Sex differentiated responses to intranasal trigeminal stimuli

Exposure limits for indoor volatile substances concerning the general population: The role of population-based differences in sensory irritation of the eyes and airways for assessment factors

Assessment factors (AFs) are essential in the derivation of occupational exposure limits (OELs) and indoor air quality guidelines. The factors shall accommodate differences in sensitivity between subgroups, i.e., workers, healthy and sick people, and occupational exposure versus life-long exposure for the general population. Derivation of AFs itself is based on empirical knowledge from human and animal exposure studies with immanent uncertainty in the empirical evidence due to knowledge gaps and experimental reliability. Sensory irritation in the eyes and airways constitute about 30-40% of OELs and is an abundant symptom in non-industrial buildings characterizing the indoor air quality and general health. Intraspecies differences between subgroups of the general population should be quantified for the proposal of more 'empirical' based AFs. In this review, we focus on sensitivity differences in sensory irritation about gender, age, health status, and vulnerability in people, based solely on human exposure studies. Females are more sensitive to sensory irritation than males for few volatile substances. Older people appear less sensitive than younger ones. However, impaired defense mechanisms may increase vulnerability in the long term. Empirical evidence of sensory irritation in children is rare and limited to children down to the age of six years. Studies of the nervous system in children compared to adults suggest a higher sensitivity in children; however, some defense mechanisms are more efficient in children than in adults. Usually, exposure studies are performed with healthy subjects. Exposure studies with sick people are not representative due to the deselection of subjects with moderate or severe eye or airway diseases, which likely underestimates the sensitivity of the group of people with diseases. Psychological characterization like personality factors shows that concentrations of volatile substances far below their sensory irritation thresholds may influence the sensitivity, in part biased by odor perception. Thus, the protection of people with extreme personality traits is not feasible by an AF and other mitigation strategies are required. The available empirical evidence comprising age, lifestyle, and health supports an AF of not greater than up to 2 for sensory irritation. Further, general AFs are discouraged for derivation, rather substance-specific derivation of AFs is recommended based on the risk assessment of empirical data, deposition in the airways depending on the substance's water solubility and compensating for knowledge and experimental gaps. Modeling of sensory irritation would be a better 'empirical' starting point for derivation of AFs for children, older, and sick people, as human exposure studies are not possible (due to ethical reasons) or not generalizable (due to self-selection). Dedicated AFs may be derived for environments where dry air, high room temperature, and visually demanding tasks aggravate the eyes or airways than for places in which the workload is balanced, while indoor playgrounds might need other AFs due to physical workload and affected groups of the general population.

Comparable responses to a wide range of olfactory stimulation in women and men

The evidence for differences between women and men in terms of olfactory abilities is contradictory. We analyzed women and men's performance and reactions to a wider range of odour exposure outcomes than usually studied, to assess possible differences and similarities between sexes. Measures of sensitivity and sensory decision rule were established in 37 women and 39 men. Perceptual, cognitive, symptom-related and autonomic nervous system (skin conductance level and heart-rate variability) reactions were also assessed during extended ambient odour exposure, as well as participants' self-rated chemical intolerance. Bayesian analyses consistently revealed greater support for sex-related similarities than differences, suggesting that women and men perform and react comparably not only in terms of basic olfactory measures, but also to environmental odour exposure mimicking everyday situations.

Trigeminal stimulation is required for neural representations of bimodal odor localization: A time-resolved multivariate EEG and fNIRS study

Whereas neural representations of spatial information are commonly studied in vision, olfactory stimuli might also be able to create such representations via the trigeminal system. We explored in two independent multi-method electroencephalography-functional near-infrared spectroscopy (EEG+fNIRS) experiments (n1=18, n2=14) if monorhinal odor stimuli can evoke spatial representations in the brain. We tested whether this representation depends on trigeminal properties of the stimulus, and if the retention in short-term memory follows the "sensorimotor recruitment theory", using multivariate representational similarity analysis (RSA). We demonstrate that the delta frequency band up to 5 Hz across the scull entail spatial information of which nostril has been stimulated. Delta frequencies were localized in a network involving primary and secondary olfactory, motor-sensory and occipital regions. RSA on fNIRS data showed that monorhinal stimulations evoke neuronal representations in motor-sensory regions and that this representation is kept stable beyond the time of perception. These effects were no longer valid when the odor stimulus did not sufficiently stimulate the trigeminal nerve as well. Our results are first evidence that the trigeminal system can create spatial representations of bimodal odors in the brain and that these representations follow similar principles as the other sensory systems.

Allergic rhinitis children with obesity are more vulnerable to air pollution: a cross sectional study

The association between air pollution, allergic rhinitis (AR), and obesity has not been studied. From 2007 to 2011, 52 obese and 152 non-obese children (7-17 years old) with AR were recruited. Pediatric-Rhinoconjunctivitis-Quality-of-Life Questionnaire (PRQLQ) and nasal peak expiratory flow (NPEF) were tested. Association between the scores and rates of the two tests and mean air pollutant concentrations within 7 days before the tests were compared. When exposed to higher concentrations of CO, PM₁₀, and PM_2.5, the rates of worse nasal discomfort were 39.4%, 44.4% and 39.3% in obese children; and 18.0%, 21.9% and 19.7% in non-obese children, respectively. Compare to non-obese children, the rates in obese children were higher for CO (odds ratio (OR) 3.54, 95% confidence interval (CI) 1.15 ~ 10.92); PM₁₀ (OR 3.26, 95% CI 1.01 ~ 10.57) and PM2.5 (OR 3.30; 95% CI 1.03 ~ 10.54). In obese children, correlations between higher concentrations of CO, PM₁₀, PM_2.5 and higher nasal discomfort (higher PRQLQ); and correlations between higher concentrations of CO, PM₁₀, PM_2.5, NMHC (non-methane hydrocarbon) and higher nasal mucosa inflammation (lower NPEF) were noted. Obesity negatively affected AR severity when AR children experienced higher concentrations of CO, PM₁₀, and PM_2.5. Increased nasal inflammation induced by air pollutants might be the underlying mechanism.

Examination of Sex Differences in a Chronic Rhinosinusitis Surgical Cohort

OBJECTIVE

Sex discrepancies have been reported in chronic rhinosinusitis (CRS), but limited data exist exploring sex-specific biological processes and sinonasal quality of life.

STUDY DESIGN

Prospective cohort.

SETTING

Academic medical center.

METHODS

Demographics, clinical data, and sinonasal mucus were collected from patients with CRS presenting for surgical consideration over a 5-year period. A random forest model and linear regression were used to assess predictor variables for the 22-item Sino-Nasal Outcome Test (SNOT-22) and subdomains. Enzyme-linked immunosorbent assays were used to measure substance P and tryptase in a subset of mucus samples to explore biological differences by sex.

RESULTS

In total, 520 patients were studied (mean age 48.3 years, 50.9% female). Males were older (50.1 vs 46.6 years, P = .008), had more polyp disease (48.2% vs 35.5%, P = .004), and had higher mean Lund-Mackay CT score (11.3 vs 9.5, P = .004). Females had a higher overall mean SNOT-22 (40.9 vs 46.9, P = .001) and higher scores in ear/facial, psychological, and sleep domains (P < .01). Age, objective disease measures, and sex were top predictors for total SNOT-22. Neither mucus substance P or tryptase, alone or paired with sex, correlated with total SNOT-22. Analysis of mucus biomarkers by sex revealed correlation between mucus tryptase in females with the extranasal subdomain (P = .01).

CONCLUSION

Sex differences exist in CRS disease manifestations and presentation for surgical consideration. Detection of mucus (substance P and tryptase) was reliable, but in this exploratory study, we were not able to establish neurogenic or allergic inflammatory processes as a large source of differential disease features between sexes.

The Effects of Gender, Functional Condition, and ADL on Pressure Pain Threshold in Stroke Patients

Background

Somatosensory impairments and pain are common symptoms following stroke. However, the condition of perception and pain threshold for pressure stimuli and the factors that can influence this in individuals with stroke are still unclear. This study aimed to investigate the gender differences in pressure pain threshold (PPT) and positive somatosensory signs for pressure stimuli, and explore the effects of joint pain, motor function, and activities of daily living (ADL) on pain threshold in post-stroke patients.

Design

A cross-sectional study.

Methods

A total of 60 participants with stroke were recruited, and their pain condition, motor functions, and ADL were evaluated by the Fugl-Meyer assessment of joint pain scale, motor function scale, and Barthel index, respectively. PPTs in eight tested points at the affected and unaffected sides were assessed.

Results

Significant differences in PPTs were found between male and female patients in all measured muscles (p < 0.05). Positive somatosensory signs for pressure stimuli, including hypoalgesia and hyperalgesia, were frequently found at the affected side, particularly in the extremity muscles, but such signs were not significantly influenced by gender (p > 0.05). More equal PPTs between both sides and relatively lower PPTs at the affected side in the trunk and medial gastrocnemius muscles (p < 0.05) were observed in patients with less pain, better motor functions, and ADL.

Conclusion

Gender differences widely exist in post-stroke survivors either at the affected or unaffected side, which are multifactorial. Sensory loss and central and/or peripheral sensitization, such as hypoalgesia and hyperalgesia for pressure stimuli, caused by a brain lesion are common signs in male and female stroke patients. Moreover, patients who are in a better condition show a more symmetrical pain sensitivity between both sides in the trunk and in female lower extremities, indicating the bidirectional improvement of somatosensory abnormalities caused by a possible neural plasticity.

Some like it, some do not: behavioral responses and central processing of olfactory-trigeminal mixture perception

Exploring the potential of eucalyptol as a masking agent for aversive odors, we found that eucalyptol masks the olfactory but not the trigeminal sensation of ammonia in a previous study. Here, we further investigate the processing of a mixture consisting of eucalyptol and ammonia, two olfactory–trigeminal stimuli. We presented the two pure odors and a mixture thereof to 33 healthy participants. The nostrils were stimulated alternately (monorhinal application). We analyzed the behavioral ratings (intensity and pleasantness) and functional brain images. First, we replicated our previous finding that, within the mixture, the eucalyptol component suppressed the olfactory intensity of the ammonia component. Second, mixture pleasantness was rated differently by participants depending on which component dominated their mixture perception. Approximately half of the volunteers rated the eucalyptol component as more intense and evaluated the mixture as pleasant (pleasant group). The other half rated the ammonia component as more intense and evaluated the mixture as unpleasant (unpleasant group). Third, these individual differences were also found in functional imaging data. Contrasting the mixture either to eucalyptol or to both single odors, neural activation was found in the unpleasant group only. Activation in the anterior insula and SII was interpreted as evidence for an attentional shift towards the potentially threatening mixture component ammonia and for trigeminal enhancement. In addition to insula and SII, further regions of the pain matrix were involved when assessing all participant responses to the mixture. Both a painful sensation and an attentional shift towards the unpleasant mixture component complicates the development of an efficient mask because a pleasant perception is an important requirement for malodor coverage.

Risk assessment for irritating chemicals - Derivation of extrapolation factors

Irritation of the eyes and the upper respiratory tract are important endpoints for setting guide values for chemicals. To optimize the use of the often-limited data, we analysed controlled human exposure studies (CHS) with 1-4 h inhalation of the test substance, repeated dose inhalation studies in rodents, and Alarie-Tests and derived extrapolation factors (EF) for exposure duration, inter- and intraspecies differences. For the endpoint irritating effects in the respiratory tract in rodents, geometric mean (GM) values of 1.9 were obtained for the EF for subacute→subchronic (n = 16), 2.1 for subchronic→chronic (n = 40), and 2.9 for subacute→chronic (n = 10) extrapolation. Based on these data we suggest an EF of 2 for subchronic→chronic and of 4 for subacute→chronic extrapolation. In CHS, exposure concentration determines the effects rather than exposure duration. Slight reversible effects during 4 h exposure indicate that an EF of 1 can be considered for assessing chronic exposures. To assess species extrapolation, 10 chemicals were identified with both, reliable rat inhalation studies and CHS. The GM of the ratio between the No Observed Adverse Effect Concentration (NOAEC) in rats and humans was 2.3 and increased to 3.6 when expanding the dataset to all available EF (n = 25). Based on these analyses, an EF of 3 is suggested to extrapolate from a NOAEC in a chronic rat study to a NOAEC in a CHS. The analysis of EFs for the extrapolation from a 50% decrease in respiratory frequency in the Alarie test in mice (RD₅₀) to a NOAEC in a CHS resulted in a GM of 40, for both, the reliable (n = 11) and the overall dataset (n = 19). We propose to use the RD₅₀ from the Alarie test for setting guide values and to use 40 as EF. Efs for intraspecies differences in the human population must account for susceptible persons, most importantly for persons with chemical intolerance (CI), who show subjective signs of irritation at low concentrations. The limited data available do not justify to deviate from an EF of 10 - 20 as currently used in different regulatory settings.

Thermo-behavioural responses to orally applied l-menthol exhibit sex-specific differences during exercise in a hot environment

AIMS

This study investigated the efficacy of l-menthol mouth-rinsing on thermal sensation and perceived effort in females and males, using a fixed-rating of perceived exertion (RPE) exercise protocol in a hot environment.

METHODS

Twenty-two participants (eleven females, eleven males) completed two trials using a fixed-RPE protocol at an exercise intensity between 'hard' and 'very hard', equating to 16 on the RPE scale at ~35 °C. Participants adjusted power output to maintain RPE-16. In a randomised, double-blind, crossover design, l-menthol or a control mouthwash was administered at an orally neutral temperature (~32 °C) prior to exercise and at 10 min intervals thereafter. Measures of mechanical power output, core temperature, heart rate, perception of thermal sensation and thermal comfort, and whole-body sweat loss are reported.

RESULTS

Thermal sensation was lowered by l-menthol in both sexes (P < 0.05), however during exercise this was only maintained for 40% of the trial duration in females. Thermal comfort did not differ between conditions (P > 0.05). No differences in exercise duration were observed compared to control, despite a ~4% and ~6% increase in male and females respectively. Power output increased by ~6.5% males (P = 0.039) with no difference in females ~2.2% (P = 0.475), compared to control. Core temperature, heart rate and whole-body sweat loss was not different between condition or sex.

CONCLUSIONS

l-menthol lowered perceptual measures of thermal sensation in females, but did not attenuate a greater rate of rise in thermal sensitivity when exercising in a hot environment, compared to males. Males appeared to adopt a higher risk strategy by increasing power output following l-menthol administration in contrast to a more conservative pacing strategy in females. Therefore, there appear to be sex-specific differences in l-menthol's non-thermal cooling properties and subsequent effects on thermo-behavioural adjustments in work-load when exercising in a hot environment.

Sexually dimorphic behavioral and neural responses to a predator scent

Male and female C57BL/6 J mice were tested on the predator odor response task, where they needed to cross through a chamber of scented bedding to reach a sucrose reward. Following the behavioral testing, mouse brains were immunohistochemically labeled for expression of the immediate early gene c-fos. In the presence of the novel odorant methyl valerate (MV), both males and females exhibited increased exploration behaviors and delayed rewards compared to control bedding. However, in the presence of the predator odor phenylethylamine (PEA), males exhibited increased exploration that strongly resembled their behavior in MV (a non-predator odor) while females behaved very similarly to the clean bedding controls, quickly traversing the chamber to achieve the reward. Expression of c-fos exhibited significant sex by odor condition interactions overall across brain regions and in the anterior piriform cortex, cingulate cortex, and dorsomedial hypothalamus specifically. In all three regions we observed the general pattern that PEA exposure evoked elevated c-fos expression in females but suppressed c-fos expression in males. Taken together these data suggest that males and females may adopt different behavioral strategies in the presence of predator threat.

Ambient Odor Exposure Affects Food Intake and Sensory Specific Appetite in Obese Women

Food odors are important in food perception not only during consumption, but also in anticipation of food. Even though it is well established that smell is involved in eating behavior, its role in affecting actual food consumption is still unclear, especially in morbidly obese subjects, who are reported to be more affected by sensory cues than lean subjects. The aim of the present study was to investigate the influence of ambient odor exposure on ad libitum food intake and on sensory specific appetite in obese women. Thirty obese women (BMI: 34.9 ± 0.8 kg m^-2; age: 50.8 ± 1.8) attended two sessions in which they were exposed to a bread odor dispersed, in a detectable but mild concentration, in the test room (“scented” condition) and to a control condition (“unscented” condition). Participants filled out a questionnaire on general appetite before entering the test room and completed a sensory specific appetite questionnaire (including 12 specific products) about 10 min after entering the test room. After approximately 15 min of exposure, the ad libitum intake of a low energy dense food product (vegetable soup) was measured. The “scented” condition significantly (p < 0.01) increased the amount of soup eaten compared to the “unscented” condition (466.4 ± 33.1 g; 368.9 ± 33.2 g, respectively). Moreover, the odor exposure induced sensory specific appetite for congruent food products in term of taste and energy density, as well as a significant increase in general appetite scores (p < 0.001). In conclusion, ambient odor exposure to a food odor affected the intake of a low energy food in obese women and stimulated appetite for congruent products. This could have important implications for influencing energy intake of individuals.

Human exposure to acrolein: Time-dependence and individual variation in eye irritation

The aim of the study was to examine the time dependence on sensory irritation detection following exposure to threshold levels of acrolein, in humans. The exposures occurred in an exposure chamber and the subjects were breathing fresh air through a mask that covered the nose and mouth. All participants participated in four exposure conditions, of which three consisted of a mixture of acrolein and heptane and one of only heptane. Exposure to acrolein at a concentration half of the TLV-C lead to sensory irritation. The perceived sensory irritation resulted in both increased detectability and sensory irritation after about 6.8min of exposure in 58% of the participants. The study confirm the previously suggested LOAEL of about 0.34mg/m(3) for eye irritation due to acrolein exposure. The sensory irritation was still significant 10min after exposure. These results have implications for risk assessment and limit setting in occupational hygiene.

Sex differences in chemosensation: sensory or emotional?

Although the first sex-dependent differences in chemosensory processing were reported in the scientific literature over 60 years ago, the underlying mechanisms are still unknown. Generally, more pronounced sex-dependent differences are noted with increased task difficulty or with increased levels of intranasal irritation produced by the stimulus. Whether differences between the sexes arise from differences in chemosensory sensitivity of the two intranasal sensory systems involved or from differences in cognitive processing associated with emotional evaluation of the stimulants is still not known. We used simultaneous and complementary measures of electrophysiological (EEG), psychophysiological, and psychological responses to stimuli varying in intranasal irritation and odorousness to investigate whether sex differences in the processing of intranasal irritation are mediated by varying sensitivity of the involved sensory systems or by differences in cognitive and/or emotional evaluation of the irritants. Women perceived all stimulants more irritating and they exhibited larger amplitudes of the late positive deflection of the event-related potential than men. No significant differences in sensory sensitivity, anxiety, and arousal responses could be detected. Our findings suggest that men and women process intranasal irritation differently. Importantly, the differences cannot be explained by variation in sensory sensitivity to irritants, differences in anxiety, or differences in physiological arousal. We propose that women allocate more attention to potentially noxious stimuli than men do, which eventually causes differences in cognitive appraisal and subjective perception.

Statistical localization of human olfactory cortex

Functional neuroimaging methods have been used extensively during the last decades to explore the neural substrates of olfactory processing. While a general consensus on the functional anatomy of olfactory cortex is beginning to emerge, the mechanisms behind the functions of individual processing nodes still remain debated. Further, it remains unclear to which extent divergent findings result from differences in methodological approaches. Using Activation Likelihood Estimation (ALE), the aim of the present study was to statistically combine all published data on functional neuroimaging of olfaction to provide a probability map reflecting the state of the field to date. Additionally, we grouped studies according to various methodological approaches to investigate whether these systematically affected the reported findings. A total of 45 studies (69 contrasts, 594 foci) met our inclusion criteria. Significant ALE peaks for odor against baseline were observed in areas commonly labeled as primary and secondary olfactory cortex, such as the piriform and orbitofrontal cortex, amygdala, anterior insula, and ventral putamen. In addition, differences were observed in the extent to which different methods were able to induce activation in these different nodes of the olfactory network.

A systematic literature review of 10 years of research on sex/gender and experimental pain perception - part 1: are there really differences between women and men?

The purpose of this systematic review was to summarize and critically appraise the results of 10 years of human laboratory research on pain and sex/gender. An electronic search strategy was designed by a medical librarian and conducted in multiple databases. A total of 172 articles published between 1998 and 2008 were retrieved, analyzed, and synthesized. The first set of results (122 articles), which is presented in this paper, examined sex difference in the perception of laboratory-induced thermal, pressure, ischemic, muscle, electrical, chemical, and visceral pain in healthy subjects. This review suggests that females (F) and males (M) have comparable thresholds for cold and ischemic pain, while pressure pain thresholds are lower in F than M. There is strong evidence that F tolerate less thermal (heat, cold) and pressure pain than M but it is not the case for tolerance to ischemic pain, which is comparable in both sexes. The majority of the studies that measured pain intensity and unpleasantness showed no sex difference in many pain modalities. In summary, 10 years of laboratory research have not been successful in producing a clear and consistent pattern of sex differences in human pain sensitivity, even with the use of deep, tonic, long-lasting stimuli, which are known to better mimic clinical pain. Whether laboratory studies in healthy subjects are the best paradigm to investigate sex differences in pain perception is open to question and should be discussed with a view to enhancing the clinical relevance of these experiments and developing new research avenues.

A systematic literature review of 10 years of research on sex/gender and pain perception - part 2: do biopsychosocial factors alter pain sensitivity differently in women and men?

This systematic review summarizes the results of 10 years of laboratory research on pain and sex/gender. An electronic search strategy was designed by a medical librarian to access multiple databases. A total of 172 articles published between 1998 and 2008 were retrieved, analyzed, and synthesized. The second set of results presented in this review (129 articles) examined various biopsychosocial factors that may contribute to differences in pain sensitivity between healthy women and men. The results revealed that the involvement of hormonal and physiological factors is either inconsistent or absent. Some studies suggest that temporal summation, allodynia, and secondary hyperalgesia may be more pronounced in women than in men. The evidence to support less efficient endogenous pain inhibitory systems in women is mixed and does not necessarily apply to all pain modalities. With regard to psychological factors, depression may not mediate sex differences in pain perception, while the role of anxiety is ambiguous. Cognitive and social factors appear to partly explain some sex-related differences. Finally, past individual history may be influential in female pain responses. However, these conclusions must be treated with much circumspection for various methodological reasons. Furthermore, some factors/mechanisms remain understudied in the field. There is also a need to assess and improve the ecological validity of findings from laboratory studies on healthy subjects, and perhaps a change of paradigm needs to be considered at this point in time to better understand the factors that influence the experience of women and men who suffer from acute or chronic pain.

Thresholds and chemosensory event-related potentials to malodors before, during, and after puberty: differences related to sex and age

A sex-related difference in olfactory sensitivity to androstadienone and androstenone has been reported to occur during puberty. The study reported here extends this work to investigate whether sex and age differences exist before, during, and after puberty for 2-methyl, 3-mercapto-butanol (2M3M; a malodorous component of human sweat), carbon disulfide/hydrogen sulfide (H2S), and androstadienone. A total of 121 participants took part in the study (58 females, 63 males; age range 9-20 years). Participants were divided into 3 groups (i) pre-puberty, (ii) puberty and (iii) post-puberty. Threshold measurements for (i) androstadienone, (ii) 2M3M and (iii) carbon disulfide were recorded. Chemosensory event-related potentials (CSERPs) were recorded using air-dilution olfactometry. The results revealed that female thresholds for the three odorants were stable between the three age groups. Pubescent males had higher thresholds (less sensitive) for all three odorants. In the post-puberty group, sex differences were only observed for 2M3M. These differences are mirrored by significant sex differences for CSERP latencies. The latency increase in male pubescents may be due to the production of sweat by the apocrine glands resulting in increased levels of 2M3M and androstadienone, resulting in adaptation. To conclude, based on the present study performed in a relatively large sample, the processing of malodors in males is different from that of females during puberty. This observation not only relates to a reduced sensitivity towards odors typially found in axillary sweat but also extends towards other malodors. While the underlying cause may be partly due to specific adaptation towards malodors present in axillary sweat it might also reflect hormonal changes modifying the perception of odors.

Lateralisation of intranasal trigeminal chemosensory event-related potentials

AIM OF THE STUDY

To determine whether or not chemosensory event-related brain potentials (CSERP) elicited by nociceptive unilateral intranasal (CO2) trigeminal stimulation are lateralized and, if they are, whether this hemispheric lateralization is related to the side of the stimulated nostril.

METHODS

Nine healthy right-handed subjects participated to the study. CSERPs were recorded after left or right monorhinal CO2 stimulation. Latency and baseline-to-peak amplitude of each CSERP component were compared across stimulation conditions (left and right nostril), scalp locations (lower-frontal, frontal, mid-temporal, central, posterior-temporal, parietal) and hemispheres (left or right), using a three-factor analysis of variance (ANOVA) for repeated measures.

RESULTS

Intranasal trigeminal CO2 stimulation elicited a large N400-P550 complex. This complex was preceded by an earlier N300 component. Whatever the stimulated nostril, N300, N400 and P550 amplitudes were significantly higher on the right as compared to the left hemisphere, at lower-frontal recording sites. The side of chemosensory stimulation (left or right nostril) did not significantly affect CSERP components.

CONCLUSIONS

This study showed that in healthy right-handed volunteers with normal olfactory ability, intranasal chemosensory trigeminal stimulation may elicit a series of event-related brain potentials, which all display a significant right-hemisphere predominance, irrespective of the stimulated nostril. The observed lateralization was maximal at lower-frontal recording sites.

Cross-modal integration of intranasal stimuli: a functional magnetic resonance imaging study

Most odorants, in addition to the olfactory system, also activate the intranasal trigeminal system. Recent studies have shown that pure trigeminal stimulation activates somatosensory regions as well as regions traditionally thought of as primary olfactory areas. As a main aim of this study we wished to a) ascertain which brain regions are responsive to an "artificially" bimodal odor composed of a trigeminal (CO(2)) and an olfactory stimulant (phenyl ethyl alcohol, PEA) and b) determine if presenting CO(2) and PEA simultaneously activates different brain regions than when presenting them individually. Fifteen men were scanned using functional magnetic resonance imaging while smelling PEA, CO(2), and a mixture of both stimuli (CO(2)PEA) presented simultaneously. Odors were presented monorhinally to the right nostril in a block design. The contrast between CO(2)PEA and baseline revealed areas implicated in the processing of both olfactory and trigeminal stimuli. When the mixture was contrasted with the sum of its single components (CO(2)PEA-{CO(2)+PEA}), activations in integration centers (left superior temporal and right intraparietal sulcus) and in orbitofrontal areas (left medial and lateral orbitofrontal cortex) were detected. The opposite contrast ({CO(2)+PEA}-CO(2)PEA) did not reveal any significant activation. In contrast to studies which have used natural mixed olfactory/trigeminal stimuli, we have shown that the perception of an artificial mixed olfactory/trigeminal stimulus activates, as opposed to inhibiting the olfactory cortex. Further, we also conclude that a mixed olfactory/trigeminal stimulus appears to lead to higher cortical activations than the sum of its parts.

Signal-to-noise ratio of chemosensory event-related potentials

OBJECTIVE

We investigated the influence of the number of stimuli on signal-to-noise (S/N) ratio of CSERP.

METHODS

CSERP from 20 normosmic subjects were obtained in response to stimulation with two olfactory (H(2)S and PEA) and a trigeminal (CO(2)) stimulant. For each of these odors, 160 stimuli were delivered into the right nostril (duration 200ms, mean ISI 30s) using a constant-flow, air-dilution olfactometer. For each EEG recording site (Fz, Cz, Pz, C3, C4), peak-to-peak amplitude N1P2 and noise amplitude levels were determined. Subsequently, S/N ratios were calculated.

RESULTS

The S/N ratios for olfactory ERP generally improved for H(2)S and PEA. For responses to PEA, S/N ratios increased significantly up to 80 averages (S/N ratio=5.6). The number of stimuli for an optimal S/N ratio for trigeminal ERP was slightly lower, i.e. 60 averages (S/N ratio=7.9).

CONCLUSIONS

S/N N1P2 ratios in olfactory and trigeminal ERP significantly improve with an increasing number of responses averaged under these experimental conditions. This is mainly due to a reduction of noise level. Applying more stimuli has little additional effect on S/N ratio due to a concomitant decrease in signal amplitude.

SIGNIFICANCE

An optimal S/N ratio is essential when recording CSERP in neurodegenerative disorders, where responses may be of low amplitude, and for medico-legal purposes.

Chemosensory specific reduction of trigeminal sensitivity in subjects with olfactory dysfunction

Humans with olfactory loss have been found to exhibit a decreased sensitivity of the chemosensory trigeminal system. It is not clear, whether the reduced trigeminal sensitivity is restricted to the chemosensitive properties of the trigeminal nerve, or whether it reflects a general decrease of trigeminal sensitivity which is also found for cutaneous afferents. To investigate the relationship between cutaneous somatosensory and intranasal chemosensory trigeminal sensitivity, 91 subjects were investigated. Forty-five of them were considered healthy controls, whereas 46 subjects had olfactory dysfunction. Subjects with olfactory dysfunction were found to have higher thresholds for CO2 than controls indicating lower trigeminal chemosensory sensitivity in subjects with olfactory dysfunction. Both etiology and degree of olfactory dysfunction appeared to have an impact on CO2 thresholds. In contrast, no such differences were found with regard to detection thresholds for electrical cutaneous stimulation. These results indicate that the decrease of trigeminal sensitivity in subjects with olfactory dysfunction is specific for chemosensory sensations.

Normative data for the “Sniffin’ Sticks” including tests of odor identification, odor discrimination, and olfactory thresholds: an upgrade based on a group of more than 3,000 subjects

"Sniffin' Sticks" is a test of nasal chemosensory function that is based on pen-like odor dispensing devices, introduced some 10 years ago by Kobal and co-workers. It consists of tests for odor threshold, discrimination, and identification. Previous work established its test-retest reliability and validity. Results of the test are presented as "TDI score", the sum of results obtained for threshold, discrimination, and identification measures. While normative data have been established they are based on a relatively small number of subjects, especially with regard to subjects older than 55 years where data from only 30 healthy subjects have been used. The present study aimed to remedy this situation. Now data are available from 3,282 subjects as compared to data from 738 subjects published previously. Disregarding sex-related differences, the TDI score at the tenth percentile was 24.9 in subjects younger than 15 years, 30.3 for ages from 16 to 35 years, 27.3 for ages from 36 to 55 years, and 19.6 for subjects older than 55 years. Because the tenth percentile has been defined to separate hyposmia from normosmia, these data can be used as a guide to estimate individual olfactory ability in relation to subject's age. Absolute hyposmia was defined as the tenth percentile score of 16-35 year old subjects. Other than previous reports the present norms are also sex-differentiated with women outperforming men in the three olfactory tests. Further, the present data suggest specific changes of individual olfactory functions in relation to age, with odor thresholds declining most dramatically compared to odor discrimination and odor identification.

Assessment of olfactory and trigeminal function using chemosensory event-related potentials

GOALS

To give an overview on the theoretical and practical applications of chemosensory event-related potentials.

METHODS

Chemosensory event-related potentials (ERPs) may be elicited by brief and precisely defined odorous stimuli. Based on the principles of air-dilution olfactometry, a stimulator was developed in the late 1970s, which allows stimulation of the olfactory neuroepithelium and the nasal mucosa with no concomitant mechanical stimulation. Chemosensory ERPs were obtained after stimulation of the olfactory nerve (olfactory ERPs) or the trigeminal nerve (somatosensory or trigeminal ERPs). The characteristics of the stimulator for chemosensory research as well as the variables influencing the responses are discussed in this paper.

RESULTS

Implementation and normative data from our department are reported with different clinical examples from otorhinolaryngologic clinic. The bulk of the evoked response consists of a large negative component (often referred to as N1), which occurs between 320 and 450 ms after stimulus onset. This component is followed by a large positive component, often referred to as P2, occurring between 530 and 800 ms after stimulus onset. Absence of olfactory ERPs and presence (even with subtle changes) of somatosensory ERPs is a strong indicator of the presence of an olfactory dysfunction.

CONCLUSIONS

This review examines and discusses the methods of chemosensory stimulation as well as the electrophysiological correlates elicited by such stimuli. The clinical applications of chemosensory ERPs in neurology and otorhinolaryngology are outlined.