Identification of an object based on its odor alone is inherently difficult, but becomes easier when other senses provide supporting cues. This suggests that crossmodal sensory input facilitates neural processing of olfactory object information; however, direct evidence is still lacking. Here, we tested the effect of multisensory stimulation on information processing in the human posterior piriform cortex (PPC), a region linked to olfactory object encoding. Participants were exposed to familiar objects in the form of uni-, bi-, and trimodal combinations of odors, videos, and sounds. We hypothesized that the PPC would respond to non-olfactory object information, and that activity would increase linearly with the number of senses providing relevant object information. As predicted, visual object information activated the PPC and activity increased linearly with the number of relevant sensory channels. The crossmodal response pattern thus indicates that the PPC does not exclusively respond to olfactory information, but also to crossmodal object information important for olfactory processing. The continuous activity increase suggests that the PPC further acts as a multisensory binding site where pertinent input from multiple senses results in an increased neural response to the odor object. This potentially represents a neural mechanism for the well-known behavioral improvement present in odor object recognition during concurrent crossmodal sensory stimulation.

Auditory and visual sensory loss has repeatedly been shown to alter abilities in remaining sensory modalities. It is, however, unclear whether sensory loss also impacts multisensory integration; an ability that is fundamental for the perception of the world around us. We determined effects of olfactory sensory deprivation on multisensory perception by assessing temporal as well as semantic aspects of audio-visual integration in 37 individuals with anosmia (complete olfactory sensory loss) and 37 healthy, matched controls. Participants performed a simultaneity judgement task to determine the temporal binding window, and a multisensory object identification task with individually degraded, dynamic visual, auditory, and audio-visual stimuli. Individuals with anosmia demonstrated an increased ability to detect multisensory temporal asynchronies, represented by a narrowing of the audio-visual temporal binding window. Furthermore, individuals with congenital, but not acquired, anosmia demonstrated indications of greater benefits from bimodal, as compared to unimodal, stimulus presentation when faced with degraded, semantic information. This suggests that the absence of the olfactory sense alters multisensory integration of remaining senses by sharpening the perception of cross-modal temporal violations, independent of sensory loss etiology. In addition, congenital sensory loss may further lead to increased gain from multisensory, compared to unisensory, information. Taken together, multisensory compensatory mechanisms at different levels of perceptual complexity are present in individuals with anosmia.