Odors assist the categorization of ambiguous visual stimuli

Open access meeting abstract CC-BY-NC-ND.Visual categorization is the rapid and automatic ability to provide a similar response to different exemplars of a single category despite widely variable sensory inputs. Whether visual categorization is solely visually-driven or can be influenced by other sensory modalities remains unclear. Here we test whether odor contexts can modulate visual categorization in humans, expecting that congruent odors facilitate the categorization of visual objects as a function of their ambiguity. Scalp electroencephalogram (EEG) was recorded in N = 26 participants while naturalistic object stimuli were displayed in rapid 12-Hz streams (i.e., 12 images / second, leading to a 12-Hz general response in the EEG frequency spectrum). Variable exemplars of a target category (human faces, cars, or facelike objects in different sequences) were interspersed every 9 stimuli to tag category-selective EEG responses at 12/9 = 1.33 Hz. During visual stimulation, odor contexts (body, gasoline or baseline odors) were implicitly diffused. Category-selective responses to every category are clearly isolated over the occipito-temporal cortex, with the largest response for human faces and the lowest for facelike objects. Importantly, body odors enhance the right-hemispheric response to the ambiguous category, i.e., facelike objects, which are either perceived as nonface objects or faces. This odor effect is especially found in participants who noticed the illusory faces during visual stimulation. In contrast, odor contexts do not modulate other category-selective responses, nor the 12-Hz general response, revealing a selective facilitation of the visual categorization of congruent ambiguous stimuli. Altogether, these observations indicate that the visual system can rely on non-visual cues for efficient categorization, and that olfaction, which is often considered as poor in humans, is ideally suited to assist visual categorization