Striking parallel between Tonotopy in Auditory Cortex and Retinotopy in Visual Cortex: A human fMRI study at 7 Tesla

Abstract

In contrast to visual retinotopic mapping with fMRI, auditory tonotopic mapping has not yet provided a clear picture of the human auditory cortex because map details are just below the spatial resolution of standard functional imaging techniques. As a result, the exact number and location of tonotopic maps in the human auditory cortex (including the primary and secondary regions) remains largely unknown. Here, using ultra-high field strength fMRI (7T) with voxel volumes as low as 1.7 mm³, we have imaged tonotopic maps in 10 human subjects, and provide the clearest measures of human tonotopy to-date. A phase-mapping approach was used, similar to that commonly used in retinotopic mapping. The results are highly consistent in 20 out of 20 hemispheres and clearly demonstrate that iso-frequency lines run parallel to the long-axis of Heschl's gryus (settling a long-standing debate about the orientation of the primary maps). Furthermore, the results suggest a striking and previously unknown organizational parallel with early visual cortex. Specifically, the low frequency union of two mirror-symmetric tonotopic maps (border between primary areas A1 and R) is consistently located on the crown of the gyrus, thus bringing common frequency bands on the two maps closer together in space along the sides of the gyrus. A similar pattern is known to exist in the visual cortex, where the unions of mirror-symmetric retinotopic maps (borders between V1 and V2) also occur on a cortical fold thus shortening cortical distances between common retinotopic points on the two maps. This phenomenon in the visual system has been the primary argument for an influential hypothesis (Van Essen 1997) that cortical folds occur as a result of axonal tension between interconnected regions. In summary, our results significantly clarify the organization of human auditory cortex, and also suggest a common pattern with early-visual cortex.

Swiss National Science Foundation.