Sustained effects of pleasant and unpleasant smells on resting state brain activity

Corticolimbic structures activation during preparation and execution of respiratory manoeuvres in voluntary olfactory sampling: An intracranial EEG study

Volitional respiratory manoeuvres such as sniffing and apnoea play a key role in the active olfactory exploration of the environment. Their impairment by neurodegenerative processes could thus impair olfactory abilities with the ensuing impact on quality of life. Functional brain imaging studies have identified brain networks engaged in sniffing and voluntary apnoea, comprising the primary motor and somatosensory cortices, the insula, the anterior cingulate cortex and the amygdala. The temporal organization and the oscillatory activities of these networks are not known. To elucidate these aspects, we recorded intracranial electroencephalograms in six patients during voluntary sniffs and short apnoeas (12 s). The preparation phase of both manoeuvres involved increased alpha and theta activity in the posterior insula, amygdala and temporal regions, with a specific preparatory activity in the parahippocampus for the short apnoeas and the hippocampus for sniff. Subsequently, it narrowed to the superior and median temporal areas, immediately after the manoeuvres. During short apnoeas, a particular dynamic was observed, consisting of a rapid decline in alpha and theta activity followed by a slow recovery and increase. Volitional respiratory manoeuvres involved in olfactory control involve corticolimbic structures in both a preparatory and executive manner. Further studies are needed to determine whether diseases altering deep brain structures can disrupt these mechanisms and if such disruption contributes to the corresponding olfactory deficits. KEY POINTS: Both sniff manoeuvres and short apnoeas are associated with oscillatory activity predominantly in low-frequency bands (alpha and theta). Preparation of sniff manoeuvres and short apnoeas involve activities in low-frequency bands in the posterior insula and temporal regions that extend to amygdala during the execution of both manoeuvres. During short apnoeas, activities in low-frequency bands initially decline before continuously increasing until the apnoeas end.

MRI evidence of gray matter loss in COVID-19 patients with cognitive and olfactory disorders

OBJECTIVE

The aim of this study was to assess COVID-19-related gray matter (GM) structural alterations in two distinct groups of patients presenting with the prevailing and distinctive COVID-19-related neurological symptoms - isolated olfactory disorders as sole neurological manifestation (COVID-OD) and cognitive disorders (COVID-CD) - as compared to a control group of unaffected individuals.

METHODS

The study included 61 COVID-CD patients (57 [60-63] years, 62% females), 84 COVID-OD patients (49 [35-57] years, 60% females), and 17 controls (51 [41-52] years, 41% females). Region-based morphometry (RBM) and voxel-based morphometry (VBM) were performed on T1-weighted MRI scans to assess GM regional volume and voxel-wise density differences between COVID-19 patients and controls. Surface-based morphometry (SBM) was applied to investigate cortical thickness alterations. The statistical models built to assess GM structural differences among groups included total intracranial volume and age as nuisance variables.

RESULTS

The multi-morphometric analysis revealed statistically significant (p < 0.05 corrected for multiple comparisons) reduction in GM regional volumes, in voxel-wise GM density and in cortical thickness in both COVID-CD and COVID-OD patient groups as compared to controls. Across all three analyses, COVID-CD patients showed more distributed and severe GM loss than COVID-OD patients. The most prominently affected GM regions in the COVID-CD group included the hippocampus, putamen, cingulate gyrus, precuneus, precentral and postcentral gyri, amygdala, lingual gyrus, and caudate nucleus.

INTERPRETATION

Our MRI findings show that COVID-19-related olfactory and cognitive disorders both induce GM atrophy, although at different degrees of severity, likely indicative of neurodegeneration and neuroinflammation.

An fMRI-based investigation of the effects of odors on the functional connectivity network underlying the working memory

In the human brain, the regions responsible for emotion processing, motivation, and memory are heavily influenced by olfaction, whose neural pathway is directly exposed to the outer world. In this study, we used fMRI to examine how different olfactory conditions might affect the functional connectivity circuit underlying working memory in the brain. To this end, 30 adults (aged 20-35), 13 males and 17 females, with high educational levels were chosen. Participants were screened for potential olfactory issues before undergoing the Sniffin' sticks test, which was part of the inclusion criteria. Before imaging, each participant was given the required level of training and was then asked to complete four olfactory tests involving pleasant and unpleasant odors, air, and null stimulation. The results of Seed-based analysis suggested a function connection between the inferior parietal region and the left frontal pole region upon olfactory stimulation with vanilla scent in contrast to null stimulation in this comparison, ROI-based analysis revealed an inverse synchronous among the entorhinal cortex, orbitofrontal cortex, and dorsolateral prefrontal cortex (dlPFC). Both dlPFC and hippocampus were involved in olfactory discrimination between two different stimulants. Our findings indicate the presence of inverse correlations between several regions associated with olfaction and working memory, with pleasant scents leaving a stronger impact on the working memory-related areas, particularly the inferior parietal region.

Olfactory deficit: a potential functional marker across the Alzheimer's disease continuum

Alzheimer's disease (AD) is a prevalent form of dementia that affects an estimated 32 million individuals globally. Identifying early indicators is vital for screening at-risk populations and implementing timely interventions. At present, there is an urgent need for early and sensitive biomarkers to screen individuals at risk of AD. Among all sensory biomarkers, olfaction is currently one of the most promising indicators for AD. Olfactory dysfunction signifies a decline in the ability to detect, identify, or remember odors. Within the spectrum of AD, impairment in olfactory identification precedes detectable cognitive impairments, including mild cognitive impairment (MCI) and even the stage of subjective cognitive decline (SCD), by several years. Olfactory impairment is closely linked to the clinical symptoms and neuropathological biomarkers of AD, accompanied by significant structural and functional abnormalities in the brain. Olfactory behavior examination can subjectively evaluate the abilities of olfactory identification, threshold, and discrimination. Olfactory functional magnetic resonance imaging (fMRI) can provide a relatively objective assessment of olfactory capabilities, with the potential to become a promising tool for exploring the neural mechanisms of olfactory damage in AD. Here, we provide a timely review of recent literature on the characteristics, neuropathology, and examination of olfactory dysfunction in the AD continuum. We focus on the early changes in olfactory indicators detected by behavioral and fMRI assessments and discuss the potential of these techniques in MCI and preclinical AD. Despite the challenges and limitations of existing research, olfactory dysfunction has demonstrated its value in assessing neurodegenerative diseases and may serve as an early indicator of AD in the future.

Local Brain Network Alterations and Olfactory Impairment in Alzheimer's Disease: An fMRI and Graph-Based Study

Alzheimer's disease (AD) is associated with the abnormal connection of functional networks. Olfactory impairment occurs in early AD; therefore, exploring alterations in olfactory-related regions is useful for early AD diagnosis. We combined the graph theory of local brain network topology with olfactory performance to analyze the differences in AD brain network characteristics. A total of 23 patients with AD and 18 normal controls were recruited for resting-state functional magnetic resonance imaging (fMRI), clinical neuropsychological examinations and the University of Pennsylvania Smell Identification Test (UPSIT). Between-group differences in the topological properties of the local network were compared. Pearson correlations were explored based on differential brain regions and olfactory performance. Statistical analysis revealed a correlation of the degree of cognitive impairment with olfactory recognition function. Local node topological properties were significantly altered in many local brain regions in the AD group. The nodal clustering coefficients of the bilateral temporal pole: middle temporal gyrus (TPOmid), degree centrality of the left insula (INS.L), degree centrality of the right middle temporal gyrus (MTG.R), and betweenness centrality of the left middle temporal gyrus (MTG.L) were related to olfactory performance. Alterations in local topological properties combined with the olfactory impairment can allow early identification of abnormal olfactory-related regions, facilitating early AD screening.

Neural Processing of Odors with Different Well-Being Associations—Findings from Two Consecutive Neuroimaging Studies

Much is known about the effect of odors on mood, cognition and behavior, but little is known about the relationship between odors and well-being. We investigated the neural processing of odors with different degrees of association with well-being (WB) through two large independent datasets. The study encompassed pre-testing and fMRI. During pre-testing, 100 and 80 (studies 1 and 2) young, healthy subjects participated, rating intensity, valence, and WB association for 14 (study 1) and 8 (study 2) different odors. Pre-testing resulted in the selection of two odors with high WB association (WB-associated) and two odors with lower WB association (neutral odors) for each study. Odors were delivered intranasally to the subjects who underwent fMRI scanning (44 and 41 subjects, respectively, for studies 1 and 2). We assessed brain activity for subjects when they experienced WB-associated versus neutral odors. In study 1, WB-associated odors showed increased activation in the right angular gyrus whereas in study 2, increased activity in the left angular gyrus existed, together with increased activity in the anterior cingulate cortex and posterior orbitofrontal cortex. The increased activity of higher-order cognitive and emotional regions during the processing of WB-associated odors in the two independent studies suggests a role of odors in influencing individual well-being. Moreover, the consistent activation of the angular gyrus might suggest its key role in shifting attention toward relevant emotional stimuli.

Attenuated facial movement in depressed women is associated with symptom severity, and nucleus accumbens functional connectivity

It is assumed that mood can be inferred from one's facial expression. While this association may prove to be an objective marker for mood disorders, few studies have explicitly evaluated this linkage. The facial movement responses of women with major depressive disorder (n = 66) and healthy controls (n = 46) under emotional stimuli were recorded using webcam. To boost facial movements, the naturalistic audio-visual stimuli were presented. To assess consistent global patterns across facial movements, scores for facial action units were extracted and projected onto principal component using principal component analysis. The associations of component for facial movements with functional brain circuitry was also investigated. Clusters of mouth movements, such as lip press and stretch, identified by principal component analysis, were attenuated in depressive patients compared to those in healthy controls. This component of facial movements was associated with depressive symptoms, and the strengths of resting brain functional connectivity between nucleus accumbens and both posterior insular cortex and thalamus. The evaluation of facial movements may prove to be a promising quantitative marker for assessing depressive symptoms and their underlying brain circuitry.

Functional Connectivity of the Chemosenses: A Review

Functional connectivity approaches have long been used in cognitive neuroscience to establish pathways of communication between and among brain regions. However, the use of these analyses to better understand how the brain processes chemosensory information remains nascent. In this review, we conduct a literature search of all functional connectivity papers of olfaction, gustation, and chemesthesis, with 103 articles discovered in total. These publications largely use approaches of seed-based functional connectivity and psychophysiological interactions, as well as effective connectivity approaches such as Granger Causality, Dynamic Causal Modeling, and Structural Equation Modeling. Regardless of modality, studies largely focus on elucidating neural correlates of stimulus qualities such as identity, pleasantness, and intensity, with task-based paradigms most frequently implemented. We call for further "model free" or data-driven approaches in predictive modeling to craft brain-behavior relationships that are free from a priori hypotheses and not solely based on potentially irreproducible literature. Moreover, we note a relative dearth of resting-state literature, which could be used to better understand chemosensory networks with less influence from motion artifacts induced via gustatory or olfactory paradigms. Finally, we note a lack of genomics data, which could clarify individual and heritable differences in chemosensory perception.

Changes in the Intranetwork and Internetwork Connectivity of the Default Mode Network and Olfactory Network in Patients with COVID-19 and Olfactory Dysfunction

Olfactory dysfunction (OD) is a common symptom in coronavirus disease 2019 (COVID-19) patients. Moreover, many neurological manifestations have been reported in these patients, suggesting central nervous system involvement. The default mode network (DMN) is closely associated with olfactory processing. In this study, we investigated the internetwork and intranetwork connectivity of the DMN and the olfactory network (ON) in 13 healthy controls and 22 patients presenting with COVID-19-related OD using independent component analysis and region of interest functional magnetic resonance imaging (fMRI) analysis. There was a significant correlation between the butanol threshold test (BTT) and the intranetwork connectivity in ON. Meanwhile, the COVID-19 patients with OD showed significantly higher intranetwork connectivity in the DMN, as well as higher internetwork connectivity between ON and DMN. However, no significant difference was found between groups in the intranetwork connectivity within ON. We postulate that higher intranetwork functional connectivities compensate for the deficits in olfactory processing and general well-being in COVID-19 patients. Nevertheless, the compensation process in the ON may not be obvious at this stage. Our results suggest that resting-state fMRI is a potentially valuable tool to evaluate neurosensory dysfunction in COVID-19 patients.