The fate of the inner nose: odor imagery in patients with olfactory loss

The relationship between survey-based subjective olfactory awareness and objective olfactory function

KEY POINTS

Correlation between symptom-based surveys and objective olfactory testing is variable. For diagnosis and symptom monitoring, surveys should correlate with objective testing. The Odor Awareness Scale (OAS) and Affective Importance of Odor Scale (AIO) showed significant but moderate positive correlations with University of Pennsylvania Scent Identification Test (UPSIT) score.

Neural mechanisms of odour imagery induced by non-figurative visual cues

Odour imagery, the ability to experience smell when an appropriate stimulus is absent, has widely been documented as being particularly difficult. However, previous studies have shown the beneficial effect of visual cues (e.g., pictures or words) to facilitate performance in numerous tasks of olfactory nature. Therefore, the use of visual cues to evoke odours seems relevant. In this study, our interest is directed towards non-figurative coloured arrangements, which result from a patented technology and aim at chromatically representing any smell from its chemical composition and sensory description. The aim of this study was to characterise the neural mechanisms of odour imagery facilitated by these non-figurative coloured arrangements. Using functional magnetic resonance imaging, we recorded and compared hemodynamic responses during odour imagery facilitated by non-figurative coloured arrangements and pictures. Our findings reveal that the use of non-figurative coloured arrangements during odour imagery solicits olfactory and non-olfactory brain regions (orbitofrontal cortex, insula, hippocampus, thalamus, dorsolateral prefrontal cortex and supplementary motor area), which are mainly involved in olfactory processing and multimodal integration. Moreover, very similar cortical activity was found between the use of non-figurative coloured arrangements and pictures during odour imagery, with increased activity in the supplementary motor area during the use of coloured arrangements only. Overall, non-figurative coloured arrangements could become a robust tool to visually evoke odours without requiring prior familiarity with the depicted odour. Future studies should use psychometric measures to determine the relationships between brain activation, odour imagery ability and vividness of the generated odour images.

fNIRS a novel neuroimaging tool to investigate olfaction, olfactory imagery, and crossmodal interactions: a systematic review

Olfaction is understudied in neuroimaging research compared to other senses, but there is growing evidence of its therapeutic benefits on mood and well-being. Olfactory imagery can provide similar health benefits as olfactory interventions. Harnessing crossmodal visual-olfactory interactions can facilitate olfactory imagery. Understanding and employing these cross-modal interactions between visual and olfactory stimuli could aid in the research and applications of olfaction and olfactory imagery interventions for health and wellbeing. This review examines current knowledge, debates, and research on olfaction, olfactive imagery, and crossmodal visual-olfactory integration. A total of 56 papers, identified using the PRISMA method, were evaluated to identify key brain regions, research themes and methods used to determine the suitability of fNIRS as a tool for studying these topics. The review identified fNIRS-compatible protocols and brain regions within the fNIRS recording depth of approximately 1.5 cm associated with olfactory imagery and crossmodal visual-olfactory integration. Commonly cited regions include the orbitofrontal cortex, inferior frontal gyrus and dorsolateral prefrontal cortex. The findings of this review indicate that fNIRS would be a suitable tool for research into these processes. Additionally, fNIRS suitability for use in naturalistic settings may lead to the development of new research approaches with greater ecological validity compared to existing neuroimaging techniques.

Olfactory dysfunction: A plausible source of COVID-19-induced neuropsychiatric symptoms

Olfactory dysfunction and neuropsychiatric symptoms are commonly reported by patients of coronavirus disease 2019 (COVID-19), a respiratory infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Evidence from recent research suggests linkages between altered or loss of smell and neuropsychiatric symptoms after infection with the coronavirus. Systemic inflammation and ischemic injury are believed to be the major cause of COVID-19-related CNS manifestation. Yet, some evidence suggest a neurotropic property of SARS-CoV-2. This mini-review article summarizes the neural correlates of olfaction and discusses the potential of trans-neuronal transmission of SARS-CoV-2 or its particles within the olfactory connections in the brain. The impact of the dysfunction in the olfactory network on the neuropsychiatric symptoms associated with COVID-19 will also be discussed.

'Tasting Imagination': What Role Chemosensory Mental Imagery in Multisensory Flavour Perception?

A number of perplexing phenomena in the area of olfactory/flavour perception may fruitfully be explained by the suggestion that chemosensory mental imagery can be triggered automatically by perceptual inputs. In particular, the disconnect between the seemingly limited ability of participants in chemosensory psychophysics studies to distinguish more than two or three odorants in mixtures and the rich and detailed flavour descriptions that are sometimes reported by wine experts; the absence of awareness of chemosensory loss in many elderly individuals; and the insensitivity of the odour-induced taste enhancement (OITE) effect to the mode of presentation of olfactory stimuli (i.e., orthonasal or retronasal). The suggestion made here is that the theory of predictive coding, developed first in the visual modality, be extended to chemosensation. This may provide a fruitful way of thinking about the interaction between mental imagery and perception in the experience of aromas and flavours. Accepting such a suggestion also raises some important questions concerning the ecological validity/meaning of much of the chemosensory psychophysics literature that has been published to date.

Losing the sense of smell does not disrupt processing of odor words

Whether language is grounded in action and perception has been a key question in cognitive science, yet little attention has been given to the sense of smell. We directly test whether smell is necessary for comprehension of odor language, by comparing language processing in a group of participants with no sense of smell (anosmics) to a group of control participants. We found no evidence for a difference in online comprehension of odor and taste language between anosmics and controls using a lexical decision task and a semantic similarity judgment task, suggesting olfaction is not critical to the comprehension of odor language. Contrary to predictions, anosmics were better at remembering odor words, and rated odor and taste words as more positively valenced than control participants. This study finds no detriment to odor language after losing the sense of smell, supporting the proposal that odor language is not grounded in odor perception.

Olfactory metacognition and memory in individuals with different subjective odor imagery abilities

Imagery vividness is one of the key indicators to evaluate the ability to generate mental images. There is large inter-individual variability in olfactory imagery (OI) abilities, however, little is known about the underlying factors for individual OI abilities. Using a word cueing imagery paradigm and the trial-by-trial imagery vividness rating method, participants with high or low OI abilities (differentiated by the Vividness of Olfactory Imagery Questionnaire) completed two OI tasks with either shorter (2 s) or longer (8 s) image generation time. Participants' olfactory function, olfactory-related working memory and episodic recognition memory were measured using validated methods. Moreover, olfactory metacognition was assessed using the Odor Awareness Scale (OAS) and the Importance of Olfaction Questionnaire (IOQ). Compared to participants with high OI abilities, those with low OI abilities reported less vivid odor images during OI tasks. For participants with low OI abilities, the imagery vividness significantly improved as the image generation time increased. There was no difference regarding olfactory perception or olfactory-related memory performances between the high and the low OI ability groups. However, participants with higher OI abilities had significant higher scores on the OAS and the IOQ, indicating a superior olfactory-related metacognition. These results provide evidences supporting the underlying factors that related to variances of subjective ability of generating vivid odor mental images.

Multisensory mental representation in covid-19 patients and the possibility of long-lasting gustatory and olfactory dysfunction in the CNS

Gustatory (GD) and olfactory (OD) dysfunctions are the most frequent neurological manifestations of COVID-19. We used mental imagery as an experimental psychological paradigm to access olfactory and gustatory brain representations in 80 Italian COVID-19 adult patients (68.75% reported both OD and GD). COVID-19 patients with OD + GD have a significantly and selectively decreased vividness of odor and taste imagery, indicating that COVID-19 has an effect on their chemosensory mental representations. OD + GD length and type influenced the status of mental chemosensory representations. OD + GD were become all COVID-19 negative at the time of testing. Data suggest that patients are not explicitly aware of long-term altered chemosensory processing. However, differences emerge when their chemosensory function is implicitly assessed using self-ratings. Among patients developing OD + GD, self-ratings of chemosensory function (taste, flavor) were significantly lower as compared to those who did not. At the level of mental representation, such differences can be further detected, in terms of a reduced ability to mentally activate an odor or taste mental image. Our study shows that COVID-19 infection not only frequently causes hyposmia and dysgeusia, but that may also alter the mental representations responsible for olfactory and gustatory perception.

A three-factor benefits framework for understanding consumer preference for scented household products: psychological interactions and implications for future development

Humans have deliberately scented their environment for purpose or pleasure for millennia. In the contemporary marketplace most consumers prefer and purchase scented versions of common household products. However, the drivers of this consumer preference have not been elucidated. To explain the attraction to scent in household products we propose a novel three-factor framework, comprising functional benefits (malodor mitigation, base odor coverage, freshening), in-use experience benefits (cleanliness, efficacy, pleasure), and emotional benefits (increasing in confidence, mood and nostalgia). To support this framework, we present new data from a market research survey on US consumer purchasing habits and attitudes towards home cleaning, laundry, and air freshening products. Further substantiating our framework, a focused review of olfactory psychological science illustrating the central role of scent in cognition, wellbeing, motivated behavior, and social behavior, as well as sensory marketing research highlights the benefits and implications of scent in consumer household products. Based on our three-factor framework we go on to discuss the potential for scent to influence health and raise issues to consider (such as potential negative responding to fragranced products). We conclude by showcasing new opportunities for future research in olfactory science and on scented household products that can advance the positive impacts of scent.

Generating odour imagery enhances brain activity in individuals with low subjective olfactory imagery ability

Olfactory imagery (OI) is defined as the generation of odour images in the mind. There are large inter-individual differences regarding OI abilities. However, the neural representations of OI among individuals with high or low OI abilities are less understood. Participants with high or low OI abilities evaluated using the Vividness of Olfactory Imagery Questionnaire were recruited in this study. Brain activations were measured during a word cueing OI and visual imagery (VI) tasks using functional magnetic resonance imaging (fMRI). In addition, the OI task was divided into two parts. In one part, OI was performed for 8 s (long imagery generation time) and in the other part for 2 s (short imagery generation time). Ratings of the overall imagery vividness were collected at the end of each task. The vividness of OI during short OI was lower among participants with low OI abilities compared to participants with high OI abilities. Brain imaging results showed that participants with low OI ability had stronger brain activation of the supplementary motor area and the superior frontal cortex, compared to participants with higher OI abilities during the short versus long imagery generation time conditions. These results suggest that when generating odour images in a relatively short period of time (e.g., 2 s), people with either high or low OI abilities may have adopted different approaches, resulting in diverse brain activation.

Verbally Induced Olfactory Illusions Are Not Caused by Visual Processing: Evidence From Early and Late Blindness

Olfactory perception is malleable and easily modulated by top-down processes such as those induced by visual and verbal information. A classic example of this is olfactory illusions where the perceived pleasantness of an odor is manipulated by the valence of a verbal label that is either visually or auditorily presented together with the odor. The mechanism behind this illusion is still unknown, and it is not clear if it is driven only by verbal information or if there is an interaction between language functions and visual mental imagery processes. One way to test this directly is to study early blind individuals who have little or no experience of visual information or visual mental imagery. Here, we did this by testing early blind, late blind, and sighted individuals in a classical paradigm where odors were presented with negative, neutral, and positive labels via speech. In contrast to our hypothesis-that the lack of visual imagery would render early blind individuals less susceptible to the olfactory illusion-early and late blind participants showed more amplified illusions than sighted. These findings demonstrate that the general mechanism underlying verbally induced olfactory illusions is not caused by visual processing and visual mental imagery per se.

Chemosensory Loss during a Traumatic Brain Injury Suggests a Central Pathway for the Rehabilitation of Anosmia

Currently, no method has been developed for rehabilitating olfaction in anosmic patients following a traumatic brain injury (TBI). Here a method for rehabilitation is described which is based on a recent finding that the human posterior pyriform cortex (PPC) generates predictive odor "search images" in advance of an encounter with an olfactory stimulus. The search image enhances perceptual sensitivity and allows the odor it represents to be identified without input occurring from the olfactory receptors or bulbs. Furthermore, based on the finding here that anosmics with a TBI often have normal trigeminal and gustatory function, it is proposed that normality in these chemosensory systems may indicate that key cortical regions including the PPC are intact in anosmics and capable of processing olfactory information. In addition, the results of chemosensory tests of the olfactory, gustatory, and intranasal trigeminal systems of 18 patients with a TBI are given that identify which patients would most likely benefit from the rehabilitation procedure.

French Vividness of Olfactory Imagery Questionnaire: A Potential Tool for Diagnosing Olfactory Loss by Assessing Olfactory Imagery?

Several studies have shown a significant relationship between smelling and olfactory imagery abilities. The primary aim of the present study was to validate a French version of the Vividness of Olfactory Imagery Questionnaire (VOIQ). The secondary aim was to investigate its capability to differentiate individuals with smell loss from healthy individuals. After having elaborated a French translation of the VOIQ (fVOIQ), we evaluated olfactory imagery abilities of 387 French participants who anonymously self-completed the fVOIQ: 121 pathologic individuals (hyposmic and anosmic), 244 normosmic individuals (healthy non-expert), and 22 fragrance experts. Significant split-half reliability as expressed by Spearman correlation coefficients for the global sample, as well as for each group separately, indicated the excellent internal consistency of the fVOIQ. Moreover, results revealed a significant effect of the smelling ability group on fVOIQ score, suggesting that daily olfactory stimulation is fundamental to maintaining the ability to create a vivid image and that severe loss of smell may result in progressive impairment of olfactory imagery. Our fVOIQ and the original English version seemingly have similarly high benefit in differentiating experts and normosmic individuals based on their olfactory imagery ability. Moreover, the fVOIQ seems capable of differentiating individuals with loss of smell from healthy individuals. These findings demonstrate the reliability and validity of the fVOIQ, and its capability to differentiate individuals’ smelling ability according to their olfactory imagery ability.

Brain Responses to Food Odors Associated With BMI Change at 2-Year Follow-Up

The understanding of food cue associated neural activations that predict future weight variability may guide the design of effective prevention programs and treatments for overeating and obesity. The current study investigated the association between brain response to different food odors with varied energy density and individual changes of body mass index (BMI) over 2 years. Twenty-five participants received high-fat (chocolate and peanut), low-fat (bread and peach) food odors, and a nonfood odor (rose) while the brain activation was measured using functional magnetic resonance imaging (fMRI). BMIs were calculated with participant’s self-reported body weight and height collected at the time of the fMRI scan and again at 2 years later. Regression analyses revealed significant negative correlations between BMI increase over 2 years and brain activation of the bilateral precuneus and the right posterior cingulate cortex (PCC) in response to high-fat vs. low-fat food odors. Also, brain activation of the right supplementary motor area (SMA) in response to food vs. non-food odor was negatively correlated to subsequent BMI increase over 2 years. Taken together, the current findings suggest that individual differences in neural responsivity to (high calorie) food odors in brain regions of the default mode and motor control network serve as a neural marker for future BMI change.

Neural processing of olfactory-related words in subjects with congenital and acquired olfactory dysfunction

Olfactory loss can be acquired (patients with a history of olfactory experiences), or inborn (patients without olfactory experiences/life-long inability to smell). Inborn olfactory loss, or congenital anosmia (CA), is relatively rare and there is a knowledge gap regarding the compensatory neural mechanisms involved in this condition. The study aimed to investigate the top-down olfactory processing in patients with CA or idiopathic acquired anosmia (IA) in comparison to normosmia controls (NC) during expectancy and reading of odor-associated words. Functional magnetic resonance imaging was used to assess brain activations in 14 patients with CA, 8 patients with IA, and 16 NC healthy participants during an expectancy and reading task. Words with strong olfactory associations (OW) (e.g. “banana”) or with little or no olfactory associations (CW) (e.g. “chair”) were used as stimuli and were presented with a block design Analyses were conducted to explore the brain activation in response to OW expectancy or OW reading between groups (CW as baseline). During the expectancy condition of OW, IA and NC groups showed stronger activation in posterior OFC extending to right insula, caudate region and frontal medial OFC respectively. Whereas during the reading condition of OW, CA patients showed stronger activation in posterior OFC extending to the insula. Increased activation of higher-order brain regions related to multisensory integration among CA patients suggests a compensatory mechanism for processing semantic olfactory cues.

Limitations in odour simulation may originate from differential sensory embodiment

Across diverse lineages, animals communicate using chemosignals, but only humans communicate about chemical signals. Many studies have observed that compared with other sensory modalities, communication about smells is relatively rare and not always reliable. Recent cross-cultural studies, on the other hand, suggest some communities are more olfactorily oriented than previously supposed. Nevertheless, across the globe a general trend emerges where olfactory communication is relatively hard. We suggest here that this is in part because olfactory representations are different in kind: they have a low degree of embodiment, and are not easily expressed as primitives, thereby limiting the mental manipulations that can be performed with them. New exploratory data from Dutch children (9-12 year-olds) and adults support that mental imagery from olfaction is weak in comparison with vision and audition, and critically this is not affected by language development. Specifically, while visual and auditory imagery becomes more vivid with age, olfactory imagery shows no such development. This is consistent with the idea that olfactory representations are different in kind from representations from the other senses. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

Magnetic Resonance Imaging of Human Olfactory Dysfunction

Olfactory dysfunctions affect a larger portion of population (up to 15% with partial olfactory loss, and 5% with complete olfactory loss) as compared to other sensory dysfunctions (e.g. auditory or visual) and have a negative impact on the life quality. The impairment of olfactory functions may happen at each stage of the olfactory system, from epithelium to cortex. Non-invasive neuroimaging techniques such as the magnetic resonance imaging (MRI) have advanced the understanding of the advent and progress of olfactory dysfunctions in humans. The current review summarizes recent MRI studies on human olfactory dysfunction to present an updated and comprehensive picture of the structural and functional alterations in the central olfactory system as a consequence of olfactory loss and regain. Furthermore, the review also highlights recent progress on optimizing the olfactory functional MRI as well as new approaches for data processing that are promising for future clinical practice.

Retronasal olfactory function in patients with smell loss but subjectively normal flavor perception

OBJECTIVES

The human sense of smell constitutes the main part of flavor perception. Typically, patients with loss of olfactory function complain of diminished perception during eating and drinking. However, some patients with smell loss still report normal enjoyment of foods. The aim of the present study was to compare orthonasal and retronasal olfactory function in patients with non-sinonasal smell loss and subjectively normal flavor perception.

METHODS

Nineteen patients (mean age [range] 52.0 [8-83 years]) with self-reported olfactory impairment but subjective normal flavor perception were included. Olfactory performance was assessed using the Sniffin' Sticks (TDI) for orthonasal and the Candy Smell Test (CST) for retronasal function. Visual analogue scales were used for self-assessment of odor (SOP), taste (STP), and flavor perception (SFP), ranging from 0 (no perception) to 10 (excellent perception).

RESULTS

Mean (SD) SFP was 8.0 (1.8). Mean (SD) orthonasal TDI-score of all patients was 14.4 (5.3, range 6-25.3) with 11 patients classified as anosmic and eight as hyposmic. Mean/SD retronasal CST-score was 8.8 (2.7, range 3-13) within the range of anosmia/hyposmia. No correlation was found between SFP and the CST (P = .62).

CONCLUSION

The present results showed that despite claiming normal flavor perception, our patients were ortho- and retronasally dysosmic using standard tests for olfactory function. Although other explanations could be possible, we suggest that this subjective flavor perception might be due to unconscious memory recall from previously experienced cross-modal sensory interactions.

LEVEL OF EVIDENCE

4 Laryngoscope, 130:1629-1633, 2020.

Measuring Multisensory Imagery of Wine: the Vividness of Wine Imagery Questionnaire

When we imagine objects or events, we often engage in multisensory mental imagery. Yet, investigations of mental imagery have typically focused on only one sensory modality - vision. One reason for this is that the most common tool for the measurement of imagery, the questionnaire, has been restricted to unimodal ratings of the object. We present a new mental imagery questionnaire that measures multisensory imagery. Specifically, the newly developed Vividness of Wine Imagery Questionnaire (VWIQ) measures mental imagery of wine in the visual, olfactory, and gustatory modalities. Wine is an ideal domain to explore multisensory imagery because wine drinking is a multisensory experience, it involves the neglected chemical senses (smell and taste), and provides the opportunity to explore the effect of experience and expertise on imagery (from wine novices to experts). The VWIQ questionnaire showed high internal consistency and reliability, and correlated with other validated measures of imagery. Overall, the VWIQ may serve as a useful tool to explore mental imagery for researchers, as well as individuals in the wine industry during sommelier training and evaluation of wine professionals.

Severity of olfactory deficits is reflected in functional brain networks-An fMRI study

Even though deficits in olfactory function affect a considerable part of the population, the neuronal basis of olfactory deficits remains scarcely investigated. To achieve a better understanding of how smell loss affects neural activation patterns and functional networks, we set out to investigate patients with olfactory dysfunction using functional magnetic resonance imaging (fMRI) and olfactory stimulation. We used patients' scores on a standardized olfactory test as continuous measure of olfactory function. 48 patients (mean olfactory threshold discrimination identification (TDI) score = 16.33, SD = 6.4, range 6 - 28.5) were investigated. Overall, patients showed piriform cortex activation during odor stimulation compared to pure sniffing. Group independent component analysis indicated that the recruitment of three networks during odor stimulation was correlated with olfactory function: a sensory processing network (including regions such as insula, thalamus and piriform cortex), a cerebellar network and an occipital network. Interestingly, recruitment of these networks during pure sniffing was related to olfactory function as well. Our results support previous findings that sniffing alone can activate olfactory regions. Extending this, we found that the severity of olfactory deficits is related to the extent to which neural networks are recruited both during olfactory stimulation and pure sniffing. This indicates that olfactory deficits are not only reflected in changes in specific olfactory areas but also in the recruitment of occipital and cerebellar networks. These findings pave the way for future investigations on whether characteristics of these networks might be of use for the prediction of disease prognosis or of treatment success.

Advances of Molecular Imaging for Monitoring the Anatomical and Functional Architecture of the Olfactory System

The olfactory system of organisms serves as a genetically and anatomically model for studying how sensory input can be translated into behavior output. Some neurologic diseases are considered to be related to olfactory disturbance, especially Alzheimer's disease, Parkinson's disease, multiple sclerosis, and so forth. However, it is still unclear how the olfactory system affects disease generation processes and olfaction delivery processes. Molecular imaging, a modern multidisciplinary technology, can provide valid tools for the early detection and characterization of diseases, evaluation of treatment, and study of biological processes in living subjects, since molecular imaging applies specific molecular probes as a novel approach to produce special data to study biological processes in cellular and subcellular levels. Recently, molecular imaging plays a key role in studying the activation of olfactory system, thus it could help to prevent or delay some diseases. Herein, we present a comprehensive review on the research progress of the imaging probes for visualizing olfactory system, which is classified on different imaging modalities, including PET, MRI, and optical imaging. Additionally, the probes' design, sensing mechanism, and biological application are discussed. Finally, we provide an outlook for future studies in this field.

The inability to self-evaluate smell performance. How the vividness of mental images outweighs awareness of olfactory performance

To rate one’s individual olfactory performance is difficult and in many cases differs clearly from validated objective olfactory performance measures. This study aimed to investigate the basis for this measurement drift between objective and subjective olfactory performance evaluation. In absence of an actual odor, one may imagine an olfactory stimulus to evaluate his subjective olfactory performance. Therefore, the impact of the vividness of mental images on self-evaluation of smell performance in patients with mild to severe olfactory dysfunction and healthy controls was investigated. Fifty-nine patients with peripheral olfactory dysfunction ranging from reduced olfactory function (hyposmia) to complete loss of olfactory perception (anosmia) and 16 healthy controls were included. Olfactory performance was assessed using the Sniffin’ Sticks battery, the vividness of olfactory mental images was evaluated using the vividness of olfactory imagery questionnaire (VOIQ). Decreased vividness of odor images was obtained for anosmic patients, and a trend of poorer odor imagery was determined in hyposmic patients. Multiple regression analyses revealed the VOIQ score as significant predictor for olfactory self-evaluation for hyposmic patients and healthy controls. In contrast, for anosmic patients, the only significant predictor for self-rating of olfactory performance was the threshold-detection-identification (TDI) score, measuring overall olfactory performance. The results of this study indicate that sensory perception and mental images are closely related to each other. Furthermore, subjects who were able to perceive odors, even to a smaller extent, rely on the vividness of their mental odor images to evaluate their olfactory performance. In contrast, anosmic patients rather trust in their knowledge that they are not able to perceive odors. We are therefore able to subjectively rate our olfactory performance levels, if we are not able to perceive odors, but not if we are able to perceive olfactory input.