Spatial navigation entails an analysis and understanding of the structure of scenes in the broader environment. The perception of scenes in turn relies on the integration of multisensory cues such as the layout of textures and objects in front of the eyes and wafts of scents in the air. How visual and olfactory inputs are continuously constructed into unified percepts of scenes in the human brain is scantly known. Here two studies are conducted to explore the neural mechanisms underlying the synthesis of olfactory and visual cues in scene perception.
Study 1 comprises 3 experiments that collectively assess whether and how the congruency between olfactory and visual cues facilitates scene perception. Through formal psychophysical testing, we demonstrate in Experiments 1 and 2 that the presence of a congruent as opposed to incongruent odor expedites one's recognition of a scene independent of semantic priming. Experiment 3 employs functional magnetic resonance imaging (fMRI) to probe how such congruency acts upon the representations of scenes in the brain. Results show that the parahippocampal place area (PPA) exhibits enhanced responses and refined spatial activity patterns to visual scenes under the exposures of congruent as compared to incongruent odors. This is accompanied by strengthened functional connectivities between PPA and the orbitofrontal cortex一a region underlying olfactory-visual integration, and between PPA and the piriform cortex.
Study 2 examines the interplays between olfactory and visual cues in the neural computations of distance. Participants were presented with video clips that simulate movements into and away from various scenes as well as odors congruent or incongruent with the scenes that either increase or decrease in odor strength while fMRI images were acquired. Results indicate that visual distance is represented in scene processing areas including the parahippocampal place area, retrosplenial complex, and occipital place area, where enhanced neural responses are observed for receding as opposed to approaching scenes. Olfactory distance, on the other hand, is processed in the piriform and orbitofrontal cortices, where enhanced neural activities are observed for approaching rather than receding odors. The olfactory and visual distance cues are integrated in the orbitofrontal cortex.
These findings underscore the role of olfaction in the neural respresentations of scene category and distance. Moreover, they provide fresh insights into the mechanisms underlying the synthesis of multisensory cues in scene perception and spatial navigation.
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