Whither unisensory olfactory cortex: processing of visual and auditory stimuli in olfactory cortex, independently of odor associations

Primary sensory cortices process atypical sensory objects in some conditions, which brings into question their presumed sensory specificity. Whether this atypical sensory processing originates from true atypical sensory processing or from cross-modal associations to objects is not known. Visual/auditory objects typically have strong reciprocal associations; hence, it is difficult to address this question in these modalities. Here, we instead use the olfactory system as a model system. We show that even in total absence of any odor reference, the posterior piriform cortex (PPC; odor object processing) is activated by both sounds and pictures of objects. This activation is independent of the objects odor associations, thereby demonstrating that the activity is not due to cross-modal associations. In the anterior piriform cortex (APC; low-level odor processing) we found activation by sounds, again independent of odor associations. Analogously, we found clear cross-modal effects in visual and auditory brain regions: Pictures activated higher-level auditory cortex and caused a decreased signal in primary auditory cortex, whereas sounds activated primary visual cortex and caused a decreased signal in higher-level visual cortex. Our findings demonstrate an atypical sensory processing that is independent of cross-modal associations in olfactory cortices and contribute to a more nuanced view of modality specificity in olfactory, auditory, and visual cortices.