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

嗅觉是两种化学感官之一，能够探测到空气中发现的微小挥发性化合物。它对许多行为都很重要，比如食物定位，识别威胁和危险，以及可能的伴侣。从解剖学上讲，它是唯一呈现出绕过丘脑的轨迹的感觉，丘脑是以其在情绪调节中的主要作用而闻名的区域。基于这一特点，许多研究者运用多种方法对嗅觉与情绪网络之间的相互作用进行了研究。人们已经发现，大脑根据气味的享乐性价态来处理气味。大多数EEG研究表明，这种相互作用在晚期正电位(LPP)成分中的证据是面部/身体表情识别的气味价调制。此外，功能磁共振成像结果表明，梨状皮质主要与主观气味强度有关，而后眼眶额叶皮质主要与气味的主观享乐基调有关。然而，大多数研究只是使用不同享乐基调的气味来研究这种相互作用。因此，我们的目标是通过使用神经成像技术，特别关注具有相似享乐基调的令人愉快的气味如何影响情绪处理神经网络。此外，我们正在调查这种对外部刺激(面孔、物体和场景的图片)的情绪判断和自我报告的内部情绪的影响。这项研究有许多应用，特别是在工业和工程环境中以及在医学领域(例如，芳香疗法)。在第一年，进行了范围界定审查，以了解关于这种相互作用的先前发现的数量，并找出文献中的差距。符合条件的标准只需要进行基于人类的研究，包括神经成像技术，旨在专门研究气味和情绪之间的相互作用。在此基础上，提出了博士学位的总体目标和问题。之后，作为试点，进行了受试者内部的行为研究，以选择最佳刺激。这些气味是香草、玫瑰、薰衣草、柠檬和干净的空气(作为对照)；它们被用嗅觉仪传递到参与者的鼻孔。作为视觉刺激，选择了快乐和中性的面孔、情感和中性的物体以及情感和中性的风景的图片。对于每一张照片，参与者被要求在闻到每种气味时对愉悦程度进行评级。此外，他们还被要求在每一种气味中对自己的情绪状态(怀旧、快乐、厌恶、口渴、感官、放松和饥饿)进行评级。定量分析采用方差分析和线性建模。这项研究的发现将为下一项研究带来合适的刺激集，下一项研究将是一项旨在找到气味和情绪之间相互作用的神经关联的脑电研究。它将基于ERP或时频分析。接下来，在第三年，目前的目标是进行一项功能磁共振研究，以进一步研究这种相互作用并绘制相关的大脑区域。由于假设只是探索性的，还没有做出适当的预期。然而，根据最一致的发现，香草可能是对改善情绪判断和内在情绪影响最大的气味。这可能是因为与母乳喂养的气味非常相似，类似于香草的气味。

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.