Olfactory dysfunction affects thresholds to trigeminal chemosensory sensations

A nasal chemosensation-dependent critical window for somatosensory development

Nasal chemosensation is considered the evolutionarily oldest mammalian sense and, together with somatosensation, is crucial for neonatal well-being before auditory and visual pathways start engaging the brain. Using anatomical and functional approaches in mice, we reveal that odor-driven activity propagates to a large part of the cortex during the first postnatal week and enhances whisker-evoked activation of primary whisker somatosensory cortex (wS1). This effect disappears in adult animals, in line with the loss of excitatory connectivity from olfactory cortex to wS1. By performing neonatal odor deprivation, followed by electrophysiological and behavioral work in adult animals, we identify a key transient regulation of nasal chemosensory information necessary for the development of wS1 sensory-driven dynamics and somatosensation. Our work uncovers a cross-modal critical window for nasal chemosensation-dependent somatosensory functional maturation.

Trigeminal function in patients with COVID-associated olfactory loss

PURPOSE

Olfactory dysfunction (OD) can be a long-term consequence of various viral infections, including COVID-19. Dysfunction includes hyposmia/anosmia and parosmia (odor distortions). Interactions of the virus with the olfactory nerve have been extensively researched, but little is known about the interactions of the intranasal trigeminal nerve system in modulating this sensory loss.

METHODS

We investigated intranasal trigeminal function across COVID-19 OD patients with and without parosmia compared to normosmic controls, to determine whether (1) post-viral hyposmia and/or (2) post-viral hyposmia with parosmia is associated with altered trigeminal function. OD patients (n = 27) were tested for olfactory function using the extended Sniffin' Sticks olfactory test and for trigeminal function through three methods-odor lateralization, subjective ratings of nasal patency, and ammonium vapor pain intensity ratings. This group was subsequently compared to controls, normosmic subjects (n = 15).

RESULTS

Our findings revealed that post-COVID OD patients without parosmia experienced decreased sensitivity in ammonium vapor pain intensity ratings and odor lateralization scores-but similar nasal patency ratings-compared to normosmic controls. There were no significant differences in trigeminal function between OD patients with and without parosmia.

CONCLUSIONS

Based on our results, we conclude that the trigeminal nerve dysfunction may partially explain post-viral OD, but does not seem to be a major factor in the generation of parosmia pathophysiology.

Olfactory stimulation may modulate the sensation of nasal patency

BACKGROUND

The sensation of nasal patency can be induced by inhaling menthol, which predominantly produces trigeminal stimulation. It remains unclear whether olfactory stimulation can also induce or modulate the sensation of nasal patency.

METHODOLOGY

A total of 118 participants (normosmia: n=67, olfactory dysfunction: n=51) were exposed to four odors in a randomized order: 1) phenylethanol (PEA), 2) menthol, 3) a mixture of PEA and menthol, 4) nearly odorless propylene glycol. The odors were presented by nasal clips. After the nasal clip had been removed, the participants rated relative nasal patency (RNP) from - 50 to +50, and their peak nasal inspiratory flow (PNIF) was measured. Repeated measures analysis of variance was used to examine the difference of RNP and PNIF among the four conditions and the influence of olfactory function.

RESULTS

The RNPs, other than PNIFs, differed between the four conditions. Menthol induced the highest RNP, followed by the mixed solution, PEA and the odorless condition. Normosmic participants, but not those with olfactory dysfunction, responded to PEA significantly higher than odorless condition with regard to RNP. The correlation analysis showed that the better the subjective or measured olfactory performance, the greater the PEA-induced sensation of nasal patency.

CONCLUSIONS

A specific olfactory stimulant that selectively induces olfactory perception can also evoke and modulate the sensation of nasal patency. Hence, patients might benefit from exposing themselves to odors in order to relieve the annoying nasal obstruction.

Functional Imaging in Olfactory Disorders

Purpose of Review

The aim was to synthesize key findings regarding the use of functional MRI (fMRI) to assess olfactory dysfunction (OD), and thus, to evaluate whether fMRI could be a reliable clinical diagnostic tool.

Recent Findings

In response to olfactory stimulation, patients with quantitative OD display reduced activation in olfactory-related brain regions but also stronger activation in non-olfactory brain areas. Parosmic patients also seem to show both weaker and higher brain signals. As to trigeminal chemosensory system, fMRI suggests that central processing may be declined in patients with OD. Functional connectivity studies report a possible correlation between altered neuronal connections within brain networks and olfactory performances.

Summary

fMRI emerges as a valuable and promising objective method in OD evaluation. Yet, its high inter-individual variability still precludes its routine clinical use for diagnostic purpose. Future research should focus on optimizing stimulation paradigms and analysis methods.

Parkinson's Disease Affects Functional Connectivity within the Olfactory-Trigeminal Network

BACKGROUND

Olfactory dysfunction (OD) is a frequent symptom of Parkinson's disease (PD) that appears years prior to diagnosis. Previous studies suggest that PD-related OD is different from non-parkinsonian forms of olfactory dysfunction (NPOD) as PD patients maintain trigeminal sensitivity as opposed to patients with NPOD who typically exhibit reduced trigeminal sensitivity. We hypothesize the presence of a specific alteration of functional connectivity between trigeminal and olfactory processing areas in PD.

OBJECTIVE

We aimed to assess potential differences in functional connectivity within the chemosensory network in 15 PD patients and compared them to 15 NPOD patients, and to 15 controls.

METHODS

Functional MRI scanning session included resting-state and task-related scans where participants carried out an olfactory and a trigeminal task. We compared functional connectivity, using a seed-based correlation approach, and brain network modularity of the chemosensory network.

RESULTS

PD patients had impaired functional connectivity within the chemosensory network while no such changes were observed for NPOD patients. No group differences we found in modularity of the identified networks. Both patient groups exhibited impaired connectivity when executing an olfactory task, while network modularity was significantly weaker for PD patients than both other groups. When performing a trigeminal task, no changes were found for PD patients, but NPOD patients exhibited impaired connectivity. Conversely, PD patients exhibited a significantly higher network modularity than both other groups.

CONCLUSION

In summary, the specific pattern of functional connectivity and chemosensory network recruitment in PD-related OD may explain distinct behavioral chemosensory features in PD when compared to NPOD patients and healthy controls.

Seeing Beyond Your Nose? The Effects of Lifelong Olfactory Sensory Deprivation on Cerebral Audio-visual Integration

Lifelong auditory and visual sensory deprivation have been demonstrated to alter both perceptual acuity and the neural processing of remaining senses. Recently, it was demonstrated that individuals with anosmia, i.e. complete olfactory sensory deprivation, displayed enhanced multisensory integration performance. Whether this ability is due to a reorganization of olfactory processing regions to focus on cross-modal multisensory information or whether it is due to enhanced processing within multisensory integration regions is not known. To dissociate these two outcomes, we investigated the neural processing of dynamic audio-visual stimuli in individuals with congenital anosmia and matched controls (both groups, n = 33) using functional magnetic resonance imaging. Specifically, we assessed whether the previously demonstrated multisensory enhancement is related to cross-modal processing of multisensory stimuli in olfactory associated regions, the piriform and olfactory orbitofrontal cortices, or enhanced multisensory processing in established multisensory integration regions, the superior temporal and intraparietal sulci. No significant group differences were found in the a priori hypothesized regions using region of interest analyses. However, exploratory whole-brain analysis suggested higher activation related to multisensory integration within the posterior superior temporal sulcus, in close proximity to the multisensory region of interest, in individuals with congenital anosmia. No group differences were demonstrated in olfactory associated regions. Although results were outside our hypothesized regions, combined, they tentatively suggest that enhanced processing of audio-visual stimuli in individuals with congenital anosmia may be mediated by multisensory, and not primary sensory, cerebral regions.

Olfactory-Trigeminal Interactions in Patients with Parkinson's Disease

Olfactory dysfunction (OD) is a highly frequent early non-motor symptom of Parkinson's disease (PD). An important step to potentially use OD for the development of early diagnostic tools of PD is to differentiate PD-related OD from other forms of non-parkinsonian OD (NPOD: postviral, sinunasal, post-traumatic, and idiopathic OD). Measuring non-olfactory chemosensory modalities, especially the trigeminal system, may allow to characterize a PD-specific olfactory profile. We here review the literature on PD-specific chemosensory alteration patterns compared with NPOD. Specifically, we focused on the impact of PD on the trigeminal system and particularly on the interaction between olfactory and trigeminal systems. As this interaction is seemingly affected in a disease-specific manner, we propose a model of interaction between both chemosensory systems that is distinct for PD-related OD and NPOD. These patterns of chemosensory impairment still need to be confirmed in prodromal PD; nevertheless, appropriate chemosensory tests may eventually help to develop diagnostic tools to identify individuals at risks for PD.

Evaluation of the Incidence of Other Cranial Neuropathies in Patients With Postviral Olfactory Loss

Importance

Postviral olfactory loss is a common cause of olfactory impairment, affecting both quality of life as well as overall patient mortality. It is currently unclear why some patients are able to recover fully after a loss while others experience permanent deficit. There is a lack of research on the possible association between postviral olfactory loss and other cranial neuropathies.

Objective

To evaluate the incidence of other cranial nerve deficits in patients with postviral olfactory loss and determine if there is an association with neurologic injury in this group. This study also sought to determine if other known risk factors were associated with postviral olfactory loss.

Design, Setting, and Participants

A case-control study was conducted at a tertiary care rhinology clinic from January 2015 to January 2018 to review the incidence of cranial neuropathies in 2 groups of patients, those with postviral olfactory loss and those with chronic rhinosinusitis without olfactory loss used as a control group.

Exposures

The Stanford Translational Research Integrated Database Environment (STRIDE) system was used for patient identification and data extraction. Patients with a history of olfactory loss or chronic rhinosinusitis as well as incidence of cranial neuropathies were identified by using International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) codes.

Main Outcomes and Measures

This study reviewed incidence of postviral or idiopathic cranial neuropathies in both patient groups, while also evaluating for any difference in demographic characteristics, comorbidities, or other patient-related factors.

Results

There were 91 patients in the postviral olfactory loss group and 100 patients in the control group, which were age and sex matched as closely as possible. Of the 91 patients with postviral olfactory loss, mean (SD) age was 56.8 (15.3), and 58 (64%) were women; for the control group, the mean (SD) age was 57.5 (15.6) years, and 63 (63%) were women. Racial breakdown was similar across cases and controls, with white individuals making up 59% to 65%; Asian individuals, 20% to 24%; black individuals, approximately 3%; Hispanic individuals, approximately 1%; and the remaining patients being of other race/ethnicity. The incidence of other cranial neuropathies in the postviral olfactory loss group was 11% compared with 2% within the control group (odds ratio, 6.1; 95% CI, 1.3-28.4). The study also found 2 cases of multiple cranial neuropathies within a single patient within the olfactory group. Family history of neurologic disease was associated with more than 2-fold greater odds of cranial nerve deficit (odds ratio, 3.05; 95% CI, 0.59-15.68).

Conclusions and Relevance

Postviral olfactory loss appears to be associated with a higher incidence of other cranial neuropathies. It is possible that there is an inherent vulnerability to nerve damage or decreased ability for nerve recovery in patients who experience this disease process.

Specific intranasal and central trigeminal electrophysiological responses in Parkinson's disease

Olfactory dysfunction is a frequent early non-motor symptom of Parkinson's disease (PD). There is evidence that with regard to trigeminal perception, PD-related olfactory dysfunction is different from other olfactory disorders. More specifically, trigeminal sensitivity, when measured behaviorally, was unimpaired in PD patients as opposed to patients with non-Parkinsonian olfactory dysfunction (NPOD). We sought to investigate the trigeminal pathway by measuring electrophysiological recordings from the nasal epithelium and EEG-derived event-related potentials in response to a specific trigeminal stimulus in 21 PD patients and compare them to 23 patients with NPOD and 25 controls (C). The peripheral trigeminal response, as measured by the negative-mucosa potential, showed no difference between patients with PD and controls whereas PD patients showed faster responses than patients with NPOD, the latter having shown slower and larger responses than controls (18 PD, 14 NPOD, 20 C). The central trigeminal response, as measured by event-related potentials, revealed larger early component response in PD patients compared to patients with NPOD (15 PD, 21 NPOD, 23 C). As expected, psychophysical olfactory testing showed impaired olfactory function in both groups of patients as opposed to controls. Discriminant analysis revealed a model that could predict group membership for 80% of participants based on the negative-mucosa potential latency, olfactory threshold and discrimination tests. These results provide novel insights into the pattern of trigeminal activation in PD which will help to differentiate PD-related olfactory loss from NPOD, a crucial step towards establishing early screening batteries for PD including smell tests.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Olfactory function in an excitotoxic model for secondary neuronal degeneration: Role of dopaminergic interneurons

Secondary neuronal degeneration (SND) occurring in Traumatic brain injury (TBI) consists in downstream destructive events affecting cells that were not or only marginally affected by the initial wound, further increasing the effects of the primary injury. Glutamate excitotoxicity is hypothesized to play an important role in SND. TBI is a common cause of olfactory dysfunction that may be spontaneous and partially recovered. The role of the glutamate excitotoxicity in the TBI-induced olfactory dysfunction is still unknown. We investigated the effects of excitotoxicity induced by bilateral N-Methyl-D-Aspartate (NMDA) OB administration in the olfactory function, OB volumes, and subventricular zone (SVZ) and OB neurogenesis in rats. NMDA OB administration induced a decrease in the number of correct choices in the olfactory discrimination tests one week after lesions (p<0.01), and a spontaneous recovery of the olfactory deficit two weeks after lesions (p<0.05). A lack of correlation between OB volumes and olfactory function was observed. An increase in SVZ neurogenesis (Ki67+ cells, PSANCAM+ cells (p<0.01) associated with an increase in OB glomerular dopaminergic immunostaining (p<0.05) were related to olfactory function recovery. The present results show that changes in OB volumes cannot explain the recovery of the olfactory function and suggest a relevant role for dopaminergic OB interneurons in the pathophysiology of recovery of loss of smell in TBI.

Olfactory Loss and Regain: Lessons for Neuroplasticity

For the visual and auditory senses, an array of studies has reported on neuronal reorganization processes after sensory loss. In contrast to this, neuroplasticity has been investigated only scarcely after loss of the olfactory sense. The present review focuses on the current extent of literature on structural and functional neuroplasticity effects after loss, with a focus on magnetic resonance imaging-based studies. We also include findings on the regain of the olfactory sense, for example after successful olfactory training. Existing studies indicate that widespread structural changes beyond the level of the olfactory bulb occur in the brain after loss of the olfactory sense. Moreover, on a functional level, loss of olfactory input not only entails changes in olfaction-related brain regions but also in the trigeminal system. Existing evidence should be strengthened by future longitudinal studies, a more thorough investigation of the neuronal consequences of congenital anosmia, and the application of state-of-the-art neuroimaging methods, such as connectivity analyses and joint analyses of brain structure and function.

Odor-Specific Loss of Smell Sensitivity with Age as Revealed by the Specific Sensitivity Test

The perception of odor mixtures plays an important role in human food intake, behavior, and emotions. Decline of smell acuity with normal aging could impact food perception and preferences at various ages. However, since the landmark Smell Survey by National Geographic, little has been elucidated on differences in the onset and extent of loss in olfactory sensitivity toward single odorants. Here, using the Specific Sensitivity test, we show the onset and extent of loss in both identification and detection thresholds of odorants with age are odorant-specific. Subjects of Chinese descent in Singapore (186 women, 95 men), aged 21-80 years, were assessed for olfactory sensitivity of 10 odorants from various odor groups. Notably, subjects in their 70s required 179 times concentration of rose-like odorant (2-phenylethanol) than subjects in the 20s, while thresholds for onion-like 2-methyloxolane-3-thiol only differed by 3 times between the age groups. In addition, identification rate for 2-phenylethanol was negatively correlated with age throughout adult life whereas mushroom-like oct-1-en-3-ol was equally identified by subjects across all ages. Our results demonstrated the girth of differentiated olfactory loss due to normal ageing, which potentially affect overall perception and preferences of odor mixtures with age.

Olfactory training induces changes in regional functional connectivity in patients with long-term smell loss

Recently, olfactory training has been introduced as a promising treatment for patients with olfactory dysfunction. However, less is known about the neuronal basis and the influence on functional networks of this training. Thus, we aimed to investigate the neuroplasticity of chemosensory perception through an olfactory training program in patients with smell loss. The experimental setup included functional MRI (fMRI) experiments with three different types of chemosensory stimuli. Ten anosmic patients (7f, 3m) and 14 healthy controls (7f, 7m) underwent the same testing sessions. After a 12-week olfactory training period, seven patients (4f, 3m) were invited for follow-up testing using the same fMRI protocol. Functional networks were identified using independent component analysis and were further examined in detail using functional connectivity analysis. We found that anosmic patients and healthy controls initially use the same three networks to process chemosensory input: the olfactory; the somatosensory; and the integrative network. Those networks did not differ between the two groups in their spatial extent, but in their functional connectivity. After the olfactory training, the sensitivity to detect odors significantly increased in the anosmic group, which was also manifested in modifications of functional connections in all three investigated networks. The results of this study indicate that an olfactory training program can reorganize functional networks, although, initially, no differences in the spatial distribution of neural activation were observed.

Making sense of the chemical senses

We review our recent behavioural and imaging studies testing the consequences of congenital blindness on the chemical senses in comparison with the condition of anosmia. We found that congenitally blind (CB) subjects have increased sensitivity for orthonasal odorants and recruit their visually deprived occipital cortex to process orthonasal olfactory stimuli. In sharp contrast, CB perform less well than sighted controls in taste and retronasal olfaction, i.e. when processing chemicals inside the mouth. Interestingly, CB do not recruit their occipital cortex to process taste stimuli. In contrast to these findings in blindness, congenital anosmia is associated with lower taste and trigeminal sensitivity, accompanied by weaker activations within the 'flavour network' upon exposure to such stimuli. We conclude that functional adaptations to congenital anosmia or blindness are quite distinct, such that CB can train their exteroceptive chemical senses and recruit normally visual cortical areas to process chemical information from the surrounding environment.

Same same but different. Different trigeminal chemoreceptors share the same central pathway

Intranasal trigeminal sensations are important in everyday life of human beings, as they play a governing role in protecting the airways from harm. Trigeminal sensations arise from the binding of a ligand to various sub-types of transient receptor potential (TRP) channels located on mucosal branches of the trigeminal nerve. Which underlying neural networks are involved in the processing of various trigeminal inputs is still unknown. To target this unresolved question fourteen healthy human subjects were investigated by completing three functional magnetic resonance imaging (fMRI) scanning sessions during which three trigeminal substances, activating varying sub-types of chemoreceptors and evoking different sensations in the nose were presented: CO2, menthol and cinnamaldehyde. We identified similar functional networks responding to all stimuli: an olfactory network, a somatosensory network and an integrative network. The processing pathway of all three stimulants was represented by the same functional networks, although CO2 evokes painful but virtually odorless sensations, and the two other stimulants, menthol and cinnamaldehyde are perceived as mostly non painful with a clear olfactory percept. Therefore, our results suggest a common central processing pathway for trigeminal information regardless of the trigeminal chemoreceptor and sensation type.

Subjective changes in nasal patency after chewing a menthol-containing gum in patients with olfactory loss

CONCLUSION

The subjective increase in nasal patency after oral menthol application depends on olfactory function. In patients with relatively low olfactory sensitivity, this effect is small or absent. It was hypothesized that this may also modulate the sensation of nasal blockage.

OBJECTIVE

Menthol stimulates the nasal trigeminal nerve, resulting in a subjectively increased nasal airflow and patency without objectively measurable changes. Patients suffering from olfactory impairment also express a reduced nasal trigeminal sensitivity. The aim of this investigation was to examine the effect of menthol on nasal patency ratings in a group of patients suffering from olfactory dysfunction.

METHODS

A group of 92 patients with olfactory impairment was included in this study. Olfactory function was assessed using the 'Sniffin' Sticks.' Patients were instructed to chew a menthol-containing gum for 30 s, after which they rated the change in nasal patency on a categorical scale.

RESULTS

Patients who rated the change in nasal patency as pronounced scored higher on the Sniffin' Sticks olfactory test compared to patients who experienced little or no change in nasal patency after chewing the menthol-containing gum. On average, similar changes could be observed for all etiologies of olfactory impairment included in this study.

Neural correlates of taste perception in congenital olfactory impairment

Olfaction and gustation contribute both to the appreciation of food flavours. Although acquired loss of smell has profound consequences on the pleasure of eating, food habits and body weight, less is known about the impact of congenital olfactory impairment on gustatory processing. Here we examined taste identification accuracy and its neural correlates using functional magnetic resonance imaging (fMRI) in 12 congenitally olfactory impaired individuals and 8 normosmic controls. Results showed that taste identification was worse in congenitally olfactory impaired compared to control subjects. The fMRI results demonstrated that olfactory impaired individuals had reduced activation in medial orbitofrontal cortex (mOFC) relative to normosmic subjects while tasting. In addition, olfactory performance as measured with the Sniffin' Sticks correlated positively with taste-induced blood-oxygen-level dependent (BOLD) signal increases in bilateral mOFC and anterior insula. Our data provide a neurological underpinning for the reduced taste perception in congenitally olfactory impaired individuals.

Memories evoked by odors stimulating the olfactory nerve versus odors stimulating both the olfactory and trigeminal nerves: possible qualitative differences?

The purpose of the study was to establish whether autobiographical memories differ when a stimulus producing olfactory or/and trigeminal sensations was used as memory cue. The following hypothesis was formulated: memories evoked by odors activating the trigeminal and olfactory nerves would be subjectively assessed as more detailed, more clear, more important and less happy, as compared to memories evoked by odors activating exclusively the olfactory nerve. The hypothesis was based on the assumption that trigeminal odors are perceived as signaling potential threats for the organism. 30 Polish psychology students (M age = 22 yr.; 20 women, 10 men) were tested using six odors: three stimulating the olfactory nerve only and three stimulating both the olfactory and trigeminal nerves. Participants were asked whether a particular odor evoked any memories, and if they answered "yes," they were to respond to four questions related to the qualities of the memory. Ratings of memories evoked by odors that stimulated the olfactory nerve and those that stimulated both the olfactory and trigeminal nerves differed in clarity. Odors stimulating the trigeminal nerve may induce less happy memories. The results are promising as to the role of the trigeminal system in coding and retrieval of survival-related memories.

Intranasal trigeminal chemosensitivity in patients with postviral and post-traumatic olfactory dysfunction

CONCLUSIONS

The olfactory and intranasal trigeminal systems are closely connected. With regard to intranasal trigeminal event-related potential (ERP), patients with olfactory dysfunction (OD) showed longer latency and lower amplitude, which indicated decreased trigeminal sensitivity. Different age, etiology, and olfactory status also affect trigeminal sensitivity differently.

OBJECTIVE

OD is a common symptom in the rhinology clinic. ERP is considered an important method to evaluate chemosensitivity. The aim of this study was to investigate changes of intranasal trigeminal sense in patients with postviral OD (PVOD) and post-traumatic OD (PTOD).

METHODS

A total of 96 participants (30 healthy adults and 66 patients with OD) aged 20-65 years were investigated. The T&T olfactometer, the Sniffin' Sticks olfactory test, and trigeminal ERPs (tERPs) were used. We evaluated trigeminal sensitivity influenced by different factors (age, etiology, and olfactory status) in healthy subjects with normal olfactory function and in patients with PVOD and PTOD.

RESULTS

Patients with OD showed higher trigeminal thresholds than normal controls. Compared with controls, N1/P2 latencies of tERPs increased and amplitudes decreased in patients with OD. Older subjects showed longer latencies and lower amplitudes than young subjects in both controls and the OD group. Patients with PTOD exhibited worse psychophysical olfactory function and decreased trigeminal sensitivity.

Examination of chemosensory functions in patients with dysosmia

Background

To examine changes of chemical sensory functions in patients with dysosmia.

Material/Methods

The 272 study subjects included 98 healthy volunteers, 86 subjects with hyposmia and 88 subjects with functional anosmia. Their chemical sensory functions were examined using olfactory event-related potentials (oERPs), trigeminal event-related potentials (tERPs), T&T olfactometer and triple drop method, respectively.

Results

The T&T results showed that the difference between patients and healthy subjects had statistical significance. The oERPs and tERPs results showed that patients with functional anosmia had N1 and P2 waves of prolonged latency and reduced amplitude when compared to healthy subjects with the difference of statistical significance. When compared to healthy subjects, patients with functional anosmia had clear hypogeusia and the difference had statistical significance. For the younger group there was significant difference between healthy subjects and patients in T&T, oERPs and tERPs results.

Conclusions

It is suggested by the apparently concomitant trigeminal nerve dysfunction and hypogeusia in patients with functional anosmia in this study that olfactory and nasal trigeminal function in young patients was clearly decreased. Our study suggests the possible application of oERPs, tERPs and three drops method in clinical diagnosis in Chinese populations and provides scientific evidence for treatment.

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.

Crossmodal plasticity in sensory loss

In this review, we describe crossmodal plasticity following sensory loss in three parts, with each section focusing on one sensory system. We summarize a wide range of studies showing that sensory loss may lead, depending of the affected sensory system, to functional changes in other, primarily not affected senses, which range from heightened to lowered abilities. In the first part, the effects of blindness on mainly audition and touch are described. The latest findings on brain reorganization in blindness are reported, with a particular emphasis on imaging studies illustrating how nonvisual inputs recruit the visually deafferented occipital cortex. The second part covers crossmodal processing in deafness, with a special focus on the effects of deafness on visual processing. In the last portion of this review, we present the effects that the loss of a chemical sense have on the sensitivity of the other chemical senses, that is, smell, taste, and trigeminal chemosensation. We outline how the convergence of the chemical senses to the same central processing areas may lead to the observed reduction in sensitivity of the primarily not affected senses. Altogether, the studies reviewed herein illustrate the fascinating plasticity of the brain when coping with sensory deprivation.

Dynamics of nasal irritation from pulsed homologous alcohols

Relatively, few studies have focused on how nasal irritation changes over time. To simulate the rhythm of natural respiration, subjects received 3-s pulses of volatile organic compounds interspersed with 3-s pulses of clean air. Each trial, subjects received 9 pulses of a chemical vapor over about 1 min. Subjects rated nasal irritation from each pulse using magnitude estimation. Within a trial, compound and concentration were fixed. Compound (ethanol, n-butanol, or n-hexanol) and concentration (4 levels for each compound) varied across trials. For all stimuli, rated irritation decreased over time (adaptation). Plots of log-rated intensity versus elapsed time were approximately linear (intensity decreased by a fixed ratio per unit time). Interestingly, the slopes of intensity versus time functions differed very little: Regardless of concentration and compound, rated irritation decreased by about 32% over the 9 pulses. The basic mechanism of short-term adaptation may be the same for the 3 alcohols studied. Regardless, these data suggest that very simple models might be able to describe some aspects of perceptual dynamics quite well.