Considerable research shows that olfactory stimulations affect other modalities in high-level cognitive functions such as emotion. However, little known fact is that olfaction modulates low-level perception of other sensory modalities. Although some studies showed that olfaction had influenced on the other low-level perception, all of them required specific experiences like perceptual training. To test the possibility that olfaction modulates low-level perception without training, we conducted a series of psychophysical and neuroimaging experiments. From the results of a visual task in which participants reported the speed of moving dots, we found that participants perceived the slower motions with a lemon smell and the faster motions with a vanilla smell, without any specific training. In functional magnetic resonance imaging (fMRI) studies, brain activities in the visual cortices [V1 and human middle temporal area (hMT)] changed based on the type of olfactory stimulation. Our findings provide us with the first direct evidence that olfaction modulates low-level visual perception without training, thereby indicating that olfactory-visual effect is not an acquired behavior but an innate behavior. The present results show us with a new crossmodal effect between olfaction and vision, and bring a unique opportunity to reconsider some fundamental roles of olfactory function.

In our previous experiments, it was shown that odors (rose, sandalwood) might influence the haptic perception (stiffness, roughness). In particular, subjects felt a smooth surface even smoother with rose odor and a stiff surface even harder with sandalwood odor. This study investigated whether the effects of odors could be explained by their categories that are derived from the verbal descriptions of odors. We used jasmine that is categorized as "flower", which includes rose, and peppermint that is categorized as "herb" close to sandalwood for our experiments. Subjects were requested to adjust the roughness or stiffness of a test surface to match that of a standard surface just shown before it using a force-feedback device. Standard surfaces were presented with different odors (jasmine, peppermint, odorless) which was projected to the noses of subjects. The results showed that jasmine made subjects feel the surfaces softer than those in odorless condition, but had no effect on roughness perception. Meanwhile peppermint did not show significant effect on either stiffness or roughness perception. According to the inconsistency between the results of present and previous experiments, we argue that odors in similar categories do not necessarily influence haptic perception in the same way.

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We receive different impressions from different visual images or odors, but it was not clear what kinds of impression the combination of visual images and odors elicits. Thus, we conducted experiments where we compared affective impressions when only olfactory information, only visual information, and combinations of olfactory and visual information were presented. In this experiment, the subjects were asked to rate the impression in ‘mild-sharp’ and ‘clear-dense’ dimensions using a visual-analogue-scale method when they were shown each of 8 different odors, 8 different visual images, or combinations of these different types of visual images and odors. The subjects were also asked to judge how much each specific combination of odor and visual image was harmonious on a 7-point scale. The results show some averaging effects when visual and olfactory information fell into different categories (i.e., mild, sharp, clear or dense) in the impression space. On the other hand, the impression of image–odor combination tended to be enhanced when the impression of odor or image alone fell into the same category along with the condition when they were judged as being harmonious. These results suggest that the way combination of visual and olfactory information elicits the impression may depend on the impression of individual visual and olfactory information and whether they are harmonious.