Approaching the Ground Truth: Revealing the Functional Organization of Human Multisensory STC Using Ultra-High Field fMRI

Integrating inputs across sensory systems is a property of the brain that is vitally important in everyday life. More than two decades of fMRI research have revealed crucial insights on multisensory processing, yet the multisensory operations at the neuronal level in humans have remained largely unknown. Understanding the fine-scale spatial organization of multisensory brain regions is fundamental to shed light on their neuronal operations. Monkey electrophysiology revealed that the bimodal superior temporal cortex (bSTC) is topographically organized according to the modality preference (visual, auditory, and bimodal) of its neurons. In line with invasive studies, a previous 3 Tesla fMRI study suggests that the human bSTC is also topographically organized according to modality preference (visual, auditory, and bimodal) when analyzed at 1.6 x 1.6 x 1.6 mm³ voxel resolution. However, it is still unclear whether this resolution is able to unveil an accurate spatial organization of the human bSTC. This issue was addressed in the present study by investigating the spatial organization of functional responses of the bSTC in 10 participants (from both sexes) at 1.5 x 1.5 x 1.5 mm³ and 1.1 x 1.1 x 1.1 mm³ using ultra-high field fMRI (at 7 Tesla). Relative to 1.5 x 1.5 x 1.5 mm³, the bSTC at 1.1 x 1.1 x 1.1 mm³ resolution was characterized by a larger selectivity for visual and auditory modalities, stronger integrative responses in bimodal voxels, and it was organized in more distinct functional clusters indicating a more precise separation of underlying neuronal clusters. Our findings indicate that increasing the spatial resolution may be necessary and sufficient to achieve a more accurate functional topography of human multisensory integration.

SIGNIFICANCE STATEMENT The bimodal superior temporal cortex (bSTC) is a brain region that plays a crucial role in the integration of visual and auditory inputs. The aim of the present study was to investigate the fine-scale spatial organization of the bSTC by using ultra-high magnetic field fMRI at 7 Tesla. Mapping the functional topography of bSTC at a resolution of 1.1 x 1.1 x 1.1 mm³ revealed more accurate representations than at lower resolutions. This result indicates that standard-resolution fMRI may lead to wrong conclusions about the functional organization of the bSTC, whereas high spatial resolution is essential to more accurately approach neuronal operations of human multisensory integration.