Olfactory notes in the mind space

Olfaction is one of our most archaic senses but its central mechanisms are lesser known than those of other senses. In this work, we address a possible link between olfactory stimuli and mental spatial representations. Although olfactory percepts are not commonly related to space (as are visual or auditory percepts), perfumiers tend to describe scents in terms of top/head or base notes, using spatial terms, and arrange notes vertically on olfactory pyramids, with more volatile notes on top. Based on preliminary evidence, we tested in a well-powered, preregistered study whether odors evoke in naïve participants a mental vertical representation dependent on odor quality, in the absence of any explicit references to elevation. In a speeded choice classification task, 110 participants were asked to press one of two vertically aligned buttons in response to a series of olfactory stimuli. A spatial stimulus-response compatibility (SRC) effect was expected to emerge from alternative mappings (compatible vs incompatible) of stimuli to responses, due to the hypothesised dimensional overlap between odors and response alternatives. However, the preregistered contrast on means of median correct responses neither confirmed the presence of a vertical SRC effect at the group level, nor provided conclusive evidence for its absence. An analogous exploratory test on restricted means did support the presence of the predicted effect at the group level, whereas its Bayesian counterpart found the outcome inconclusive. Exploratory analyses revealed the presence of three distinct clusters of participants with regards to the vertical SRC effect for odors, with two clusters (N = 61 and N = 19) showing a significant effect in the expected direction and one cluster (N = 30) showing a significant effect in the opposite direction. These results call for replications with an approach that factors in potential sources of individual differences.