Tonotopic organization in monkey and human auditory cortex using phase-encoded functional MRI
Tonotopic organization is the main topographic feature of the primary auditory cortex. Importantly, tonotopy allows to define borders of cortical areas as reversals of the frequency gradient (i.e the spatial derivative of the preferred frequency map). Very different tonotopic organisations have been described with respect to Heschl’s gyrus in humans. Furthermore, human tonotopy remains difficult to relate to the organization in nonhuman primates and a comparison would require the same measurement, design and analysis in both species. Here, we measured blood oxygen leveldependent signal with functional MRI (fMRI) in humans and monkeys. We applied phase-encoded designs ('travelling wave') and performed surface-based analyses using high-resolution individual anatomical MRIs. Thirteen humans and three rhesus monkeys were scanned during passive listening. Monkeys were scanned at 4.7 Tesla and humans were scanned at 3T and 7T. Two run types with either low-to-high or high-to-low progression of tone bursts were alternated. Crosscorrelation between time-series from both run types were computed and time delay between the two signals revealed the preferred frequency. In monkeys, we found a tonotopic pattern with a posteroanterior axis of high-low-high preference located at the coordinates of A1-R according to monkey atlas. In humans, we also observed a tonotopic pattern of high-low-high preference along an overall postero-anterior axis, running across Heschl’s gyrus. Our findings suggest that the precise 3D representation of the individual cortical surface is critical to better appreciate the organization in both species. This first comparative fMRI study of the tonotopy would contribute to the definition of a unified primate model of core and belt fields (Baumann et al., 2013).