebral impairments involved in these disorders. METHODS: Olfactory sensitivity, detection, identification, self-evaluation of intensity and pleasantness have been recorded in a control group of healthy subjects (N = 58) and in three sample populations admitted to a Psychiatry Department: depressive patients (N = 49), anorectic patients (N = 17), and patients suffering from addiction to alcohol or drugs (N = 21). RESULTS: Depressive patients have a poor sensitivity, poor detection abilities but over-evaluate the pleasantness of odors. Anorectic patients have a high sensitivity, over-evaluate the intensities of the odors but under-evaluate their pleasantness. Alcoholic/drug addicted patients showed impairments in identification. LIMITATION: This study does not identify inter-individual differences in olfactory perception. CONCLUSION: The psychiatric diseases, here at hand although every one of them may be characterized by depressive components, show diverging impairments in olfactory perception. When variations in sensitivity are usually attributed to peripheral cues, impairments in emotional and cognitive aspects of olfaction are typically related to specific brain structures and processes which could be particularly involved in these diseases.     

Discovering Alzheimer's disease and bipolar disorder white matter effects building computer aided diagnostic systems on brain diffusion tensor imaging features

Responses to affective stimuli are usually studied in just one sensory system at a time. However, this is rarely the way they are experienced. We were interested in how combining affective stimuli of similar intensities across two sensory modalities (smell and vision) would affect both behavioral responses (ratings) and psychophysiological responses (skin conductance). We studied this using olfactory stimuli delivered birhinally while the subjects viewed affectively laden scenes on a computer screen. Bilateral skin conductance recordings were taken throughout. Subjects rated the pleasantness of the images that they were viewing. We found a particularly salient effect of unpleasant smells, which enhanced the pleasantness ratings of certain images and also the skin conductance responses to unpleasant images.     

Effets du repas sur l''agrément d''odeurs alimentaires et nonalimentaires chez l''homme

Food-related and nonfood-related olfactory stimuli were presented to human subjects before and after they ate a meal. The subjects were asked to judge the pleasantness or the unpleasantness of the stimuli using a category scale. The responses of ten control subjects, who remained fasted, did not change during a 120 min period. With the other subjects on ad lib meals there followed these two characteristic phenomena: (1) there was a significant drop in the pleasantness of the food-related odors (negative alliesthesia); and, (2) there was no change in the pleasantness of the non-food-related odors (no alliesthesia). Therefore negative alliesthesia is specific to food-related odors. Negative alliesthesia reached a maximum within 60 min after the meals, then decreased and disappeared within the next 60 min. In three other subjects, with abnormal body weight, negative alliesthesia did not occur after the meal. These results favor the view that olfactory alliesthesia plays a role in satiety and the control of intermeal interval. They are also compatible with the existence of a ponderostat.     

Neural coding of stimulus concentration in the human olfactory and intranasal trigeminal systems

Nasal chemical sensations are mediated principally by the olfactory and the trigeminal systems. Over the last few years brain structures involved in processing of trigeminal stimuli have been more and more documented. However, the exact role of individual regions in stimulus intensity processing is unclear. The present study set out to examine the neural network involved in encoding stimulus intensity in the trigeminal system and the olfactory system of humans. Participants were presented with two concentrations of relatively specific trigeminal stimuli (CO2) and olfactory (H2S), respectively. Responses were assessed by functional magnetic resonance imaging (fMRI). Whereas brain responses to stimulus intensity in the olfactory modality involved a wide neural network including cerebellum, entorhinal cortex, visual areas, and frontal regions, contrasting high and low CO2 concentrations revealed activation in a less complex network including various sub-regions of the cingulate cortex. Taken together, these results suggest separate but overlapping neural networks involved in encoding stimulus intensity in the two chemosensory systems.     

Olfactory and trigeminal interaction of menthol and nicotine in humans

The purpose of the study was to investigate the interactions between two stimuli—menthol and nicotine—both of which activate the olfactory and the trigeminal system. More specifically, we wanted to know whether menthol at different concentrations modulates the perception of burning and stinging pain induced by nicotine stimuli in the human nose. The study followed an eightfold randomized, double-blind, cross-over design including 20 participants. Thirty phasic nicotine stimuli at one of the two concentrations (99 and 134 ng/mL) were applied during the entire experiment every 1.5 min for 1 s; tonic menthol stimulation at one of the three concentrations (0.8, 1.5 and 3.4 μg/mL) or no-menthol (placebo control conditions) was introduced after the 15th nicotine stimulus. The perceived intensities of nicotine’s burning and stinging pain sensations, as well as perceived intensities of menthol’s odor, cooling and pain sensations, were estimated using visual analog scales. Recorded estimates of stinging and burning sensations induced by nicotine initially decreased (first half of the experiment) probably due to adaptation/habituation. Tonic menthol stimulation did not change steady-state nicotine pain intensity estimates, neither for burning nor for stinging pain. Menthol-induced odor and cooling sensations were concentration dependent when combined with low-intensity nicotine stimuli. Surprisingly, this dose dependency was eliminated when combining menthol stimuli with high-intensity nicotine stimuli. There was no such nicotine effect on menthol’s pain sensation. In summary, we detected interactions caused by nicotine on menthol perception for odor and cooling but no effect was elicited by menthol on nicotine pain sensation.     

Relationship between obesity,weight loss,and taste responsiveness

Obese, mildly overweight and normal weight females rated glucose solutions of increasing concentrations for perceived intensity and pleasantness. Obese and mildly overweight subjects found increasingly sweet solutions more pleasant than did normals. Weight loss by dieting did not affect this relationship. Weight loss due to intestinal bypass surgery altered ratings of the pleasantness of glucose solutions, making them appear more similar to ratings given by normal weight individuals. Finally, after weight loss by dieting, all weight groups found the sweet taste of milkshake pleasant even after a preload and consumed large amounts of the milkshake. Prior to weight loss, ingestion of a preload had produced lowered pleasantness ratings and reduced consumption.     

Time-course of trigeminal versus olfactory stimulation: Evidence from chemosensory evoked potentials

Habituation of responses to chemosensory signals has been explored in many ways. Strong habituation and adaptation processes can be observed at the various levels of processing. For example, with repeated exposure, amplitudes of chemosensory event-related potentials (ERP) decrease over time. However, long-term habituation has not been investigated so far and investigations of differences in habituation between trigeminal and olfactory ERPs are very rare. The present study investigated habituation over a period of approximately 80 min for two olfactory and one trigeminal stimulus, respectively. Habituation was examined analyzing the N1 and P2 amplitudes and latencies of chemosensory ERPs and intensity ratings. It was shown that amplitudes of both components – and intensity ratings – decreased from the first to the last block. Concerning ERP latencies no effects of habituation were seen. Amplitudes of trigeminal ERPs diminished faster than amplitudes of olfactory ERPs, indicating that the habituation of trigeminal ERPs is stronger than habituation of olfactory ERPs. Amplitudes of trigeminal ERPs were generally higher than amplitudes of olfactory ERPs, as it has been shown in various studies before. The results reflect relatively selective central changes in response to chemosensory stimuli over time.     

The influence of stimulus duration on odor perception.

Although different parameters are known to alter the shape of olfactory event related potentials (ERP), ERP parameters are generally thought to be independent from stimulus duration. Evidence from recent studies investigating trigeminal ERP indicates that this may not be true. Aim of the present study was to investigate the relationship of stimulus duration and ERP. A total of 20 young healthy subjects participated. Subjects were investigated on 5 occasions on 5 different days. ERP were recorded to olfactory stimuli of two different concentrations and 3 different durations (100 ms, 200 ms, 300 ms). In two sessions olfactory ERP to PEA were recorded, in another two sessions H2S was applied. During the same sessions, intensity ratings were recorded. In the fifth session, subjects were asked to rate the duration of H2S stimuli and PEA stimuli. Whereas at weak stimulus concentrations no effect of stimulus duration could be observed, there was a clear effect of "duration" in ERP amplitudes following stimuli with higher concentrations: the longer the stimulus duration the larger the ERP amplitudes. No effect was found on ERP latencies. With regard to intensity ratings, strong stimuli and longer lasting stimuli lead to higher ratings. Similarly, ratings of stimulus duration were dependent from stimulus concentration and stimulus duration. Results of the present study showed that similar to trigeminal ERP, information about stimulus duration is encoded in olfactory ERP, mainly in amplitudes.     

Event-related brain potentials to attended and ignored olfactory and trigeminal stimuli.

Event-related brain potentials (ERPs) were recorded from 26 young adults, with equal numbers of male and female subjects, using attended and ignored, olfactory and trigeminal stimuli. The amplitudes and latencies of the N1, P2, and P3 components were recorded using a single-stimulus paradigm, with an inter-stimulus interval of 60 s, employing amyl acetate as the olfactory stimulus and ammonia as the trigeminal stimulus. Subjects estimated stimulus intensity in the attend condition or continued with a visual tracking task in the ignore condition. Results indicate that olfactory information is processed 30-70 ms faster than trigeminal information for the N1 and P2 potential and 100 ms faster for the P3 ERP component. N1/P2 interpeak amplitude was greater for the trigeminal than the olfactory stimuli, and greater in the attended than ignored condition. P3 amplitude was greater in the attend than ignore condition for olfactory information processing and equivalent for trigeminal information processing. These findings suggest that neuronal resource allocation is greatest for attended stimuli and that a painful stimulus demands neuronal resources even when ignored.     

Thermal taste, PROP responsiveness, and perception of oral sensations

Differences between 6-n-propylthiouracil (PROP) taster groups have long been the focus of studies on individual variation in perception of oral sensation. Recently, "thermal taste" was described, the phenomenon whereby some individuals perceive "phantom" taste sensations after thermal stimulation of small areas of the tongue. As with PROP taster status (PTS), thermal taster status (TTS) has been proposed as a proxy for general responsiveness to oral stimuli. Here we examined the influence of PTS and TTS, independently, on the perceived intensity of sweet, sour, salty, bitter, astringent, and metallic stimuli, and temperature on heating or cooling the tongue. Interactions between PTS and TTS were also examined, and fungiform papillae (FP) density and salivary flow rate (SFR) were determined. Both PTS and TTS were associated with perceived stimulus intensities. PROP super-tasters (pSTs) rated all oral stimuli as more intense than PROP non-tasters (pNTs). Thermal tasters (TTs) gave higher logged ratings than thermal non-tasters (TnTs) for all oral sensations including temperature, with the exception of metallic flavour (at low concentration) and PROP. Examination of ETA-squared values showed that PTS had a greater effect on perceived intensities than did TTS for most sensations. No PTSTTS interaction was found for any oral stimuli. In contrast with PTS, TTS was not associated with FP density, and neither PTS nor TTS were associated with SFR. We conclude that pSTs and TTs possess greater responsiveness across a range of taste and trigeminal stimuli and concentrations.     

Localisation of unilateral nasal stimuli across sensory systems

Odor stimuli presented to one nostril can only be localised if they additionally activate the trigeminal nerve's chemosensitive fibers. In this study we aimed to investigate characteristics in the localisation of unilateral trigeminal, olfactory and somatosensory nasal stimuli. We compared the ability of healthy young subjects to localise monorhinally presented (a) pure olfactory stimuli (phenyl ethyl alcohol), (b) mixed olfactory trigeminal stimuli (eucalyptol), and (c) somatosensory stimuli (air puffs). As expected, subjects could localise the air puffs and eucalyptol, but could not phenyl ethyl alcohol. Interestingly, we observed a significant correlation between localisation performance for eucalyptol and phenyl ethyl alcohol but not between the ability to localise somatosensory and trigeminal or olfactory stimuli. These observations show that on a behavioural level, the trigeminal chemosensory system is more intimately connected to the olfactory system than to the somatosensory system despite the fact that anatomically its information is conveyed via same nerve as the latter. Furthermore, they show that the trigeminal chemosensory system should therefore be considered a self-confined contributor to chemosensory perception.     

Neuronal generator patterns of olfactory event‐related brain potentials in schizophrenia

To better characterize neurophysiologic processes underlying olfactory dysfunction in schizophrenia, nose–referenced 30–channel electroencephalogram was recorded from 32 patients and 35 healthy adults (18 and 18 male) during detection of hydrogen sulfide (constant‐flow olfactometer, 200 ms unirhinal exposure). Event‐related potentials (ERPs) were transformed to reference–free current source density (CSD) waveforms and analyzed by unrestricted Varimax–PCA. Participants indicated when they perceived a high (10 ppm) or low (50% dilution) odor concentration. Patients and controls did not differ in detection of high (23% misses) and low (43%) intensities and also had similar olfactory ERP waveforms. CSDs showed a greater bilateral frontotemporal N1 sink (305 ms) and mid‐parietal P2 source (630 ms) for high than low intensities. N1 sink and P2 source were markedly reduced in patients for high intensity stimuli, providing further neurophysiological evidence of olfactory dysfunction in schizophrenia.     

Olfactory lateralization: odor intensity but not the hedonic estimation is lateralized

An earlier study in humans comparing the olfactory sensitivity of both nostrils revealed a small but significant advantage of the right nostril for detection and for olfactory quality discrimination. However lateralization was not evaluated for the perception of odor intensity and hedonic evaluation (pleasantness/unpleasantness). Thus we investigated lateralization of olfactory intensity and hedonic evaluation in right-handed healthy volunteers (n=186) from the HeDoS-F database (Hedonic Database of Smell-Franconia). For olfactory evaluation the Sniffin' Stick Test was employed with the parameters detection, discrimination, identification and extended by analogue hedonic and intensity rating scales. Over all odors subjects rated the perceived intensity significantly higher following stimulation of the right compared to the left nostril. The analysis of the single odors of the Sniffin' Stick Test consistently confirmed higher intensity ratings for the right compared to the left nostril reaching a statistically significant difference for 10 out of 16 odors. In contrast we found no significant differences between the nostrils for the hedonic estimates over all odors. Differences in odor detection, discrimination and identification did not reach a statistically significant level, but for all these parameters the scores of the right nostril were slightly higher compared to the left nostril. For odor identification, however, a statistical tendency was observed. Based on our results we concluded that olfactory intensity estimates represent the most sensitive parameter of olfactory lateralization.     

Chemosensory perception and event-related potentials in self-reported chemical hypersensitivity.

Anormal chemosensory perception has been identified as a possible mechanism underlying odor intolerance, but research in this domain has yet been rather limited. The main objective of the present study was to investigate total perceived intensity, unpleasantness, sensory irritation, and cortical activity assessed with chemosensory event-related potentials (ERPs) for three concentrations of pyridine ranging from predominantly olfactory to trigeminal in activation. Results from 19 individuals with self-reported chemical hypersensitivity and 19 controls with self-reported normal chemical sensitivity show that the hypersensitive group, compared to controls, rated the pyridine stimuli to be more intense and unpleasant, and that these group differences increased with pyridine concentration. Sensory irritation was also the perceptual dimension found to correlate strongest with score on the chemical sensitivity scale. However, no group differences were found in ERP amplitudes or latencies. These findings suggest that self-reported chemical hypersensitivity (1) can be associated with anormal chemosensory perception, (2) may be more closely related to trigeminal function than to olfaction, and (3) has a neural basis at a higher cortical level than that captured by chemosensory ERPs.