The role of Odor Pleasantness in human learning: an electrophysiological approach
Abstract
Odors are a fundamental aspect of human perception, bridging the external chemical world with subjective experiences. Unlike other sensory modalities, odors lack a universal classification system. Reflecting the inherent complexity of olfactory perception. Odors are generated through interactions between chemical molecules present in the environment, known as odorants, and specialized olfactory receptors located at the beginning of the olfactory circuit. However, their perception is also shaped by different dimensions, including pleasantness, intensity, and trigeminal stimulation.
This thesis focuses on the odor dimension of pleasantness, investigating its electrophysiological correlates and their role in learning processes. While most studies on human olfaction have relied on neuroimaging techniques such as PET or fMRI to explore the brain regions involved in processing dimensions like intensity, odor identity, and pleasantness, these methods are limited by high costs and low temporal resolution. To address these limitations, this thesis employs electroencephalography (EEG), a cost-effective method with high temporal resolution. EEG captures the electrical activity generated by cortical neurons through electrodes placed on the scalp.
To understand the electrophysiological correlates of odor pleasantness and their interaction with different learning processes, a preliminary step was required: the development of an olfactory laboratory capable of delivering odors with precise temporal resolution. To achieve this, two versions of a device called an olfactometer were designed and built as part of this thesis.
Olfactometers are computer-controlled machines capable of delivering different odorants to a participant’s nose with high precision, enabling the conduction of event-related potential (ERP) studies involving odors. To ensure proper time-locking of events, the olfactometer was equipped with a sensor that synchronized the ERP recordings with the arrival of the odors at the participant’s nose.
Using the two olfactometers developed in this thesis, three studies were conducted to address different questions about the processing of odor pleasantness and its interaction with learning processes.
In Study 1, the role of odor-induced neural persistence in humans was investigated by examining how brief exposure to pleasant and unpleasant odors influences memory formation through neural persistence. Event-related potential (ERP) analyses revealed that unpleasant odors significantly enhance memory confidence for visually encoded images. Furthermore, ERP results confirmed that odor cues elicit sustained neural activity in humans, with the magnitude of persistence depending on the odor’s valence. These findings highlight the critical role of odor valence in neural persistence and its impact on memory confidence.
Study 2 explored the role of odors as reinforcers in an associative learning task, focusing on their hedonic properties (pleasantness/unpleasantness) and associated brain activity. In this study participants performed a modified version of a monetary reward task in which pleasant and unpleasant odors were used as feedback, replacing monetary rewards. Behavioral results revealed that pleasant odors facilitated faster and greater learning compared to unpleasant odors. Scalp EEG analyses revealed beta-band activity (11–30 Hz) localized to centro-parietal regions in response to pleasant odor feedback, similar to patterns observed in monetary reward studies, but with distinct spatial distribution. Electrobulbogram (EBG) data showed beta activity for unpleasant odors and theta band activity for pleasant odors. These findings suggest that odors act as rewards in humans, with pleasantness and unpleasantness affecting learning in different ways.
Finally, Study 3 proposed and tested a potential application in the perfume industry. A proof-of-concept protocol was developed that leverages EEG activity to identify and recommend perfumes most likely to be preferred by an individual. By analyzing brain responses to olfactory stimuli, this approach successfully recommended perfumes aligned with individual preferences.
The three studies presented in this thesis contribute to our understanding of how olfactory pleasantness is reflected in EEG activity. Furthermore, they demonstrate how pleasantness, the primary dimension of odors, influences different types of learning processes.