Multimodal Frequency Representations Are Embedded In Modality-defined Cortical Sensory Systems
Sensory information is represented and elaborated in hierarchical cortical systems that are thought to be dedicated to individual sensory modalities. This traditional view of sensory cortex organization has been challenged by recent evidence of multimodal responses in primary and association sensory areas. Although it is indisputable that sensory areas respond to multiple modalities, it remains unclear whether these multimodal responses reflect selective information processing for particular stimulus features. Here, we used fMRI adaptation to identify brain regions that are sensitive to the temporal frequency information contained in auditory, tactile, and audiotactile stimulus sequences. A number of brain regions distributed over the parietal and temporal lobes exhibited frequency-selective temporal response modulation for both auditory and tactile stimulus events, as indexed by repetition suppression effects. A smaller set of regions responded to crossmodal adaptation sequences in a frequency-dependent manner. Despite an extensive overlap of multimodal frequency-selective responses across the parietal and temporal lobes, representational similarity analysis revealed a cortical “regional landscape” that clearly reflected distinct somatosensory and auditory processing systems that converged on modality-invariant areas. These structured relationships between brain regions were also evident in spontaneous signal fluctuation patterns measured at rest. Our results reveal that multimodal processing in human cortex can be feature-specific and that multimodal frequency representations are embedded in the intrinsically hierarchical organization of cortical sensory systems.
A hallmark of traditional brain organization models is the segregation of signals from the different senses in modality-dedicated brain regions. Recent evidence showing multimodal activity in brain regions thought to be dedicated to a single modality have challenged the traditional sensory cortex model. Notably, few studies have explored the feature-specificity of multimodal responses found in sensory cortex. Here, we used fMRI adaptation to identify parietal and temporal cortex regions which exhibited sensitivity to both tactile and auditory frequency information. These univariate results demonstrate that multimodal processing in sensory cortex can be feature-specific. Using the same data, though, we found clear evidence of modality-based cortical organization estimated from multivariate response patterns and spontaneous BOLD signal fluctuations. Thus, our results reveal an embedding of feature-specific multimodal processing in traditionally-defined cortical systems.