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Age-related sensory impairment is a slow and gradual progress, which affects multiple modalities. Two contradictory hypotheses exist about the age-related decline of sensory thresholds. The common factor theory assumes one underlying factor-which accounts for the loss of several sensory modalities simultaneously-and the specific factor theory predicts that the sensory decline is uncorrelated between different modalities. In this study, we aimed to explore whether (i) there is a common factor of sensory thresholds in older people, (ii) older people assume that sensory decline in one modality also affects other modalities, (iii) there is a relation between sensory threshold and the subjective assessment of sensory function. This was accomplished by collecting both threshold measures and self-reported ratings for smell, hearing, taste, vision, and touch function in a group of 104 older people (mean age: 67.2 years; SD: 9.85; range: 50-100 years). Results indicated that there was no common factor of sensory thresholds, hence an impairment in one modality did not necessarily imply a shortfall in other modalities. In contrast, our results suggested one or two common factor(s) for the participants' ratings. Participants who reported a diminished function in one sense tended to generalize this rating to the other senses as well. The correspondence between subjective ratings and sensory thresholds was relatively good for vision and audition, although no correlations were observed for the other domains. These findings have implications for clinicians, suggesting that subjective measures should be combined with sensory threshold measurements when evaluating sensory dysfunction. Also, these data convey a positive message for older people and their physicians by showing that loss in one sensory modality does not necessarily generalize to losses across all sensory modalities.

•Habitual spicy eating is related to shift in hedonic but not intensity perception of oral capsaicin.•Habitual spicy eaters had greater left postcentral gyrus responses to high-concentration capsaicin.•Reduced right hippocampus activation to high-concentration capsaicin in habitual eaters.•Pleasantness for high-level capsaicin correlates positively with hippocampus activation in non-habitual eaters.•Altered hippocampus-somatosensory/affective connectivity in habitual eaters in response to low-concentration of capsaicin. Repeated exposure to spicy flavor can shift individuals’ preferences for capsaicin-containing stimuli, however, the neural mechanisms underlying the hedonic processing of oral capsaicin remain unclear. Here, we investigated how habitual spicy food consumption modulates brain responses involved in hedonic perceptions of oral capsaicin. Using functional magnetic resonance imaging, we compared brain activity between high-frequency (HF, N = 19) and low-frequency (LF, N = 17) spicy food consumers during oral administration of three concentrations of capsaicin (10, 30, 60 μM) and water as a control stimulus. Participants rated the pleasantness and intensity after each stimulus. Compared with the LF group, the HF group reported significantly higher pleasantness ratings across all capsaicin concentrations, an effect that was independent of perceived intensity. At the neural level, a significant group × concentration interaction was observed. For the high-concentration of capsaicin stimulation (60 μM), the HF group exhibited increased activation in the left postcentral gyrus and decreased activation in the right hippocampus relative to the LF group. In the LF group, pleasantness for the high concentration of capsaicin was positively correlated with activation in the right hippocampus. For the low-concentration of capsaicin (10 μM), the LF group showed weakened hippocampus–postcentral and hippocampus–caudate connectivity, but stronger insula–parahippocampal connectivity compared to the HF group. In addition, pleasantness ratings for low-concentration of capsaicin were positively correlated with hippocampus–postcentral connectivity in LF participants. Together, these findings suggest that the neural substrates subserving the hedonic processing of oral capsaicin vary as a function of habitual spicy food intake and stimulus concentration, reflecting experience-dependent plasticity within chemosensory and affective brain networks.

Olfactory loss is associated with symptoms of depression. The present study, conducted on a large cohort of mostly dysosmic patients, aimed to investigate whether improvement in olfactory performance would correspond with a decrease in depression severity. In 171 participants (157 dysosmic), we assessed olfactory function and severity of depression before and after an average interval of 11 months, with many patients showing improvement in olfactory function. Separate analyses were conducted for (a) the whole group of patients and (b) the group of dysosmic patients using both classic and Bayesian approaches. For odor identification, Student t test demonstrated that the whole sample improved consistently, especially within the group of dysosmic patients. The dysosmic group also improved in odor threshold and overall olfactory function. Pearson correlation showed that an increase in olfactory function was associated with a decrease in depression severity, particularly in dysosmic patients. To conclude, the present results indicate that symptoms of depression change with olfactory function in general and odor identification in particular.

This study offers insights into the complex relationship between chemical species constituting air pollution and chemosensory function. We examined the relationship between chemical species known to contribute to air pollution and assault human health and chemosensory sensitivity. Chemosensory sensitivity data was retrieved from a large-scale study involving 711 urban-dwelling participants inhabiting 10 different regions of the globe. Their olfactory threshold towards phenyl ethyl alcohol (PEA) and olfactory/trigeminal threshold towards Eucalyptol was measured in a multicentre study. We matched the individual chemosensory data with the levels of PM2.5, PM10, O3, NO2, SO2, CO at the location of testing sites, on the exact date of the test, using EMAC (ECHAM5/MESSy for Atmospheric Chemistry) model. Our findings indicate that air pollution negatively affects olfactory function and has cumulative negative effects with aging. The reported patterns are seasonal and increase during Autumn and Winter, and interact with medical conditions related to poorer olfactory function. We extend the current knowledge by demonstrating that olfactory/trigeminal perception is also disrupted by toxic air, albeit in a slightly different manner. The analyzed models promote a more complex perspective on the relationship between air composition and chemosensory sensitivity, but delineate problems related to the interdependence of the levels of chemical species constituting air pollution and using them together to predict chemosensory sensitivity. Conclusions point to the need to investigate the problem of air pollution and chemosensory health from a global perspective, as air quality partly accounts for the differences in chemosensory perception in different regions of the world.

•Reduced brain responses to pleasant odor, but not to neural or unpleasant odor, after acute stress.•Activation in the amygdala/piriform, OFC, and insula in response to the pleasant odor was correlated with stress-related cortisol responses.•Increased right piriform-insula coupling in response to pleasant odor was correlated with subjective stressful ratings after stress. Acute stress alters olfactory perception. However, little is known about the neural processing of olfactory stimuli after acute stress exposure and the role of cortisol in such an effect. Here, we used an event-related olfactory fMRI paradigm to investigate brain responses to odors of different valence (unpleasant, pleasant, or neutral) in healthy young adults following an acute stress (Trier Social Stress Test, TSST) induction (N = 22) or a non-stressful resting condition (N = 22). We obtained the odor pleasantness, intensity, and familiarity ratings after the acute stress induction or resting condition. We also measured the participants' perceived stress and salivary cortisol at four time points during the procedure. We found a stress-related decrease in brain activation in response to the pleasant, but not to the neutral or unpleasant odor stimuli in the right piriform cortex extending to the right amygdala, the right orbitofrontal cortex, and the right insula. In addition, activation of clusters within the regions of interest were negatively associated with individual baseline-to-peak increase in salivary cortisol levels after stress. We also found increased functional connectivity between the right piriform cortex and the right insula after stress when the pleasant odor was presented. The strength of the connectivity was positively correlated with increased perceived stress levels immediately after stress exposure. These results provide novel evidence for the effects of acute stress in attenuating the neural processing of a pleasant olfactory stimulus. Together with previous findings, the effect of acute stress on human olfactory perception appears to depend on both the valence and the concentration (e.g., peri-threshold or suprathreshold levels) of odor stimuli.

The olfactory system contributes significantly to human social behavior and especially to mate choice and empathic functioning. In this context, previous research examining individuals with impaired olfactory function indicated an influence of the sense of smell on different aspects of sexuality. However, the applied samples, methods, and results are diverse and an involvement of confounding factors, such as breathing problems, depression or social insecurity cannot be ruled out. The present study examined the potential correlation between odor threshold in healthy participants and their sexual desire, sexual experience, and sexual performance. In 70 adults (28 male, 42 female; mean age 24.8 ± 4.1 years), odor threshold was assessed using the “Sniffin’ Sticks.” The participants also responded to a battery of questions on sexual desire (Sexual Desire Inventory), sexual experience (orgasm frequency, perceived pleasantness of sexual activities on a visual analogue scale) as well as sexual performance (frequency of having sex, average duration of sexual intercourse). Odor sensitivity correlated positively with sexual experience: Participants with high olfactory sensitivity reported higher pleasantness of sexual activities. Further, women with high olfactory sensitivity reported a higher frequency of orgasms during sexual intercourse. These findings were exclusively present for sexual experience; no significant correlations were detected for sexual desire or sexual performance. The experience of sexual interactions appears to be enriched by olfactory input. We discuss that the perception of certain body odors may contribute to the concept of sexual pleasure by enhanced recruitment of reward areas.

Purpose of ReviewThe sense of smell is today one of the focuses of interest in aging and neurodegenerative disease research. In several neurodegenerative diseases, such as Parkinson’s disease and Alzheimer’s disease, the olfactory dysfunction is one of the initial symptoms appearing years before motor symptoms and cognitive decline, being considered a clinical marker of these diseases’ early stages and a marker of disease progression and cognitive decline. Overall and under the umbrella of precision medicine, attention to olfactory function may help to improve chances of success for neuroprotective and disease-modifying therapeutic strategies.Recent FindingsThe use of olfaction, as clinical marker for neurodegenerative diseases is helpful in the characterization of prodromal stages of these diseases, early diagnostic strategies, differential diagnosis, and potentially prediction of treatment success. Understanding the mechanisms underlying olfactory dysfunction is central to determine its association with neurodegenerative disorders. Several anatomical systems and environmental factors may underlie or contribute to olfactory loss associated with neurological diseases, although the direct biological link to each disorder remains unclear and, thus, requires further investigation.SummaryIn this review, we describe the neurobiology of olfaction, and the most common olfactory function measurements in neurodegenerative diseases. We also highlight the evidence for the presence of olfactory dysfunction in several neurodegenerative diseases, its value as a clinical marker for early stages of the diseases when combined with other clinical, biological, and neuroimage markers, and its role as a useful symptom for the differential diagnosis and follow-up of disease. The neuropathological correlations and the changes in neurotransmitter systems related with olfactory dysfunction in the neurodegenerative diseases are also described.

Random walks describe stochastic processes characterized by a sequence of unpredictable changes in a random variable with no correlation to past changes. This report describes the random walk component of a clinical sensory test of olfactory performance. The precise definition of this stochastic process allows the establishment of precise diagnostic cut-offs for the identification of olfactory loss. Within the Sniffin`Sticks olfactory test battery, odor discrimination (D) and odor identification (I) are assessed by four- and three-alternative forced-choice designs, respectively. Meanwhile, the odor threshold (T) test integrates a three-alternative forced-choice paradigm within a staircase paradigm with seven turning points. We explored this paradigm through computer simulations and provided a formal description. The odor threshold assessment test consists of two sequential components, the first of which sets the starting point for the second. Both parts can be characterized as biased random walks with significantly different probabilities of moving to higher (11%) or lower (89%) values. The initial odor concentration step for the first phase of the test and the length of the subsequent random walk in the second phase significantly affect the probability of randomly achieving high test scores. Changing the odor concentration from where the starting point determination for the second test part begins has raised the current cut-off for anosmia, represented as T + D + I < 16, from the 87th quantile of random test scores to the 97th quantile. Analogous findings are likely applicable to other sensory tests that use the staircase paradigm characterized as random walk.

Depression is associated with reduced olfactory function. This relationship is assumed to be based on either a reduced olfactory bulb volume or diminished functioning of higher cortical areas. As previous results are controversial, we aimed to re-evaluate central olfactory processing in depression. We recorded the BOLD signal of 21 patients with Major Depressive Disorder and 21 age and gender matched healthy controls during odor presentation. In addition, we measured the individual olfactory bulb volume, tested odor identification and odor threshold, and asked for hedonic odor perception. In both groups, odor presentation led to a pronounced activation of primary olfactory areas. However, secondary olfactory areas were significantly less activated in depressed individuals. The two groups did not differ in olfactory bulb volume. Our results point towards altered olfactory processing in patients in those regions that relate to sensory integration and attention allocation. Difficulties in cognitive processing could impact olfactory function in depression. We are therefore in favor of a top-down mechanism originating in higher cortical areas explaining parts of the relation between depression and olfaction.

•Odors are sequentially processed in the primary and secondary olfactory cortices.•Diffusion MRI helps in visualization white matter streamlines connecting olfactory brain regions.•Piriform cortex and orbitofrontal cortex are intensively connected and have higher streamline number in the right hemisphere and correlate with thresholds for irritating (trigeminal) odors.•The number of streamlines connecting piriform cortex and thalamus is higher on the left side and correlates with odor intensity for trigeminal odors. Odorous sensations are based on trigeminal and olfactory perceptions. Both trigeminal and olfactory stimuli generate overlapping as well as distinctive activations in the olfactory cortex including the piriform cortex. Orbitofrontal cortex (OFC), an integrative center for all senses, is directly activated in the presence of olfactory stimulations. In contrast, the thalamus, a very important midbrain structure, is not directly activated in the presence of odors, but rather acts as a relay for portions of olfactory information between primary olfactory cortex and higher-order processing centers. The aims of the study were (1) to examine the number of streamlines between the piriform cortex and the OFC and also between the piriform cortex and the thalamus and (2) to explore potential correlations between these streamlines and trigeminal and olfactory chemosensory perceptions. Thirty-eight healthy subjects were recruited for the study and underwent diffusion MRI using a 3T MRI scanner with 67 diffusion directions. ROIs were adapted from two studies looking into olfaction in terms of functional and structural properties of the olfactory system. The “waytotal number” was used which corresponds to number of streamlines between two regions of interests. We found the number of streamlines between the piriform cortex and the thalamus to be higher in the left hemisphere, whereas the number of streamlines between the piriform cortex and the OFC were higher in the right hemisphere. We also found streamlines between the piriform cortex and the thalamus to be positively correlated with the intensity of irritating (trigeminal) odors. On the other hand, streamlines between the piriform cortex and the OFC were correlated with the threshold scores for these trigeminal odors. This is the first studying the correlations between streamlines and olfactory scores using tractography. Results suggest that different chemosensory stimuli are processed through different networks in the chemosensory system involving the thalamus.