The layers of the neocortex each have a unique anatomical connectivity and functional role. Their exploration in the human brain, however, has been severely restricted by the limited spatial resolution of non-invasive measurement techniques. Here, we exploit the sensitivity and specificity of ultra-high field fMRI at 7 Tesla to investigate responses to natural sounds at deep, middle, and superficial cortical depths of the human auditory cortex. Specifically, we compare the performance of computational models that represent different hypotheses on sound processing inside and outside the primary auditory cortex (PAC). We observe that while BOLD responses in deep and middle PAC layers are equally well represented by a simple frequency model and a more complex spectrotemporal modulation model, responses in superficial PAC are better represented by the more complex model. This indicates an increase in processing complexity in superficial PAC, which remains present throughout cortical depths in the non-primary auditory cortex. These results suggest that a relevant transformation in sound processing takes place between the thalamo-recipient middle PAC layers and superficial PAC. This transformation may be a first computational step towards sound abstraction and perception, serving to form an increasingly more complex representation of the physical input.
Bibliographical noteFunding Information:
This work was supported by the Netherlands Organization for Scientific Research (NWO, Rubicon grant number 446-12-010 to M.M., VENI grant number 451-15-012 to M.M., VIDI grant number 864-13-012 to F.D.M., and VICI grant 453-12-002 to E.F.); the National Institutes of Health (NIH grant numbers P41 EB015894, P30 NS076408, and S10 RR026783); European Research Council (ERC grant number 269853), the WM KECK Foundation; and the Dutch Province of Limburg.
© 2019, The Author(s).