Abstract
Recent neuroimaging studies in primates aim to define the functional properties of auditory cortical areas, especially areas beyond A1, in order to further our understanding of the auditory cortical organization. Precise mapping of functional magnetic resonance imaging (fMRI) results and interpretation of their localizations among all the small auditory subfields remains challenging. To facilitate this mapping, we combined here information from cortical folding, micro-anatomy, surface-based atlas and tonotopic mapping. We used for the first time, phase-encoded fMRI design for mapping the monkey tonotopic organization. From posterior to anterior, we found a high-low-high progression of frequency preference on the superior temporal plane. We show a faithful representation of the fMRI results on a locally flattened surface of the superior temporal plane. In a tentative scheme to delineate core versus belt regions which share similar tonotopic organizations we used the ratio of T1-weighted and T2-weighted MR images as a measure of cortical myelination. Our results, presented along a co-registered surface-based atlas, can be interpreted in terms of a current model of the monkey auditory cortex.
Highlights
The auditory cortex is located in the temporal lobe of the primate brain and in macaque monkeys it lies mainly on the superior temporal plane (Figures 1A,B)
Phase-encoded functional magnetic resonance imaging studies (fMRI) design has been successfully applied to retinotopic mapping in humans (e.g., Sereno et al, 1995) and in monkeys (Kolster et al, 2009) and to human tonotopic mapping (Talavage et al, 2004; Da Costa et al, 2011; Striem-Amit et al, 2011)
Combined with macro-anatomical features and co-registered surface-based atlas, we present here a detailed tonotopic map of the monkey auditory cortex in good agreement with the current model of organization of the monkey auditory cortex
Summary
The auditory cortex is located in the temporal lobe of the primate brain and in macaque monkeys it lies mainly on the superior temporal plane (Figures 1A,B). The monkey auditory cortex can be divided into three core regions surrounded by seven or eight belt areas which project mainly to parabelt regions on the convexity of the superior temporal gyrus (Hackett, 2011). The current model of organization of the monkey auditory cortex consists of 12 regions which are defined on the basis of architectonic boundaries and connections (Jones et al, 1995; Hackett et al, 1998). The attribution of auditory cortical subfields in fMRI studies can be problematic, because of imaging issues (e.g., EPI distortion and partial volume effects) and problems with the distinction between adjacent core and narrow belt areas that have the same tonotopic preference. Imaging issues related to field definition using fMRI are manifold: (1) Spatial resolution is currently between 1 and 2 mm which is about the width of medial belt regions. (2) Echo-planar images (T2∗-weighted fMRI) typically show geometric distortions which are only partially corrected contributing to misalignment of fMRI results with anatomical images. (3) Voxel-based representation of the auditory cortex using axial slices or oblique slices aligned with the posterior part of the superior temporal plane cannot represent faithfully the anterior auditory cortex which is folded, in particular at the level of the circular sulcus (Figures 1B,C)
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