Brain imaging studies of the functional organization of human olfaction

Tempo-Spectral EEG Biomarkers for Odour Identification

Different odours evoke different activity in the brain. Among the non-invasive methods, electroencephalogram (EEG) is the most widely used mode to measure brain activity. While there has been significant work around EEG signal analysis, studies in the area of EEG with odour as stimuli is nascent. In this paper, we experiment and study different EEG biomarkers with an aim to understand which biomarker shows promise for odour identification. We show, on a widely used and publicly available data-set, through a series of experiments that it is possible to get a Subject Dependent (SD) odour classification accuracy of over 90%, using a set of tempo-spectral EEG biomarkers. We further experiment with Subject Independent (SI) odour classification, which has not been addressed and show that the performance drops to under 50% for SI odour classification.Clinical Relevance - The study shows that the same odour evoke different brain responses from the subject.

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.

The diagnostic value of the olfactory evaluation for congenital hypogonadotropic hypogonadism

OBJECTIVE

The aim of this study was to evaluate the diagnostic accuracy of different olfactory evaluation tools in congenital hypogonadotropic hypogonadism (CHH) patients.

METHODS

Seventy-one CHH patients were prospectively recruited at Peking Union Medical College Hospital between November 2020 and July 2021. The Chinese Olfactory Function Test (COFT) and Self-reported Olfactory Scale (SROS) were adapted as the subjective tools for the evaluation of olfactory function, and magnetic resonance imaging of olfactory apparatus (MRI-OA) was the objective tool. The olfactory bulb volume (OBV) and the olfactory sulcus depth (OSD) were quantified.

RESULTS

Based on the COFT, 36 patients were categorized as having normosmic CHH (nCHH), and the other 35 patients were categorized as having Kallmann syndrome (KS). Among nCHH patients, 35 patients were classified as having normal olfaction and 1 patient had abnormal olfaction by SROS. For KS patients, there were 30 patients grouped into abnormal olfaction, while 5 patients had normal olfaction by SROS. For MRI-OA, 67% (18/27) of nCHH patients showed normal olfactory apparatus, and 33% (9/27) showed bilateral or unilateral olfactory bulb aplasia or hypoplasia. Among KS patients, 96% (27/28) of patients showed bilateral olfactory bulb hypoplasia or aplasia, and 4% (1/28) of patients showed normal olfactory apparatus. All six patients with unilateral olfactory bulb aplasia and three patients with bilateral olfactory bulb aplasia showed normal olfactory function. The accuracy of the SROS in the diagnosis of nCHH and KS was 91.5%, with a sensitivity of 0.857 and a specificity of 0.972, while the accuracy of MRI-OA is 92.7%, with a sensitivity of 0.964 and a specificity of 0.889.

CONCLUSION

SROS and MRI-OA both showed high accuracy to distinguish between KS and nCHH. The abnormal structure of the olfactory apparatus was relatively common in nCHH patients. CHH patients with unilateral olfactory bulb aplasia dysplasia usually had normal olfaction. Normal olfaction without apparent olfactory bulbs is rare but occurred in male CHH patients.

Brain imaging in patients with COVID-19: A systematic review

Background

There is increasing evidence that SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) not only affects the respiratory tract but also influence the central nervous system (CNS), resulting in neurological symptoms such as loss of smell and taste. Growing literature indicates largely distributed brain alterations encompassing subcortical micro- and macro-bleeds, cerebral swelling and haemorrhage in gray and white matter tissue. A systematic review was performed to synthesise the potential evidence of the brain correlates of SARS-CoV-2.

Methods

A literature search was conducted using electronic databases for studies reporting neuroimaging abnormalities in SARS-CoV-2 infected individuals. Identified case series, cohort studies, and case control studies on SARS-CoV-2 effects on the brain were critically appraised for methodological quality. A narrative synthesis of the findings from the included studies is presented.

Results

Twenty-seven studies were included in the review, including 5 case series, 8 cohort studies and 14 case control studies. The findings revealed predominant involvement of the olfactory system with disruptions across four olfactory structures. Abnormalities also extended to the corpus callosum, cingulate cortex, and insula, jointly implicating the olfactory brain network.

Conclusion

Alterations in olfactory areas, along with neighbouring brain regions, including prefrontal and limbic regions were associated to contraction of SARS-CoV-2. Viral infection could either trigger systemic reactions, or use the olfactory's unique anatomical organisation as an environmental entry zone to directly impact on the CNS.

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A spectrum of brain changes particularly at advanced stage relates to SARS-CoV-2

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Across severity phases SARS-CoV-2 affects olfactory structures

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Emerging abnormalities in COVID-19 is associated with the olfactory brain network

Structural and metabolic brain abnormalities in COVID-19 patients with sudden loss of smell

OBJECTIVES

Sudden loss of smell is a very common symptom of coronavirus disease 19 (COVID-19). This study characterizes the structural and metabolic cerebral correlates of dysosmia in patients with COVID-19.

METHODS

Structural brain magnetic resonance imaging (MRI) and positron emission tomography with [18F]-fluorodeoxyglucose (FDG-PET) were prospectively acquired simultaneously on a hybrid PET-MR in 12 patients (2 males, 10 females, mean age: 42.6 years, age range: 23-60 years) with sudden dysosmia and positive detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on nasopharyngeal swab specimens. FDG-PET data were analyzed using a voxel-based approach and compared with that of a group of healthy subjects.

RESULTS

Bilateral blocking of the olfactory cleft was observed in six patients, while subtle olfactory bulb asymmetry was found in three patients. No MRI signal abnormality downstream of the olfactory tract was observed. Decrease or increase in glucose metabolism abnormalities was observed (p < .001 uncorrected, k ≥ 50 voxels) in core olfactory and high-order neocortical areas. A modulation of regional cerebral glucose metabolism by the severity and the duration of COVID-19-related dysosmia was disclosed using correlation analyses.

CONCLUSIONS

This PET-MR study suggests that sudden loss of smell in COVID-19 is not related to central involvement due to SARS-CoV-2 neuroinvasiveness. Loss of smell is associated with subtle cerebral metabolic changes in core olfactory and high-order cortical areas likely related to combined processes of deafferentation and active functional reorganization secondary to the lack of olfactory stimulation.

Cerebellar involvement in olfaction: An fMRI Study

BACKGROUND AND PURPOSE

The role of the cerebellum in olfactory function is not fully understood. In this study, we tried to combine resting state and task functional MRI (fMRI) to improve the understanding of the cerebellum during olfactory processing.

METHODS

A resting state and a block paradigm of olfactory stimulation fMRI were scanned in 50 subjects. The olfactory stimuli, including phenylethyl alcohol and isovaleric acid, were alternately delivered to the subject using a custom-built olfactometer through air flow. The cerebellar activations elicited by isovaleric acid were subsequently used in the seed-based resting-state functional connectivity study.

RESULTS

Phenylethyl alcohol did not induce any cerebellum activation, while isovaleric acid with a more unpleasant smell elicited significant cerebellum activations, primarily in the bilateral posterior lateral hemispheres (bilateral lobule crus I and right lobule VI). Seed-based functional connectivity analysis revealed significant within-cerebellum and corticocerebellar connections.

CONCLUSIONS

The results imply that the cerebellum is probably involved in olfactory-related responses caused by unpleasant odor but does not directly participate in olfactory perception. Our results may further improve the understanding of the cerebellum in olfactory function.

Olfaction as a Marker for Dystonia: Background, Current State and Directions

Dystonia is a heterogeneous group of hyperkinetic movement disorders. The unifying descriptor of dystonia is the motor manifestation, characterized by continuous or intermittent contractions of muscles that cause abnormal movements and postures. Additionally, there are psychiatric, cognitive, and sensory alterations that are possible or putative non-motor manifestations of dystonia. The pathophysiology of dystonia is incompletely understood. A better understanding of dystonia pathophysiology is highly relevant in the amelioration of significant disability associated with motor and non-motor manifestations of dystonia. Recently, diminished olfaction was found to be a potential non-motor manifestation that may worsen the situation of subjects with dystonia. Yet, this finding may also shed light into dystonia pathophysiology and yield novel treatment options. This article aims to provide background information on dystonia and the current understanding of its pathophysiology, including the key structures involved, namely, the basal ganglia, cerebellum, and sensorimotor cortex. Additionally, involvement of these structures in the chemical senses are reviewed to provide an overview on how olfactory (and gustatory) deficits may occur in dystonia. Finally, we describe the present findings on altered chemical senses in dystonia and discuss directions of research on olfactory dysfunction as a marker in dystonia.

Smell and taste in cervical dystonia

The pathophysiology of cervical dystonia is not completely understood. Current concepts of the pathophysiology propose that it is a network disorder involving the basal ganglia, cerebellum and sensorimotor cortex. These structures are primarily concerned with sensorimotor control but are also involved in non-motor functioning such as the processing of information related to the chemical senses. This overlap lets us hypothesize a link between cervical dystonia and altered sense of smell and taste. To prove this hypothesis and to contribute to the better understanding of cervical dystonia, we assessed olfactory and gustatory functioning in 40 adults with idiopathic cervical dystonia and 40 healthy controls. The Sniffin Sticks were used to assess odor threshold, discrimination and identification. Furthermore, the Taste Strips were applied to assess the combined taste score. Motor and non-motor deficits of cervical dystonia including neuropsychological and psychiatric alterations were assessed as cofactors for regression analyses. We found that cervical dystonia subjects had lower scores than healthy controls for odor threshold (5.8 ± 2.4 versus 8.0 ± 3.2; p = 0.001), odor identification (11.7 ± 2.3 versus 13.1 ± 1.3; p = 0.001) and the combined taste score (9.5 ± 2.2 versus 11.7 ± 2.7; p < 0.001), while no difference was found in odor discrimination (12.0 ± 2.5 versus 12.9 ± 1.8; p = 0.097). Regression analysis suggests that age is the main predictor for olfactory decline in subjects with cervical dystonia. Moreover, performance in the Montreal Cognitive Assessment is a predictor for gustatory decline in cervical dystonia subjects. Findings propose that cervical dystonia is associated with diminished olfactory and gustatory functioning.

A Masked Aversive Odor Cannot Be Discriminated From the Masking Odor but Can Be Identified Through Odor Quality Ratings and Neural Activation Patterns

Odor masking is a very prominent problem in our daily routines, mainly concerning unpleasant sweat or toilet odors. In the current study we explored the effectiveness of odor masking both on a behavioral and neuronal level. By definition, participants cannot differentiate a fully masked unpleasant odor from the pleasant pure odor used as a masking agent on a behavioral level. We hypothesized, however, that one can still discriminate between a fully masked odor mixture and the pure masking odor on a neuronal level and that, using a reinforcing feedback paradigm, participants could be trained to perceive this difference. A pleasant, lemon-like odor (citral) and a mixture of citral and minor amounts of an unpleasant, goat-like odor (caproic acid) were presented to participants repeatedly using a computer-controlled olfactometer and participants had to decide whether two presented stimuli were the same or different. Accuracy of this task was incentivized with a possible monetary reward. Functional imaging was used throughout the task to investigate central processing of the two stimuli. The participants rated both stimuli as isopleasant and isointense, indicating that the unpleasant odor was fully masked by the pleasant odor. The isolated caproic acid component of the mixture was rated less pleasant than the pleasant odor in a prior experimental session. Although the masked and pure stimuli were not discriminated in the forced-choice task, quality ratings on a dimensional scale differed. Further, we observed an increased activation of the insula and ventral striatum/putamen for the pure in contrast to the fully masked odor, hence revealing a difference in neuronal processing. Our hypothesis that perceptual discrimination and neuronal processing can be enhanced using a reinforcing feedback paradigm is not supported by our data.

Olfactory Function in SCA10

Although the main clinical manifestations of spinocerebellar ataxias (SCAs) result from damage of the cerebellum, other systems may also be involved. Olfactory deficits have been reported in other types of ataxias, especially in SCA3; however, there are no studies on olfactory deficits in SCA type 10 (SCA10). To analyze olfactory function of SCA10 patients compared with that of SCA3, Parkinson's, and healthy controls. Olfactory identification was tested in three groups of 30 patients (SCA10, SCA3, and Parkinson's disease (PD)) and 44 healthy controls using the Sniffin' Sticks (SS16) test. Mean SS16 score was 11.9 ± 2.9 for the SCA10 group, 12.3 ± 1.9 for the SCA3 group, 6.6 ± 2.8 for the PD group, and 12.1 ± 2.0 for the control group. Mean SS16 score for the SCA10 group was not significantly different from the scores for the SCA3 and control groups but was significantly higher than the score for the PD group (p < 0.001) when adjusted for age, gender, and history of smoking. There was no association between SS16 scores and disease duration in the SCA10 or SCA3 groups or number of repeat expansions. SS16 and Mini Mental State Examination scores were correlated in the three groups: SCA10 group (r = 0.59, p = 0.001), SCA3 group (r = 0.50, p = 0.005), and control group (r = 0.40, p = 0.007). We found no significant olfactory deficits in SCA10 in this large series.

Olfactory Testing in Frontotemporal Dementia: A Literature Review

Frontotemporal dementia (FTD) is a heterogeneous disorder featuring language impairment, personality changes, and executive defects, often due to the frontotemporal lobar degeneration (FTLD). Both FTD and FTLD are often associated with olfactory impairment, early biomarker for neurodegeneration, which can be evaluated with different techniques, among which low-cost olfactory tests are widely used. Therefore, we conducted a review of the literature focusing on papers published between January 1, 2007, and June 12, 2017, investigating the usefulness of olfactory testing in FTD/FTLD. A general decrease in the olfactory identification ability was seen in most of the articles and, taken together with a preserved odor discrimination, reveals a higher order impairment, possibly linked to cognitive decrease or language impairments, and not to a specific deficit of the olfactory system. This evidence could represent a useful add-on to the current literature, increasing the diagnostic value of olfactory assessment, particularly in cases where differential diagnosis is difficult.

High prevalence of olfactory dysfunction in cervical dystonia

INTRODUCTION

Olfactory dysfunction has been established as a frequent non-motor symptom in neurodegenerative and movement disorders such as Parkinson's disease, Alzheimer's disease, and hereditary ataxias. To expand knowledge of non-motor symptoms in dystonia, and to test for a potential endophenotype, we examined olfactory function in cervical dystonia (CD).

METHODS

In patients with CD, and neurologically healthy controls, olfactory function was examined by "Sniffin' Sticks", a test of nasal chemosensory function based on pen-like odor dispensing devices. This test enables to define an individual's odor threshold, odor discrimination, and odor identification. Owing to the etiological heterogeneity of olfactory dysfunction, strict exclusion criteria were applied, especially smoking, and sinonasal disease.

RESULTS

58 CD patients completed the study. Olfactory dysfunction was present in 29 patients (50.0%), significantly more frequent than in two groups of matched healthy control subjects (20.7%; 22.4%; p = 0.001). Analysis of the pattern of hyposmia revealed that odor threshold (p = 0.002), and odor identification (p < 0.001) were significantly worse in CD patients compared to controls, while odor discrimination was unchanged. Higher age was the only clinical characteristic to correlate with olfactory dysfunction in CD.

CONCLUSIONS

Our observations establish olfactory dysfunction, possibly of both peripheral and central origin, as a new non-motor, and probably motor-unrelated, symptom of CD. Additionally, the potential involvement of cerebellar functions in olfactory identification and discrimination tasks, as well as in pathophysiology of dystonia, justifies further studies of olfactory dysfunction as a possible endophenotype in dystonia.

Plasticity in the Olfactory System: Lessons for the Neurobiology of Memory

We are rapidly advancing toward an understanding of the molecular events underlying odor transduction, mechanisms of spatiotemporal central odor processing, and neural correlates of olfactory perception and cognition. A thread running through each of these broad components that define olfaction appears to be their dynamic nature. How odors are processed, at both the behavioral and neural level, is heavily dependent on past experience, current environmental context, and internal state. The neural plasticity that allows this dynamic processing is expressed nearly ubiquitously in the olfactory pathway, from olfactory receptor neurons to the higher-order cortex, and includes mechanisms ranging from changes in membrane excitability to changes in synaptic efficacy to neurogenesis and apoptosis. This review will describe recent findings regarding plasticity in the mammalian olfactory system that are believed to have general relevance for understanding the neurobiology of memory.

Human amygdala activations during nasal chemoreception

This review serves as a comprehensive discussion of chemosensory stimulation of the amygdala in healthy humans. Following an introduction of the neuroanatomy of chemosensory processing in primary and secondary olfactory structures, functional resonance magnetic imaging and positron imaging tomography studies are systematically categorized based on valence of stimuli, stimulus concentration, and paradigm-dependent amygdala activation. The amygdala shows patterns of lateralization due to stimulus valence. Main findings include pleasant odors being associated with bilateral or left amygdala activation, and unpleasant odors being associated with activation of the right amygdala, suggesting a crucial role of the right amygdala in evolutionary preservation. Potentially threatening social stimuli, however, might be processed apart from the olfactory system and tend to activate the left amygdala. Amygdala response to chemosensory stimuli correlated with simultaneous activation in the orbitofrontal cortex (OFC), piriform cortex (PC), and insula, suggesting a close-knit network of these areas during stimulus processing.

Comparative study of perception and processing of socially or sexually significant odor information in male rats with normal or accelerated senescence using fMRI

Olfaction plays an important role in mammals while aging causes olfactory dysfunction. Here the features of olfactory function in aging male rats were studied. We compared brain activity of regions involved in the perception (olfactory bulbs) and processing (cerebral cortex, hippocampus, hypothalamus) of sexually or socially significant odor stimulus with 11.7 T MR-scanner and odor perception using behavioral tests in 5-month old males with normal (Wistar rats) or accelerated senescence (d-galactose-treated Wistar rats (150 mg/kg/day, i.p., 12 weeks) or OXYS rats with hereditary defined accelerated aging). d-galactose-treated Wistar males had altered BOLD-response in the centers processing socially significant odor information and changed patterns of the functional connectivity. We detected no significant changes in the olfactory function of OXYS males probably due to compensatory processes. In saline-treated Wistar rats, the correlation of BOLD-responses to both types of stimuli in the olfactory bulbs and cerebral cortex indicated changes in odor differentiation. Behavioral tests showed no significant differences between groups. However, the time of odor exploration increased in d-galactose-treated males indicating changes in odor recognition. Thus, we first revealed that in animal model of pharmacologically induced aging olfactory dysfunction occurred at the level of the centers processing socially significant odor information while the centers of odor perception (olfactory bulbs) remained unaffected. Alterations observed in Wistar rats chronically treated with saline evidenced the influence of long-term manipulations with experimental animals on olfactory function per se.

Cortical activity during olfactory stimulation in multiple chemical sensitivity: a (18)F-FDG PET/CT study

PURPOSE

To investigate the differences in brain glucose consumption during olfactory stimulation between subjects affected by multiple chemical sensitivity (MCS) and a group of healthy individuals.

METHODS

Two (18)F-FDG PET/CT scans were performed in 26 subjects (6 men and 20 women; mean age 46.7 ± 11 years) with a clinical diagnosis of MCS and in 11 healthy controls (6 women and 5 men; mean age 45.7 ± 11 years), the first scan after a neutral olfactory stimulation (NS) and the second after a pure olfactory stimulation (OS). Differences in (18)F-FDG uptake were analysed by statistical parametric mapping (SPM2).

RESULTS

In controls OS led to an increase in glucose consumption in BA 18 and 19 and a reduction in glucose metabolism in BA 10, 11, 32 and 47. In MCS subjects, OS led to an increase in glucose consumption in BA 20, 23, 18 and 37 and a reduction in glucose metabolism in BA 8, 9 and 10.

CONCLUSION

The results of our study suggest that cortical activity in subjects with MCS differs from that in healthy individuals during olfactory stimulation.

An fMRI study on the influence of sommeliers' expertise on the integration of flavor

Flavors guide consumers' choice of foodstuffs, preferring those that they like and meet their needs, and dismissing those for which they have a conditioned aversion. Flavor affects the learning and consumption of foods and drinks; what is already well-known is favored and what is new is apprehended. The flavor of foodstuffs is also crucial in explaining some eating behaviors such as overconsumption. The "blind" taste test of wine is a good model for assessing the ability of people to convert mouth feelings into flavor. To determine the relative importance of memory and sensory capabilities, we present the results of an fMRI neuro-imaging study involving 10 experts and 10 matched control subjects using wine as a stimulus in a blind taste test, focusing primarily on the assessment of flavor integration. The results revealed activations in the brain areas involved in sensory integration, both in experts and control subjects (insula, frontal operculum, orbitofrontal cortex, amygdala). However, experts were mainly characterized by a more immediate and targeted sensory reaction to wine stimulation with an economic mechanism reducing effort than control subjects. Wine experts showed brainstem and left-hemispheric activations in the hippocampal and parahippocampal formations and the temporal pole, whereas control subjects showed activations in different associative cortices, predominantly in the right hemisphere. These results also confirm that wine experts work simultaneously on sensory quality assessment and on label recognition of wine.

Olfactory impairment in familial ataxias

The main clinical manifestations of the spinocerebellar ataxias (SCAs) result from the involvement of the cerebellum and its connections. Cerebellar activity has been consistently observed in functional imaging studies of olfaction, but the anatomical pathways responsible for this connection have not yet been elucidated. Previous studies have demonstrated olfactory deficit in SCA2, Friedreich's ataxia and in small groups of ataxia of diverse aetiology. The authors used a validated version of the 16-item smell identification test from Sniffin' Sticks (SS-16) was used to evaluate 37 patients with genetically determined autosomal dominant ataxia, and 31 with familial ataxia of unknown genetic basis. This data was also compared with results in 106 Parkinson's disease patients and 218 healthy controls. The SS-16 score was significantly lower in ataxia than in the control group (p<0.001, 95% CI for β=0.55 to 1.90) and significantly higher in ataxia than in Parkinson's disease (p<0.001, 95% CI for β=-4.58 to -3.00) when adjusted for age (p=0.001, 95% CI for β=-0.05 to -0.01), gender (p=0.19) and history of tobacco use (p=0.41). When adjusted for general cognitive function, no significant difference was found between the ataxia and control groups. This study confirms previous findings of mild hyposmia in ataxia, and further suggests this may be due to general cognitive deficits rather than specific olfactory problems.

Time course of odorant- and trigeminal-induced activation in the human brain: an event-related functional magnetic resonance imaging study

It is well known that most odorants stimulate the trigeminal system but the time course of the brain regions activated by these chemical stimulations remains poorly documented, especially regarding the trigeminal system. This functional magnetic resonance imaging (fMRI) study compares brain activations resulting from the contrast between two odorant conditions (one bimodal odor and one relatively pure olfactory stimulant) according to the duration of the stimulation (i.e. one inhalation, or three or six successive inhalations). The results show striking differences in the main brain regions activated according to these durations. The caudate nucleus and the orbitofrontal cortex are only involved in short-duration stimulations, and the posterior insular cortex and post-central gyrus (SI) are only activated by long duration stimulations. Different regions of the frontal, temporal and occipital lobe are activated depending on the duration but mainly during medium-duration stimulations. These results expand on the findings of previous studies and contribute to the description of temporal networks in trigeminal perception.

Odor perception in alexithymic patients

Alexithymia is characterized by a marked inability to identify feelings and emotional states and some studies have documented sensorial perception in response to visual or auditory cues in this disease. Although olfaction is well known for its emotional correlates, the perception of olfactory stimulations has not been previously investigated. This study compares with standard psychophysical methods the olfactory sensitivity and the self-ratings of intensity and hedonic valence of a panel of odorants in alexithymic patients, non-alexithymic patients and control subjects. Results show that alexithymics over-evaluate intensity and pleasantness of odorants compared to non-alexithymics or control subjects. This could be interpreted in the framework of a lack of inhibitory control including this particular sense.

A new trigemino-nociceptive stimulation model for event-related fMRI

Functional imaging of human trigemino-nociceptive processing provides meaningful insights into altered pain processing in head and face pain diseases. Although functional magnetic resonance imaging (fMRI) offers high temporal and spatial resolution, most studies available were done with radioligand-positron emission tomography, as fMRI requires non-magnetic stimulus equipment and fast on–off conditions. We developed a new approach for painful stimulation of the trigeminal nerve that can be implemented within an event-related design using fMRI and aimed to detect increased blood-oxygen-level-dependent (BOLD) signals as surrogate markers of trigeminal pain processing. Using an olfactometer, 20 healthy volunteers received intranasally standardized trigeminal nociceptive stimuli (ammonia gas) as well as olfactory (rose odour) and odourless control stimuli (air puffs). Imaging revealed robust BOLD responses to the trigeminal nociceptive stimulation in cortical and subcortical brain areas known to be involved in pain processing. Focusing on the trigeminal pain pathway, significant activations were observed bilaterally in brainstem areas at the trigeminal nerve entry zone, which are agreeable with the principal trigeminal nuclei. Furthermore, increased signal changes could be detected ipsilaterally at anatomical localization of the trigeminal ganglion and bilaterally in the rostral medulla, which probably represents the spinal trigeminal nuclei. However, brainstem areas involved in the endogenous pain control system that are close to this anatomical localization, such as raphe nuclei, have to be discussed. Our findings suggest that mapping trigeminal pain processing using fMRI with this non-invasive experimental design is feasible and capable of evoking specific activations in the trigeminal nociceptive system. This method will provide an ideal opportunity to study the trigeminal pain system in both health and pathological conditions such as idiopathic headache disorders.

Flavour processing in semantic dementia

The cognitive mechanisms for the analysis of flavour information remain poorly understood. Patients with semantic dementia (SD) could potentially provide a window on these mechanisms; however, while abnormal eating behaviour and altered food preferences are common in SD, flavour processing has been little studied in this disorder. Here we undertook a detailed investigation of flavour processing in three patients at different stages of SD. One patient with a clinical syndrome of logopenic aphasia (LPA) was studied as a disease control, and six healthy control subjects also participated. Olfaction was assessed using the University of Pennsylvania Smell Identification Test and processing of flavours was assessed using a novel battery to assess flavour perception, flavour identification, and congruence and affective valence of flavour combinations. Patients with SD performed equivalently to healthy controls on the perceptual subtest, while their ability to identify flavours or to determine congruence of flavour combinations was impaired. Classification of flavours according to affective valence was comparable to healthy controls. In contrast, the patient with LPA exhibited a perceptual deficit with relatively preserved identification of flavours, but impaired ability to determine flavour congruence, which did not benefit from affective valence. Olfactory and flavour identification performance was correlated in both patients and controls. We propose that SD produces a true deficit of flavour knowledge (an associative agnosia), while other peri-Sylvian pathologies may lead to deficient flavour perception. Our findings are consistent with emerging evidence from healthy subjects for a cortical hierarchy for processing flavour information, instantiated in a brain network that includes the insula, anterior temporal lobes and orbitofrontal cortex. The findings suggest a potential mechanism for the development of food fads and other abnormal eating behaviours.

Patterns of cerebral activation during olfactory and trigeminal stimulations

It is well known that most odorants stimulate both the olfactory system and the trigeminal system. However, the overlap between the brain processes involved in each of these sensorial perceptions is still poorly documented. This study aims to compare fMRI brain activations while smelling two odorants of a similar perceived intensity and pleasantness: phenyl ethyl alcohol (a pure olfactory stimulus) and iso-amyl-acetate (a bimodal olfactory-trigeminal stimulus) in a homogeneous sample of 15 healthy, right-handed female subjects. The analysis deals with the contrasts of brain activation patterns between these two odorant conditions. The results showed a significant recruitment of the right insular cortex, and bilaterally in the cingulate in response to the trigeminal component. These findings are discussed in relation to the characteristics of these odorants compared with those tested in previous studies.

Relapsing-remitting type multiple sclerosis with olfactory hallucinosis and secondary delusions

Multiple sclerosis is frequently associated with psychiatric symptoms. In more than 50% of the patients symptoms of affective disorder or cognitive decline are observed. A significantly smaller proportion of these individuals develop psychotic symptoms usually during the course of the disease. In this report the rare clinical case of a woman who developed olfactory hallucinations and delusional disorder after 13 years of relapsing-remitting type multiple sclerosis is described.

Odour identification in frontotemporal lobar degeneration

Little information is available concerning olfactory processing in frontotemporal lobar degeneration (FTLD). We undertook a case-control study of olfactory processing in three male patients fulfilling clinical criteria for FTLD. Odour identification (semantic analysis) and odour discrimination (perceptual analysis) were investigated using tests adapted from the University of Pennsylvania Smell Identification Test. General neuropsychometry and structural volumetric brain magnetic resonance imaging (MRI) were also performed. The three patients with FTLD exhibited a disorder of olfactory processing with the characteristics of a predominantly semantic (odour identification) deficit. This olfactory deficit was more prominent in patients with greater involvement of the temporal lobes on MRI. Central deficits of odour identification may be more common in FTLD than previously recognised, and these deficits may assist in clinical characterisation.

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.