Advances in Understanding Parosmia: An fMRI Study

Iannilli, Emilia; Leopold, Donald A.; Hornung, David E.; Hummel, Thomas

doi:10.1159/000500558

Cited by 43 publications

(52 citation statements)

References 27 publications

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“…The present observation of pleasantness evoked by OFC stimulation does not dispute the suggestion of Nigri et al [20] that the medial OFC is involved in integrative olfactory processing even if they tested only unpleasant odorants. A very recent fMRI paper comparing patients with olfactory impairment with versus without parosmia, a condition where unpleasant odorous percepts are elicited by normal odors, showed that parosmia patients had higher activations in the putamen and thalamus but not in the OFC [33]. This goes in line with the present data that the OFC is probably not the main generator of unpleasant olfactory symptoms.…”

Section: Discussionsupporting

confidence: 91%

Electrical stimulation of the medial orbitofrontal cortex in humans elicits pleasant olfactory perceptions

Bérard

Landis

Legrand

et al. 2021

Epilepsy & Behavior

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Background: Olfactory hallucinations can be part of epileptic seizures of orbitofrontal origin. Olfactory hallucinations, however, are rare and therefore the semiology, localization and lateralization characteristics are underdetermined. In addition, many discrepancies are found in the literature regarding olfactory processing and orbitofrontal (OF) functions and olfactory function. Particularly, the questions of laterality and affective component in coding of odors in the OF cortex remain controversial. Aims: This study explored whether cortical electrical stimulation of the OF and mesiotemporal brain can trigger olfactory hallucinations with special focus on olfactory percepts in terms of laterality and hedonics. Materials and methods: Eight patients with temporal lobe epilepsy participated in the study, at the time of invasive exploration of their epilepsy. The most distal contact of the OF and anterior hippocampus depth electrodes were stimulated (50 Hz, 0.2 ms biphasic pulse; maximal stimulation 4 mA). Patients were instructed to report any kind of sensation they might experience. Intracranial depth electrodes were localized (iElectrodes): subject-specific brain mask, subcortical segmentation and cortical parcellation based on the Destrieux atlas (FreeSurfer) were superposed to the coregistered T1-weighted MRI and CT images (SPM). The center of mass of each electrode-artifact cluster determined the electrode localization. The electrode labeling was done in patient space. To obtain the electrode coordinates in Montreal Neurological Institute (MNI) space, the images obtained previously in the patient space were first segmented and normalized (SPM). Then, the localization procedure (iElectrodes) was run again with these new normalized images in MNI space. Results: No hallucination was evoked by stimulation, neither of the right nor the left hippocampus (8/8 patients). Pleasant olfactory hallucinations were evoked by OF stimulation in 5/8 patients in either hemisphere. Patients named the percept as the smell of lemon or coffee for example. Among those 5 patients, electrodes were localized in the cortex of the olfactory sulcus, medial orbital sulcus or medial OF gyrus. Increasing stimulation amplitude changed the olfactory percept identification in 3 out of those 5 patients. No affective judgement or change in perceived odor intensity was reported by the patients. No hallucination was evoked by the stimulation of the white matter of the medial OF brain in 3/8 patients independently of the hemisphere stimulated. Conclusions: This study demonstrated that stimulation of the cortex of the medial OF brain and not of its white matter elicits specific pleasant olfactory hallucinations independently of the hemisphere stimulated, supporting one symmetrical olfactory processing in human.

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Section: Discussionsupporting

confidence: 91%

Electrical stimulation of the medial orbitofrontal cortex in humans elicits pleasant olfactory perceptions

Bérard

Landis

Legrand

et al. 2021

Epilepsy & Behavior

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“…A decrease in olfactory bulb volume 23, 24 and a significant loss of grey matter volume 25 has been demonstrated in parosmic participants. Further evidence of central mechanisms has been published recently showing different fMRI activation patterns in parosmic participants compared to hyposmics 26 . Increased activation in the thalamus and the putamen was observed in the parosmic participants, the latter being of relevance since it is connected to the olfactory cortical networks and has been associated with the perception of disgust.…”

Section: Discussionmentioning

confidence: 94%

Molecular Mechanism of Parosmia

Parker

Kelly²,

Gane

2021

Preprint

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The molecular stimuli that trigger parosmia have been identified. Parosmia is a debilitating disease in which familiar smells become distorted and unpleasant. Often a result of post infectious smell loss, incidences are increasing as the number of COVID-19 cases escalates worldwide. Little is understood of its pathophysiology, but the prevailing hypothesis for the underlying mechanism is a mis-wiring of olfactory sensory neurons. We identified 15 different molecular triggers in coffee using GC-Olfactometry as a relatively rapid screening tool for assessment of both quantitative olfactory loss and parosmia. This provides evidence for peripheral causation, but places constraints on the mis-wiring theory.

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“…Because many of these areas are critically involved in olfactory quality discrimination and odor memory, it has been speculated that the parosmia was associated with structural changes in a neuronal network crucial for odor discrimination and memory. A functional MRI study demonstrated that hyposmic patients with parosmia exhibited a strong activation in the thalamus, known to be related to directed attention in relation to the sense of smell, and the putamen that is implicated in the recognition of disgust signals (48).…”

Section: Discussionmentioning

confidence: 99%