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Distribution of publications related to olfaction, odour imagery and crossmodal visual-olfactory integration by year. For the purposes of this review, the search range was restricted to 2003–2023.
Source publication
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 emplo...
Citations
... Nonetheless, cortical regions, such as the orbitofrontal cortex (OFC), middle and inferior frontal gyri, and inferior parietal lobule, are known to be implicated in later stages of olfactory processes. 5 Therefore, they are a target of studies on olfaction accomplished with EEG and fNIRS. ...
Olfactory perception can be studied in deep brain regions at high spatial resolutions with functional magnetic resonance imaging (fMRI), but this is complex and expensive. Electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS) are limited to cortical responses and lower spatial resolutions but are easier and cheaper to use. Unlike EEG, available fNIRS studies on olfaction are few, limited in scope, and contradictory. Here, we investigated fNIRS efficacy in assessing the hedonic valence of pleasant and unpleasant odors, using ten channels on each hemisphere, covering the orbitofrontal cortex and adjacent areas involved in olfactory and cognitive tasks. Measurements on 22 subjects (11 males and 11 females) showed statistically significant higher increases in oxygenated hemoglobin concentration for the unpleasant odor, compared to the pleasant one (mean difference = 1.025 × 10⁻¹ μM). No difference in activation was found between the hemispheres. Conversely, differences were observed between the sexes: for the first time, we show that higher activations for the unpleasant odor relative to the pleasant one are detectable by fNIRS in females (mean difference = 1.704 × 10⁻¹ μM), but not in an equal-sized and equal-age group of males. Moreover, females had greater activations relative to males for the unpleasant odor (mean difference = 1.285 × 10⁻¹ μM). Therefore, fNIRS can capture peculiarities of olfactory activations, highlighting differences between odors with opposite valence and between sexes. This evidence positions fNIRS next to EEG as suitable technologies for cortical investigations of olfactory perception, providing complementary information (late and early response components, respectively), with lower costs and easier operation (albeit at lower resolutions) compared to fMRI.
... Turning now to the case of olfaction, Wiesmann et al. (2001, p. 237) note that: "Olfactory information is projected from the olfactory bulb to the primary olfactory cortex, which is composed of the anterior olfactory nucleus, the olfactory tubercle, the piriform cortex, the amygdala, the periamygdaloid region, and the entorhinal cortex. From there, the primary olfactory cortex projects to secondary olfactory regions including the hippocampus, ventral striatum and pallidum, hypothalamus, thalamus, orbitofrontal cortex, agranular insular cortex, and cingulate gyrus" (see also Boot et al., 2024). ...
... 6. Though note that in the case of the latter two studies this is more retaining an image in memory than the spontaneous generation of something from within, as in the original Perky (1910) 8. Looking to the future, it would be interesting to know whether olfactory mental imagery could be harnessed to deliver some of the well-being benefits that exposure to actual pleasant ambient odours has been shown to have (Boot et al., 2024;Bromberg and Schilder, 1934;Spence, 2020;Weber and Heuberger, 2008; see also Schlintl et al., 2022). 9. ...
The study of chemosensory mental imagery is undoubtedly made more difficult because of the profound individual differences that have been reported in the vividness of (e.g.) olfactory mental imagery. At the same time, the majority of those researchers who have attempted to study people’s mental imagery abilities for taste (gustation) have actually mostly been studying flavour mental imagery. Nevertheless, there exists a body of human psychophysical research showing that chemosensory mental imagery exhibits a number of similarities with chemosensory perception. Furthermore, the two systems have frequently been shown to interact with one another, the similarities and differences between chemosensory perception and chemosensory mental imagery at the introspective, behavioural, psychophysical, and cognitive neuroscience levels in humans are considered in this narrative historical review. The latest neuroimaging evidence show that many of the same brain areas are engaged by chemosensory mental imagery as have previously been documented to be involved in chemosensory perception. That said, the pattern of neural connectively is reversed between the ‘top-down’ control of chemosensory mental imagery and the ‘bottom-up’ control seen in the case of chemosensory perception. At the same time, however, there remain a number of intriguing questions as to whether it is even possible to distinguish between orthonasal and retronasal olfactory mental imagery, and the extent to which mental imagery for flavour, which most people not only describe as, but also perceive to be, the ‘taste’ of food and drink, is capable of reactivating the entire flavour network in the human brain.
Background
Olfactory dysfunction stands as one of the most prevalent non-motor symptoms in the initial stage of Parkinson’s disease (PD). Nevertheless, the intricate mechanisms underlying olfactory deficits in Parkinson’s disease still remain elusive.
Methods
This study collected rs-fMRI data from 30 PD patients [15 with severe hyposmia (PD-SH) and 15 with no/mild hyposmia (PD-N/MH)] and 15 healthy controls (HC). To investigate functional segregation, the amplitude of low-frequency fluctuation (ALFF) and regional homogeneity (ReHo) were utilized. Functional connectivity (FC) analysis was performed to explore the functional integration across diverse brain regions. Additionally, the graph theory-based network analysis was employed to assess functional networks in PD patients. Furthermore, Pearson correlation analysis was conducted to delve deeper into the relationship between the severity of olfactory dysfunction and various functional metrics.
Results
We discovered pronounced variations in ALFF, ReHo, FC, and topological brain network attributes across the three groups, with several of these disparities exhibiting a correlation with olfactory scores.
Conclusion
Using fMRI, our study analyzed brain function in PD-SH, PD-N/MH, and HC groups, revealing impaired segregation and integration in PD-SH and PD-N/MH. We hypothesize that changes in temporal, frontal, occipital, and cerebellar activities, along with aberrant cerebellum-insula connectivity and node degree and betweenness disparities, may be linked to olfactory dysfunction in PD patients.
Food aroma molecules have the potential to influence people's emotions through the olfactory pathway and are anticipated to emerge as a new method for regulating emotional states, owing to their simplicity and high acceptance. Current research on food aroma predominantly centres on the physicochemical properties and formation mechanisms of aroma components, neglecting the effects of aroma molecules in emotional regulation. Moreover, the evaluation of pleasantness, a pivotal dimension of emotions, lacks objective assessment methods. In this study, sensory assessments of pleasantness for 12 aroma compounds were gathered from 45 subjects, and their correlation with the brain's activity responses in the left frontal‐temporal lobe (LFT) and right frontal‐temporal lobe (RFT) using electroencephalogram (EEG) signals was analysed. The results revealed a close relationship between brain activity in the LFT and the perception of aroma pleasantness. Furthermore, a substantial correlation was observed between the α , β and γ frequency bands in the LFT and the subjective pleasantness scores. These findings demonstrate that the LFT plays a critical role in evaluating the pleasantness of aroma molecules, and that changes in the power of the α , β and γ bands serve as important evaluation indicators. Consequently, this method offers a new objective means for assessing pleasantness to find higher pleasantness aroma molecules and the emotional regulation of food aroma.
