ELECTROENCEPHALOGRAM ALTERATIONS DURING PERCEPTION OF PLEASANT AND UNPLEASANT ODORS Ashkan Yazdani 1 , Eleni Kroupi 1 , Jean-Marc Vesin 2 , Touradj Ebrahimi 1 1 Multimedia Signal Processing Group (MMSPG), Institute of Electrical Engineering, Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Lausanne, Switzerland. 2 Applied Signal Processing Group (ASPG), Institute of Electrical Engineering, Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Lausanne, Switzerland. ABSTRACT The olfactory system enables humans and many animals to recognize and categorize different odors and can determine many behavioral and social reactions. For human beings, odor stimuli are highly associated with many processes such as emotions, attraction, mood, etc. One approach to under- standing the olfaction is to monitor and analyze human brain activity during perception of odors. In this paper, we analyze the electroencephalogram (EEG) of five subjects during per- ception of unpleasant and pleasant odor stimuli. We identify the regions of the brain that are active during discrimination of unpleasant and pleasant odor stimuli. We also show that classification of EEG signals during perception of odors can reveal the pleasantness of the odor with relatively high accu- racy. Index Terms— Olfaction, Odor, EEG, Brain, Signal Pro- cessing 1. INTRODUCTION For many animals the olfactory sense is the most important and thus the most evolved sense. It allows them to inter- act better with their environment by helping them take right decisions for survival such as to identify food, mates, prays and also escape from their predators. Although human be- ings appear less driven by olfaction than many other animals, human olfaction has a particular importance and can deter- mine many behavioral and social reactions. Odor stimuli are highly associated with attraction, mood, detection of danger and they have the power to evoke distant memories and boost self-confidence. Odor stimuli are created by volatilized chemical com- pounds and are directly perceived by the human olfactory The research leading to these results has been performed in the frame- works of Swiss National Foundation for Scientific Research (FN 200020- 132673-1), and the NCCR Interactive Multimodal Information Management (IM2). Furthermore, the authors would like to thank all subjects, who kindly participated in the experiments. Last but not least, we would like thank Dr. Karin Diserens and Dr. Etienne Pralong for their useful inputs on experimen- tal paradigm design. bulb through the olfactory epithelium. In general, odors can be measured from two different aspects, namely odor inten- sity and odor pleasantness. Odor intensity is perceived as the strength of an odor, and can range from no odor to intoler- able. Furthermore, hedonic assessment of odors can scale from extremely unpleasant up to extremely pleasant. While smelling an odor, olfactory neurons located inside the nasal cavity fire and send special electrical impulses to olfactory bulb. The information then flows from the olfactory bulb to higher cortical areas, such as the piriform cortex [1]. This cortical region has a unique architecture and a unique contribution to odor encoding and perception, so it has been extensively studied [2, 3, 4]. One approach to understanding olfaction and odor per- ception is to study the changes in the brain electrical activity after smelling an odor. Several studies have tried to capture the oscillatory nature of the olfactory perception by means of EEG signal processing. However, there exists a great deal of variability in terms of the reported findings. For instance, dif- ferences in the amount of alpha activity and an increase of theta activity were observed when subjects were exposed to fragrances and the EEG was recorded from 4 electrodes [5]. However, other authors suggested that there are no changes in alpha activity but there are increases in theta activity [5, 6]. Moreover, in a more recent study [7] a decrease in the spec- tral power of theta and alpha bands was found while smelling essential oils. In the same study, an increase in the high fre- quency EEG components (11-25 Hz) was reported. One possible reason for the variability among EEG stud- ies and olfactory psychophysiology is that in addition to the direct activation of the olfactory bulb and the piriform cortex, the human olfactory path is interconnected with other corti- cal and limbic structures, such as the amygdala [1]. The role of the amygdala in emotion, memory, and autonomic control directly associates olfaction to these processes and adds com- plexity to the odor perceptual experience. Therefore it is of great importance to investigate the affective dimension of ol- factory perception. In affect recognition research, several different audio/visual