Chemosensory perception in blind and sighted populations

Compared to the other senses, research in the chemical senses (trigeminal and olfactory systems) have received far less attention than other modalities such as vision and audition. Even though it is an emerging field, there are still numerous questions left to answer. The main goal of the present thesis was to further our understanding of the impact of various internal factors on chemosensory perception for which the literature remains particularly inconclusive in hopes to advance our comprehension of this modality. To do so, four objectives were established. The first objective (Study 1) was to determine the impact of an internal state (expectancy) on olfactory perception using odor labeling. The second objective (Study 2) was to investigate whether there is a nostril dominance in chemosensory perception and whether it is linked with handedness. Given the importance of visual cues in chemosensory processing, we also set out to investigate the influence of blindness on chemosensory processing. Therefore, the third objective (Study 3) was to determine the impact of blindness on wine odor categorization, a task particularly complex and ecologic. The fourth and final goal of the present thesis (Study 4) was to investigate odor localization in blind individuals, a task that has not yet been explored in this population. In light of the results of our four studies, we further discuss their impacts on our current comprehension of the chemical senses. The results obtained in Study 1 showed that positively labeled odors were systematically rated as being more edible, and this was true even for odors carrying non-food labels. Positively labeled odors were also rated as being more pleasant than negatively labeled ones, which is in line with previous work. Labels also modulated intensity ratings and reaction times, although to a lesser degree. These results highlight that “top-down” mechanisms can influence olfactory perception, and that previous experiences or different interpretations of a labeled odor can alter our perception of it. The results acquired in Study 2 found partial support for the presence of a nostril dominance in right-handed individuals, yet no effect was found for left-handed individuals. The heterogeneity found in these results mirrors the one found in the literature. Methodological differences as well as influences of the nasal cycle could partially explain the heterogeneity in the results. The results from Study 3 displayed worse performance in early-blind individuals compared to sighted controls on a wine odor categorization task. Since early-blind individuals lacked visual input to make visual-olfactory associations, they could have compensated with a stronger verbal-olfactory association. However, since our results show that blind individuals have a worse performance in wine odor categorization, this suggests that they are penalized for their lack of visual input, and any verbal-olfactory compensation is not enough to match the visual-olfactory association. Put differently; it seems that blind individuals lack mental imagery abilities which are important when undertaking this type of task. Finally, in Study 4, results show that congenitally-blind individuals outperform late-blind individuals as well as sighted controls on an odor localization task. This finding provides support for the idea that congenitally-blind individuals rely more on their chemosensory perception to navigate throughout their environment, by retrieving spatial information from the surrounding odors.

Keyphrases: olfaction, odorat, trigeminal, odors, chemosensory perception, blindness, congenitally-blind, early-blind, late-blind, odor-labelling, nostril advantage