Event-related potentials to intranasal trigeminal stimuli change in relation to stimulus concentration and stimulus duration

Spatiotemporal Processing of Bimodal Odor Lateralization in the Brain Using Electroencephalography Microstates and Source Localization

The neuronal cascade related to the perception of either purely olfactory or trigeminal airborne chemicals has been investigated using electroencephalography (EEG) microstate analyses and source localization. However, most airborne chemicals are bimodal in nature, encompassing both properties. Moreover, there is an ongoing debate regarding whether there is one dominant nostril, and this could be investigated using these multichannel EEG methods. In this study, 18 right-handed, healthy participants (13 females) were monorhinally stimulated using an olfactometer with the bimodal component acetic acid during continuous EEG recording. Participants indicated the side of stimulation, the confidence in their decision, and rated the strength of the evoked perception. EEG microstate clustering determined four distinct maps and successive backfitting procedures, and source estimations revealed a network that evolved from visual-spatial processing areas to brain areas related to basic olfactory and trigeminal sensations (e.g., thalamus, cingulate cortex, insula, parahippocampal, and pre-/post-central gyri) and resulted in activation of areas involved in multisensory integration (e.g., frontal-temporal areas). Right-nostril stimulation was associated with faster microstate transition and longer involvement of the superior temporal gyrus, which was previously linked to chemical localization and provides evidence for a potential nostril dominance. The results describe for the first time the processing cascade of bimodal odor perception using microstate analyses and demonstrate its feasibility to further investigate potential nostril dominance.

Electrophysiologic assessment of olfactory and gustatory function

This chapter reviews approaches for assessing human and gustatory function using electrophysiologic methods. Its focus is on changes in electrical signals, including summated generator potentials that occur after nasal or oral stimulation. In the first part of the review, we describe tools available to the clinician for assessing olfactory and nasotrigeminal function, including modern electroencephalography (EEG) analysis of brain responses both in the time domain and in the time-frequency (TF) domain. Particular attention is paid to chemosensory event-related potentials (CSERPs) and their potential use in medical-legal cases. Additionally, we focus on the changes of summated generator potentials from the olfactory and respiratory nasal epithelium that could provide new diagnostic insights. In the second part, we describe gustatory event-related potentials (gCSERPs) obtained using a relatively new computer controlled gustometer. A device for presenting different pulses of electrical current to the tongue is also described, with weaker pulses likely reflecting gCSERPs and stronger ones trigeminal CSERPs. Finally, summated generator potentials from the surface of the tongue during gustatory stimulation are described that may prove useful for examining peripheral taste function.

Time frequency analysis of olfactory induced EEG-power change

OBJECTIVES

The objective of the present study was to investigate the usefulness of time-frequency analysis (TFA) of olfactory-induced EEG change with a low-cost, portable olfactometer in the clinical investigation of smell function.

MATERIALS & METHODS

A total of 78 volunteers participated. The study was composed of three parts where olfactory stimuli were presented using a custom-built olfactometer. Part I was designed to optimize the stimulus as well as the recording conditions. In part II EEG-power changes after olfactory/trigeminal stimulation were compared between healthy participants and patients with olfactory impairment. In Part III the test-retest reliability of the method was evaluated in healthy subjects.

RESULTS

Part I indicated that the most effective paradigm for stimulus presentation was cued stimulus, with an interstimulus interval of 18-20s at a stimulus duration of 1000ms with each stimulus quality presented 60 times in blocks of 20 stimuli each. In Part II we found that central processing of olfactory stimuli analyzed by TFA differed significantly between healthy controls and patients even when controlling for age. It was possible to reliably distinguish patients with olfactory impairment from healthy individuals at a high degree of accuracy (healthy controls vs anosmic patients: sensitivity 75%; specificity 89%). In addition we could show a good test-retest reliability of TFA of chemosensory induced EEG-power changes in Part III.

CONCLUSIONS

Central processing of olfactory stimuli analyzed by TFA reliably distinguishes patients with olfactory impairment from healthy individuals at a high degree of accuracy. Importantly this can be achieved with a simple olfactometer.

The Effect of Stimulus Duration on the Nostril Localization of Eucalyptol

The trigeminal system is a chemosensory system participating in the perception of most odorants, which allows for the perception of diverse sensations including the freshness of eucalyptus or the spiciness of pepper. The lateralization task, that is, the identification of the stimulated nostril in a monorhinal stimulation paradigm is only possible following trigeminal stimulation and allows therefore for the assessment of the trigeminal sensitivity also in a clinical setting. In this study, we aimed to determine the effects of the duration of stimuli on the lateralization task. To this end, we asked 32 young and healthy subjects perform the lateralization task while being exposed to eucalyptol stimuli ranging between 100 and 1250 ms. We found that participants performed on average at chance for stimuli shorter than 500 ms, and observed increasing accuracy for stimuli with longer durations. In conclusion, these data suggest that 500 ms represents a threshold for the lateralization of eucalyptol stimuli. Therefore, when trigeminal sensitivity is tested in a clinical setting, eucalyptol stimuli should have a duration of at least 500 ms.

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.

Barium versus nonbarium stimuli: differences in taste intensity, chemesthesis, and swallowing behavior in healthy adult women

PURPOSE

The authors examined the impact of barium on the perceived taste intensity of 7 different liquid tastant stimuli and the modulatory effect that these differences in perceived taste intensity have on swallowing behaviors.

METHOD

Participants were 80 healthy women, stratified by age group (<40; >60) and genetic taste status (supertasters; nontasters). Perceived taste intensity and chemesthetic properties (fizziness; burning-stinging) were rated for 7 tastant solutions (each prepared with and without barium) using the general Labeled Magnitude Scale. Tongue-palate pressures and submental surface electromyography (sEMG) were simultaneously measured during swallowing of these same randomized liquids. Path analysis differentiated the effects of stimulus, genetic taste status, age, barium condition, taste intensity, and an effortful saliva swallow strength covariate on swallowing.

RESULTS

Barium stimuli were rated as having reduced taste intensity compared with nonbarium stimuli. Barium also dampened fizziness but did not influence burning-stinging sensation. The amplitudes of tongue-palate pressure or submental sEMG did not differ when swallowing barium versus nonbarium stimuli.

CONCLUSIONS

Despite impacting taste intensity, the addition of barium to liquid stimuli does not appear to alter behavioral parameters of swallowing. Barium solutions can be considered to elicit behaviors that are similar to those used with nonbarium liquids outside the assessment situation.

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.

Separating brain processing of pain from that of stimulus intensity

Regions of the brain network activated by painful stimuli are also activated by nonpainful and even nonsomatosensory stimuli. We therefore analyzed where the qualitative change from nonpainful to painful perception at the pain thresholds is coded. Noxious stimuli of gaseous carbon dioxide (n = 50) were applied to the nasal mucosa of 24 healthy volunteers at various concentrations from 10% below to 10% above the individual pain threshold. Functional magnetic resonance images showed that these trigeminal stimuli activated brain regions regarded as the "pain matrix." However, most of these activations, including the posterior insula, the primary and secondary somatosensory cortex, the amygdala, and the middle cingulate cortex, were associated with quantitative changes in stimulus intensity and did not exclusively reflect the qualitative change from nonpainful to pain. After subtracting brain activations associated with quantitative changes in the stimuli, the qualitative change, reflecting pain-exclusive activations, could be localized mainly in the posterior insular cortex. This shows that cerebral processing of noxious stimuli focuses predominately on the quantitative properties of stimulus intensity in both their sensory and affective dimensions, whereas the integration of this information into the perception of pain is restricted to a small part of the pain matrix.

Intranasal localizability of odorants: influence of stimulus volume

When an odorant is presented to one side of the nose and air to the other, the ability to localize which side received the odorant depends upon trigeminal nerve stimulation. It has been shown that performance on this lateralization task increases as stimulus concentration increases. In this study, we determined the influences of stimulus volume and sex on the ability to localize each of 8 odorants presented at neat concentrations: anethole, geraniol, limonene, linalool, menthol, methyl salicylate, phenyl ethanol, and vanillin. At a low stimulus volume (11 mL), only menthol was localized at an above-chance level. At a high stimulus volume (21 mL), above-chance localization occurred for all odorants except vanillin. Women were significantly better than men in localizing menthol. Stimuli rated as most intense were those that were most readily localized. The detection performance measures, as well as rated intensity values, significantly correlated with earlier findings of the trigeminal detectability of odorants presented to anosmic and normosmic subjects. This study suggests that differences in stimulus volume may explain some discrepant findings within the trigeminal chemosensory literature and supports the concept that vanillin may be a "relatively pure" olfactory stimulus.

Comparison of the orthonasal and retronasal detection thresholds for carbon dioxide in humans

Several studies have investigated the orthonasal detection threshold for carbon dioxide (CO(2)) in humans. The aim of current study was to investigate whether 24 healthy young subjects exhibited differences of CO(2) detection thresholds during orthonasal or retronasal stimulation. As nasal mucosa is believed to desensitize to CO(2) concentrations at or below 4% (v/v) during expiration, the second aim of the study was to explore the influence during nasal versus oral breathing on the detection thresholds. CO(2) stimuli of varying concentrations and a duration of 1000 ms were applied with an air-dilution olfactometer in either the anterior nasal cavity or the nasopharynx during nasal respectively oral breathing. In these 4 conditions, the mean CO(2) detection thresholds using the staircase forced-choice procedure were between 3.9% and 5.3% (v/v). Statistical analysis revealed a significant difference between orthonasal and retronasal stimulation. The CO(2) detection threshold was lower in retronasal stimulation. The nasopharyngeal mucosa is more sensitive to perithreshold CO(2) stimuli than the nasal mucosa. The breathing route had no influence on the detection thresholds. The results of this study indicate that the natural contact of the nasal mucosa with approximately 4% (v/v) CO(2) during nasal expiration does not influence CO(2) detection thresholds.

Temporal integration in nasal lateralization of homologous propionates

For nasal irritation from volatile chemicals, a version of Haber's rule (k = C(n)T) can model the trade-off between concentration (C) and duration of exposure (T) to achieve a fixed sensory impact, e.g. threshold-level irritation or a fixed suprathreshold intensity. The term k is a constant. The exponent, n, represents how well the system integrates over time. An exponent of 1 indicates complete temporal integration: an x-fold increase in stimulus duration exactly compensates for cutting the concentration 1/x. An exponent greater than 1 indicates incomplete temporal integration: more than an x-fold increase in duration is needed. In a previous study of homologous alcohols, n varied systematically with number of methylene units: integration became more complete as the length of the carbon chain increased. To explore the generality of this finding, we tested homologous esters that differ in the number of methylene units: n-ethyl propionate, n-propyl propionate, and n-butyl propionate. Nasal lateralization was used to measure irritation thresholds. Human subjects received a fixed concentration of a single compound within each experimental session. Stimulus duration was varied to find the briefest stimulus that caused lateralizable irritation. Concentration and compound varied across sessions. Consistent with results with n-alcohols, integration became more complete as the number of methylene units increased. Lipid solubility varies with chain length; hence, solubility in the nasal mucosa may play a role in the dynamics of irritation. Further, preliminary analyses suggest that, for data pooled across both chemical series, n varies systematically with molecular parameters related to solubility and diffusion.

Attention bias and sensitization in chemical sensitivity

OBJECTIVE

We investigated whether persons with self-reported chemical sensitivity (CS) have an attention bias and enhanced sensitization to chemical exposure.

METHODS

Chemosomatosensory, olfactory, and auditory event-related potentials (ERPs) were recorded from 21 CS subjects and 17 controls in attend and ignore conditions. Reaction times (RTs) and magnitude estimations of perceived intensity were collected in the attend condition. ERPs were averaged over attention conditions and during the first/second part of the testing.

RESULTS

ERP patterns indicated that CS subjects did not habituate to the same extent as the controls and had difficulties ignoring the chemical exposure. CS subjects had faster overall RT, and the perceived intensities for the chemosomatosensory stimuli did not decrease with time in the CS group, which was the case for the controls.

CONCLUSIONS

These results indicating attention bias and enhanced sensitization in CS suggest alterations in central, cognitive responses to chemical exposure.

Improved smell pleasantness after odor-taste associative learning in humans

Whereas some aspects of olfactory hedonism in humans are present from birth, others form during development and throughout adulthood. Although it is generally agreed that such hedonic representations emerge by associative learning, it is not yet clear which learning parameters are prominent. The present study investigated the influence of number of trials on odor preference acquisition in human adults. Forty-eight subjects randomly assigned to three groups were tested in three sessions. In the first session, subjects ranked eight food odors from most pleasant to most unpleasant. The second session consisted in an associative learning, the two most neutral odors were randomly associated with a drink: one odor with water (CS-) and the other odor with a pleasant sweet solution (CS+). In the third session subjects ranked the eight food odors again. In group A, CS+ was paired three times with the US, and in group B only once; in group C, CS+ was paired only once with the US but with a total duration identical to that in group A. Results showed that CS+ was ranked as significantly more pleasant after learning in group A (p<.05), but not in groups B and C (p>.05). In conclusion, the study showed that a neutral smell can acquire positive emotional features after being paired with a pleasant taste, and that this learning depends on the number of associations between smell and taste.

Temporal integration in nasal lateralization of homologous alcohols

Through temporal integration, sensory systems accumulate stimulus energy, e.g., photons, acoustic energy, or molecules, over time to detect weaker signals than they otherwise could. Past studies found imperfect temporal integration in detection of nasal irritation: To maintain a fixed level of detection, one must increase stimulus duration by more than twofold to compensate for cutting concentration in half. Despite this generality, integration varied widely among compounds, from nearly perfect, i.e., an increase in duration of slightly more than twofold could compensate for cutting concentration in half, to highly imperfect. How do such differences relate to molecular parameters? Perhaps molecules that more readily dissolve into the lipid-rich perireceptor environment will accumulate, and therefore integrate, better over time. To test this hypothesis, studies compared temporal integration for three compounds that differ systematically in lipid solubility: n-ethanol, n-butanol, and n-hexanol. Subjects were healthy, adult humans. Nasal lateralization was used to measure irritation threshold. Subjects received a fixed concentration of a single compound within each experimental session, and stimulus duration was varied to find the briefest stimulus subjects could reliably lateralize. Concentration and compound varied across sessions. Consistent with the hypothesis, integration did become closer to perfect as lipid solubility increased. That just one molecular parameter can help predict degree of integration suggests that a structure-activity approach to understanding temporal integration shows promise.

The neural representation of odor is modulated by the presence of a trigeminal stimulus during odor encoding

OBJECTIVES

Odor perception does not simply consist in hierarchical processing from transduction to a single "true" cerebral representation. Odor sensation may be modulated by available sensory information during encoding. The present study set out to examine whether the presence of a pure trigeminal stimulus during odor encoding may modulate odor perception at both behavioral and cortical levels.

METHODS

Participants were tested in a 2-session within-subject design: first, an odor encoding session included a delay conditioning procedure in which relatively selective olfactory stimulants (phenyl ethyl alcohol or vanillin, Conditioned Stimulus+, CS+) were presented either with a pulse of CO(2) (Unconditioned Stimulus, US), or alone (Conditioned Stimulus-, CS-); then, in the second session, both pure odorants (CS+ and CS-) were presented alone. During this second session, olfactory event-related potentials were simultaneously recorded and analyzed at different electrode sites including Cz and Pz (sites known to have maximal amplitudes for trigeminal and olfactory stimuli, respectively). After each trial, subjects were asked to rate odor intensity and hedonics.

RESULTS

The results showed that CS+ intensity ratings increased in 8 subjects and decreased in 6. Cortically, a group effect was observed for P2 amplitude, which increased in the "CS+ intensity increase" group vs. the "CS+ intensity decrease" group at Cz (p<0.05) but not at Pz (p>0.05).

CONCLUSIONS

This result suggests that the presence of a pure trigeminal stimulus (CO(2)) during odor encoding alters the neural representation of a pure odor.

SIGNIFICANCE

The neural representation of odors comprises not only the odor itself but also contextual information (trigeminal in the present case) presented during encoding.

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.

Interactions between olfaction and the trigeminal system: what can be learned from olfactory loss

The olfactory and the trigeminal systems have a close relationship. Most odorants also stimulate the trigeminal nerve. Further, subjects with no sense of smell exhibit a decreased trigeminal sensitivity with unclear underlying mechanisms. Previous studies indicated that single stages of trigeminal processing may differently be affected by olfactory loss. A better knowledge of adaptive and compensatory changes in the trigeminal system of subjects with acquired anosmia (AA) will improve the understanding of interactive processes between the 2 sensory systems. Thus, we aimed to assess trigeminal function on different levels of processing in subjects with AA. Subjects with AA showed larger electrophysiological responses to irritants obtained from the mucosa than healthy controls. On central levels, however, they exhibited smaller event-related potentials and psychophysical measures to irritants. Over 9 months, they exhibited an increase in trigeminal sensitivity. Subjects with recovering olfactory function showed an even more increased peripheral responsiveness to irritants. These data suggest dynamic mechanisms of mixed sensory adaptation/compensation in the interaction between the olfactory and trigeminal systems, where trigeminal activation is increased on mucosal levels in subjects with AA and amplified on central levels in subjects with a functioning olfactory system.

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.

Does Haber's law apply to human sensory irritation?

Irritation of the eyes, nose, and throat by airborne chemicals--also referred to as "sensory irritation"--is an important endpoint in both occupational and environmental toxicology. Modeling of human sensory irritation relies on knowledge of the physical chemistry of the compound(s) involved, as well as the exposure parameters (concentration and duration). A reciprocal relationship between these two exposure variables is postulated under Haber's law, implying that protracted, low-level exposures may be toxicologically equivalent to brief, high-level exposures. Although time is recognized as having an influence on sensory irritation, the quantitative predictions of Haber's Law have been addressed for only a handful of compounds in human experimental studies. We have conducted a systematic literature review that includes a semiquantitative comparison of psychophysical data extracted from controlled human exposure studies versus. the predictions of Haber's law. Studies containing relevant data involved exposures to ammonia (2), chlorine (2), formaldehyde (1), inorganic dusts such as calcium oxide (1), and the volatile organic compound 1-octene (1). With the exception of dust exposure, varying exposure concentration has a proportionally greater effect on sensory irritation than does changing exposure duration. For selected time windows, a more generalized power law model (c(n) x t = k) rather than Haber's law per se (c x t = k) yields reasonably robust predictions. Complicating this picture, however, is the frequent observation of intensity-time "plateauing," with time effects disappearing, or even reversing, after a relatively short period, depending on the test compound. The implications of these complex temporal dynamics for risk assessment and standard setting have been incompletely explored to date.

Chemosensory event-related potentials in relation to side of stimulation, age, sex, and stimulus concentration

OBJECTIVE

For chemosensory event-related potentials (ERP) significant effects of age and sex have been demonstrated. The aim of the present study was to assess the effects of stimulus concentration, side of stimulation, and sex on the topographical distribution of chemosensory ERP in a large group of subjects stratified for different age groups. In addition, psychophysical measures of both olfactory and trigeminal function should be assessed in greater detail compared to previous work.

METHODS

A total of 95 healthy subjects participated in the study. Olfactory functions were tested using the 'Sniffin' Sticks' comprising tests of odor identification, odor discrimination, and odor threshold. Trigeminal sensitivity was assessed on a psychophysical level using a lateralization paradigm. ERP to the olfactory stimulant H2S and the trigeminal irritant CO2 were recorded; stimuli were presented in different concentrations to the left and right nostril.

RESULTS

Olfactory thresholds exhibited an age-related increase while the outcome of psychophysical trigeminal tests was not significantly affected by age. In contrast, there was no significant main effect of the factor 'sex' for olfactory tests, while women scored higher than men in the trigeminal task. ERP to olfactory and trigeminal stimuli exhibited a relationship to stimulus concentration, age, and sex with youngest women showing largest amplitudes and shortest latencies. There was no significant main effect of left- or right-sided stimulation on ERP. Measures of olfactory function were found to correlate with parameters of olfactory ERP even when controlling for the subject's age. In addition, correlations between scores in the lateralization task and parameters of the trigeminal ERP were found.

CONCLUSIONS

Based on electrophysiological data obtained in a large sample size the present results established an age-related loss of olfactory and trigeminal function, which appears to be almost linear. Further, the present results emphasize that responses to chemosensory stimuli are related to sex, while the side of stimulation does not play a major role in the presently used paradigm. Finally, these data establish the lateralization paradigm as a psychophysical tool to investigate intranasal trigeminal function.

SIGNIFICANCE

The present results obtained in a representative group of healthy subjects establishes a comprehensive set of data, which will serve as reference for future work in this area of research.

On the trigeminal percept of androstenone and its implications on the rate of specific anosmia

Specific anosmia is a term that describes an inability to perceive a particular odorant in the context of an otherwise normal olfactory acuity. The most common example, for the odor of androstenone, has been ascribed a prevalence ranging from 2 to 45%. In two experiments we sought to determine whether this wide range could be explained by the difference in steroid concentrations used, and by the degree to which the trigeminal system contributes to perception of androstenone. Experiment 1 demonstrated that high concentrations of androstenone stimulated the trigeminal system, as indicated by electrophysiological recordings. Experiment 2 demonstrated that conscious detection of androstenone is possible based solely on the trigeminal system. Interestingly, detection seems to interact with olfactory acuity in that subjects with a low olfactory sensitivity to androstenone were better able to detect its trigeminal component. The agreement between conscious experience and behavioral discrimination was not well calibrated, in that subjects demonstrated a clear overconfidence in their abilities. Altogether, the current study suggests that androstenone is an odorant that produces a concentration-dependent degree of trigeminal stimulation. This trigeminal component explains the diversity of the reported prevalence of specific anosmia for androstenone and might have implications on future use of specific anosmia as a tool to understand odor processing.

Temporal integration in nasal lateralization of ethanol

Two experiments examined the trade-off between concentration and stimulus duration in nasal lateralization of n-ethyl alcohol. In nasal lateralization, a common measure of irritation threshold, subjects receive chemical vapor in one nostril and clean air in the other. Subjects try to determine which nostril received the chemical. Within experimental runs, subjects received fixed concentrations (1650-5000 ppm) of ethanol, and duration was varied to find the shortest, lateralizable stimulus. In Experiment 1, a small group of subjects was tested intensively to obtain stable individual data. In Experiment 2, a larger group was studied using more rapid methods. In both cases, subjects could lateralize increasingly weaker concentrations with longer stimulus presentations. Hence integration occurred. However, more than a twofold increase in duration was required to compensate for a twofold decrease in concentration to maintain threshold lateralization. These results suggest that an imperfect, mass-integrator model can describe short-term integration of nasal lateralization of ethanol.

Temporal integration of nasal irritation from ammonia at threshold and supra-threshold levels

Two experiments examined integration of perceived irritation over short-term (approximately 100-4000 ms) delivery of ammonia into the nasal cavity of human subjects. Experiment 1 examined trade-offs between time and concentration at threshold level by means of nasal lateralization, a common measure of irritation threshold. Within experimental sessions, the duration of a fixed-concentration stimulus varied to determine the shortest, detectable pulse. Subjects could lateralize increasingly weaker concentrations with longer stimulus presentations. Experiment 2 examined an analogous trade-off for supra-threshold irritation. Subjects rated irritation from presentations of ammonia that varied both in concentration and in duration. Rated intensity for a given concentration increased with stimulus duration. Hence integration occurred at both threshold and supra-threshold levels. However, more than a twofold increase in duration was required to compensate for a twofold decrease in concentration to maintain threshold lateralization or a fixed level of perceived intensity. These results suggest that an imperfect mass-integrator model may be able to describe short-term integration of nasal irritation from ammonia at both the threshold and supra-threshold levels.

Intranasal trigeminal thresholds in healthy subjects

The trigeminal chemosensory system responds to irritation of the nasal cavity. Despite its dominant role as a sentinel in protecting the respiratory tract from harmful substances and its involvement in the perception of odorous substances, it has received relatively little attention compared to the olfactory system. Aim of the present study was the comparison of two psychophysical techniques to assess intranasal trigeminal thresholds, namely (A) responses of subjects who focused on intranasal trigeminally mediated sensations, and (B) the ability of subjects to identify the side of the nose receiving unilaterally presented stimuli. Method A (0.81>r>0.56) was found to show a higher test-retest reliability than Method B (0.48>r>0.40). Method A revealed thresholds that were approximately 32 times lower than those measured with method B. With method A women were found to have lower thresholds than men; no such difference could be detected between older and younger subjects. In conclusion, if the objective is to assess the level at which trigeminal sensations are detected with the utmost objectivity and unconfounded by smell, the obvious choice is B. If one's purpose is to assess the level at which trigeminal sensations are detected and the quality perceived, in the context of an odor one might opt for Method A. Thus, preference of one method over the other may depend on the question being asked, provided a well-instructed/trained panel of subjects is used.

How chemical information processing interferes with face processing: a magnetoencephalographic study

Magnetic field changes related to face encoding were recorded in 20 healthy young participants. Faces had to be deeply encoded under four kinds of simultaneous nasal chemical stimulation. Neutral room air, phenyl ethyl alcohol (PEA, rose flavor), carbon dioxide (CO2, pain), and hydrogen sulfide (H2S, rotten eggs flavor) were used as chemical stimuli. PEA and H2S represented odor stimuli, whereas CO2 was used for trigeminal stimulation (pain sensation). After the encoding of faces, the respective recognition performances were tested focusing on recognition effects related to specific chemical stimulation during encoding. The number of correctly recognized faces (hits) varied between chemical conditions. PEA stimulation during face encoding significantly increased the number of hits compared to the control condition. H2S also led to an increased mean number of hits, whereas simultaneous CO2 administration during face encoding resulted in a reduction. Analysis of the physiological data revealed two latency regions of interest. Compared to the control condition, both olfactory stimulus conditions resulted in reduced activity components peaking at about 260 ms after stimulus onset, whereas CO2 produced a strongly pronounced enhanced activity component peaking at about 700 ms after stimulus onset. Both olfactory conditions elicited only weak enhanced activities at about 700 ms, and CO2 did not show any difference activity at 260 ms after stimulus onset compared to the control condition. It is concluded that the early activity differences represent subconscious olfactory information processing leading to enhanced memory performances irrespective of the hedonic value, at least if they are only subconsciously processed. The later activity is suggested to reflect conscious CO2 perception negatively affecting face encoding and therefore leading to reduced subsequent face recognition. We interpret that conscious processing of nasal chemical stimulation competes with deep face encoding with respect to cortical resources, whereas subconscious processing of nasal chemical stimulation does not.