A systematic review of physiological responses to odours with a focus on current methods used in event-related study designs

Time-Dependent Analysis of Human Neurophysiological Activities during an Ecological Olfactory Experience

It has been demonstrated that odors could affect humans at the psychophysiological level. Significant research has been done on odor perception and physiological mechanisms; however, this research was mainly performed in highly controlled conditions in order to highlight the perceptive phenomena and the correlated physiological responses in the time frame of milliseconds. The present study explored how human physiological activity evolves in response to different odor conditions during an ecological olfactory experience on a broader time scale (from 1 to 90 s). Two odors, vanilla and menthol, together with a control condition (blank) were employed as stimuli. Electroencephalographic (EEG) activity in four frequency bands of interest, theta, alpha, low beta, and high beta, and the electrodermal activity (EDA) of the skin conductance level and response (SCL and SCR) were investigated at five time points taken during: (i) the first ten seconds of exposure (short-term analysis) and (ii) throughout the entire exposure to each odor (90 s, long-term analysis). The results revealed significant interactions between the odor conditions and the time periods in the short-term analysis for the overall frontal activity in the theta (p = 0.03), alpha (p = 0.005), and low beta (p = 0.0067) bands, the frontal midline activity in the alpha (p = 0.015) and low beta (p = 0.02) bands, and the SCR component (p = 0.024). For the long-term effects, instead, only one EEG parameter, frontal alpha asymmetry, was significantly sensitive to the considered dimensions (p = 0.037). In conclusion, the present research determined the physiological response to different odor conditions, also demonstrating the sensitivity of the employed parameters in characterizing the dynamic of such response during the time. As an exploratory study, this work points out the relevance of considering the effects of continuous exposure instead of short stimulation when evaluating the human olfactory experience, providing insights for future studies in the field.

Regulation of brain cognitive states through auditory, gustatory, and olfactory stimulation with wearable monitoring

Inspired by advances in wearable technologies, we design and perform human-subject experiments. We aim to investigate the effects of applying safe actuation (i.e., auditory, gustatory, and olfactory) for the purpose of regulating cognitive arousal and enhancing the performance states. In two proposed experiments, subjects are asked to perform a working memory experiment called n-back tasks. Next, we incorporate listening to different types of music, drinking coffee, and smelling perfume as safe actuators. We employ signal processing methods to seamlessly infer participants' brain cognitive states. The results demonstrate the effectiveness of the proposed safe actuation in regulating the arousal state and enhancing performance levels. Employing only wearable devices for human monitoring and using safe actuation intervention are the key components of the proposed experiments. Our dataset fills the existing gap of the lack of publicly available datasets for the self-management of internal brain states using wearable devices and safe everyday actuators. This dataset enables further machine learning and system identification investigations to facilitate future smart work environments. This would lead us to the ultimate idea of developing practical automated personalized closed-loop architectures for managing internal brain states and enhancing the quality of life.

Relaxing effects of music and odors on physiological recovery after cognitive stress and unexpected absence of multisensory benefit

Several studies have described, often separately, the relaxing effects of music or odor on the autonomic nervous system (ANS) activity. Only a few studies compared the presentation of these stimuli and their interaction within a same experimental protocol. Here, we examined whether relaxing music (slow-paced classical pieces) and odor (lavender essential oil) either presented in isolation or in combination would facilitate physiological recovery after cognitive stress. We continuously recorded the electrocardiogram to assess the high-frequency component of heart rate variability (HF-HRV), an index of parasympathetic activity, and electrodermal activity (EDA), an index of sympathetic activity, 10 min before, during and 30 min after a cognitive stress (i.e., completing timely constrained cognitively demanding tasks) in 99 participants allocated to four recovery conditions (control N = 26, music N = 23, odor N = 24, music+odor N = 26). The stressing event triggered both a significant increase in EDA and decrease in HF-HRV (compared to baseline). During the recovery period, the odor elicited a greater decrease in EDA compared to an odorless silent control, whereas no difference in HRV was observed. Conversely, during this period, music elicited a greater increase in HF-HRV compared to control whereas no difference in EDA was observed. Strikingly, in the multimodal music+odor condition, no beneficial effect was observed on ANS indexes 30 min after stress. Overall, our study confirms that both olfactory and musical stimuli have relaxing effects after stress on ANS when presented separately only, which might rely on distinct neural mechanisms and autonomic pathways.

Citrus Essential Oils in Aromatherapy: Therapeutic Effects and Mechanisms

Citrus is one of the main fruit crops cultivated in tropical and subtropical regions worldwide. Approximately half (40-47%) of the fruit mass is inedible and discarded as waste after processing, which causes pollution to the environment. Essential oils (EOs) are aromatic compounds found in significant quantities in oil sacs or oil glands present in the leaves, flowers, and fruit peels (mainly the flavedo part). Citrus EO is a complex mixture of ~400 compounds and has been found to be useful in aromatic infusions for personal health care, perfumes, pharmaceuticals, color enhancers in foods and beverages, and aromatherapy. The citrus EOs possess a pleasant scent, and impart relaxing, calming, mood-uplifting, and cheer-enhancing effects. In aromatherapy, it is applied either in message oils or in diffusion sprays for homes and vehicle sittings. The diffusion creates a fresh feeling and enhances relaxation from stress and anxiety and helps uplifting mood and boosting emotional and physical energy. This review presents a comprehensive outlook on the composition, properties, characterization, and mechanism of action of the citrus EOs in various health-related issues, with a focus on its antioxidant properties.

Measurement of Stress Relief during Scented Cosmetic Product Application Using a Mood Questionnaire, Stress Hormone Levels and Brain Activation

Nowadays, consumers’ well-being plays a decisive role in the purchase of cosmetic products. Although factors influencing consumers' well-being are very subjective, companies strive to develop their products in such a way that a positive effect is likely. Therefore, methods are required to objectively explore and scientifically prove the product’s performance on humans. In this placebo-controlled study, a method was developed to evaluate relaxation or stress relief associated with one olfactory ingredient of a cosmetic product (face cream). Our experimental protocol included product testing in 25 healthy females, while an emotion questionnaire, analysis of saliva samples regarding the concentration of the hormones cortisol and α-amylase and mobile EEG measurement for quantification of the alpha brain waves before and after stress induction were conducted. It was shown that with this experimental design, the sample with the ingredient produced significant stress relief, as evidenced by significantly less negative emotion, significantly lowered cortisol levels and showed a trend towards a significant increase in alpha activity compared to placebo application. Our data provide evidence that this method is suitable for analyzing the differences between the two samples. In the future, this method can be utilized in the current or a further optimized form to evaluate the psychophysiological effects of cosmetic products on humans.

Odour enhances the sense of presence in a virtual reality environment

Virtual reality (VR) headsets provide immersive audio-visual experiences for users, but usually neglect to provide olfactory cues that can provide additional information about our environment in the real world. This paper examines whether the introduction of smells into the VR environment enhances users’ experience, including their sense of presence through collection of both psychological and physiological measures. Using precise odour administration with an olfactometer, study participants were exposed to smells while they were immersed in the popular PlayStation VR game “Resident Evil 7”. A within-subject study design was undertaken where participants (n = 22) walked-through the same VR environment twice, with or without the introduction of associated congruent odour stimuli. Directly after each gameplay, participants completed a questionnaire to determine their sense of presence from the overall gameplay and their sense of immersion in each of the virtual scenes. Additionally, physiological measurements (heart rate, body temperature and skin electrodermal activity) were collected from participants (n = 11) for each gameplay. The results showed the addition of odours significantly increased participants’ sense of spatial presence in the VR environment compared to VR with no odour. Participants also rated the realism of VR experience with odour higher compared to no odour, however odour addition did not result in change in emotional state of participants (arousal, pleasure, dominance). Further, the participants’ physiological responses were impacted by the addition of odour. Odour mediated physiological changes were dependent on whether the VR environment was novel, as the effect of odour on physiological response was lost when participants experienced the aroma on the second gameplay. Overall, the results indicate the addition of odours to a VR environment had a significant effect on both the psychological and physiological experience showing the addition of smell enhanced the VR environment. The incorporation of odours to VR environments presents an opportunity to create a more immersive experience to increase a person’s presence within a VR environment. In addition to gaming, the results have broader applications for virtual training environments and virtual reality exposure therapy.

Food olfactory cues reactivity in individuals with obesity and the contribution of alexithymia

Obesity has been associated with increased reward sensitivity to food stimuli, but a few studies have addressed this issue by using odors. This study investigated whether obesity is associated with increased liking and wanting of food odors and whether alexithymia, a psychological construct characterized by diminished affective abilities, contributes to altered responsiveness to food. Liking and wanting for food and pleasant non-food odors were measured through explicit (self-report ratings) and implicit measures (heart rate and skin conductance) in 23 women with healthy weight (HW) and 20 women with overweight/obesity (OW/OB). Differently from the HW group, the OW/OB group explicitly liked food odors less than non-food odors; but, at the implicit level, there were no differences in heart rate response for both types of odors, indicating that they were equally liked. Moreover, at variance with the HW group, the OW/OB group did not exhibit increased skin conductance response for food compared to nonfood odors. Alexithymia was associated with increased implicit liking and explicit wanting of food odors, in particular in the HW group. These findings show that obesity is characterized by high levels of implicit food liking and low levels of implicit food wanting. Moreover, both affective and motivational responses to food reward seem to be affected by alexithymia, which should be taken into account by studies evaluating the effect of cue exposure intervention for obesity treatment.