Smell what you hardly see: Odors assist categorization in the human visual cortex

Abstract

Visual categorization is the brain ability to rapidly and automatically respond to widely variable visual inputs in a category-selective manner (i.e., distinct responses between categories and similar responses within categories). Whether category-selective neural responses are purely visual or can be influenced by other sensory modalities remains unclear. Here, we test whether odors modulate visual categorization, expecting that odors facilitate the neural categorization of congruent visual objects, especially when the visual category is ambiguous. Scalp electroencephalogram (EEG) was recorded while natural images depicting various objects were displayed in rapid 12-Hz streams (i.e., 12 images / second) and variable exemplars of a target category (either human faces, cars, or facelike objects in dedicated sequences) were interleaved every 9th stimulus to tag category-selective responses at 12/9 = 1.33 Hz in the EEG frequency spectrum. During visual stimulation, participants (N = 26) were implicitly exposed to odor contexts (either body, gasoline or baseline odors) and performed an orthogonal cross-detection task. We identify clear category-selective responses to every category over the occipito-temporal cortex, with the largest response for human faces and the lowest for facelike objects. Critically, body odor boosts the response to the ambiguous facelike objects (i.e., either perceived as nonface objects or faces) over the right hemisphere, especially for participants reporting their presence post-stimulation. By contrast, odors do not significantly modulate other category-selective responses, nor the general visual response recorded at 12 Hz, revealing a specific influence on the categorization of congruent ambiguous stimuli. Overall, these findings support the view that the brain actively uses cues from the different senses to readily categorize visual inputs, and that olfaction, which is generally considered as poorly functional in humans, is well placed to disambiguate visual information.

Olfaction in the Multisensory Processing of Faces: A Narrative Review of the Influence of Human Body Odors

“…If so, the younger they are infants might prove more prone to olfaction-vision associative learning, and the influence of olfaction on face processing might reveal negatively associated with the development of visual perceptual skills in infancy. Indirect evidence suggests such a decrease of the weight of olfaction in adulthood compared to early infancy, since in adults the implicit exposure of human body odor no longer enhances the neural categorization response of faces as it does in infants (Leleu et al, 2020;Rekow et al, 2021a). Interestingly, an effect of body odor is however found on the adults' brain response when visual perception is made more challenging (i.e., by using ambiguous face-like objects as stimuli presented for brief durations with forward-and backwardmasking, Rekow et al, 2021a).…”