Primary sensory cortices can respond not only to their defined sensory modality but also to cross-modal information. In addition to the observed cross-modal phenomenon, it is valuable to research further whether cross-modal information can be valuable for categorizing stimuli and what effect other factors, such as experience and imagination, may have on cross-modal processing. In this study, we researched cross-modal information processing in the early visual cortex (EVC, including the visual area 1, 2, and 3 (V1, V2, and V3)) and auditory cortex (primary (A1) and secondary (A2) auditory cortex). Images and sound clips were presented to participants separately in two experiments in which participants' imagination and expectations were restricted by an orthogonal fixation task and the data were collected by functional magnetic resonance imaging (fMRI). We successfully decoded categories of the cross-modal stimuli in the ROIs except for V1 by multi-voxel pattern analysis (MVPA). It was further shown that familiar sounds had the advantage of classification accuracies in V2 and V3 when compared with unfamiliar sounds. The results of the cross-classification analysis showed that there was no significant similarity between the activity patterns induced by different stimulus modalities. Even though the cross-modal representation is robust when considering the restriction of top-down expectations and mental imagery in our experiments, the sound experience showed effects on cross-modal representation in V2 and V3. In addition, primary sensory cortices may receive information from different modalities in different ways, so the activity patterns between two modalities were not similar enough to complete the cross-classification successfully.
Keywords: Auditory cortex; Cross-modal; Early visual cortex; MVPA; fMRI.