Understanding the impact of pleasant odours on the emotional processing networks in the brain

Olfaction is one of the two chemical senses able to detect small volatile compounds found in the air. It is important for many behaviours such as food localisation, identifications of threats and danger, and possible mates. Anatomically, it is the only sense presenting a trajectory that bypasses the thalamus, region known for its main role in emotional modulation. Because of this peculiarity, many researchers have investigated the interaction between olfaction and the emotional network, using a wide range of methodologies. It has been found that the brain processes odorants based on their hedonic valence. Most EEG studies have shown the evidence of this interaction in the late positive potential (LPP) components as odour-valence modulation of face/body expression recognition. Moreover, fMRI results suggested that the piriform cortex is mainly associated to subjective odour intensity, while the posterior orbitofrontal cortex is mainly associated to the subjective hedonic tone of the odorant. However, most of the research has only investigated this interaction using odours with very different hedonic tones. Hence, our aim is to focus specifically on how pleasant odours, therefore with similar hedonic tone, influence the emotional processing neural networks, by using neuroimaging techniques. Moreover, we are investigating this influence on emotional judgements towards external stimuli (pictures of faces, objects, and scenario) and on self-reported internal moods. This research has many applications, especially in the industrial and engineering setting and in the medical field (e.g., aromatherapy).
In the first year, a scoping review was conducted to understand the volume of previous findings regarding this interaction and identify the gaps in the literature. The eligible criteria required only human-based studies, involving neuroimaging techniques, aiming to investigate specifically the interaction between odours and emotions. Based on its findings, the PhD general aim and question were formulated. After, a behavioural within-subjects study was conducted as pilot, to select the best stimuli. The odours were Vanilla, Rose, Lavender, Lemon and clean air (as control); they were delivered to the participant's nostrils using an olfactometer. As visual stimuli, pictures showing happy and neutral faces, emotional and neutral objects, and emotional and neutral landscapes were selected. For each picture, participants were required to rate the pleasantness while smelling each odour. Moreover, they were required to rate their emotional states (nostalgic, happy, disgusted, thirsty, sensual, relaxed, and hungry) during each odour. The quantitative analysis is conducted using ANOVAs and linear modelling. This study's findings will lead to the proper set of stimuli for the next study, which will be an EEG research, aiming to find the neural correlates of the interaction between odours and emotions. It will be based on ERP or time-frequency analysis. Following this, during the third year, the current goal is to conduct an fMRI study to further investigate this interaction and map the brain regions involved.
Since the hypotheses are merely exploratory, proper expectations are not made yet. However, based on the most consistent findings, vanilla might be the odour with the strongest influence on improving emotional judgements and internal moods. This could due to the strict similarity with the breastfeeding odour, which resemble the vanilla one.