Human Primary Auditory Cortex Follows the Shape of Heschl's Gyrus

Abstract

The primary auditory cortex (PAC) is central to human auditory abilities, yet its location in the brain remains unclear. We measured the two largest tonotopic subfields of PAC (hA1 and hR) using high-resolution functional MRI at 7 T relative to the underlying anatomy of Heschl's gyrus (HG) in 10 individual human subjects. The data reveals a clear anatomical-functional relationship that, for the first time, indicates the location of PAC across the range of common morphological variants of HG (single gyri, partial duplications, and complete duplications). In 20/20 individual hemispheres, two primary mirror-symmetric tonotopic maps were clearly observed with gradients perpendicular to HG. PAC spanned both divisions of HG in cases of partial and complete duplications (11/20 hemispheres), not only the anterior division as commonly assumed. Specifically, the central union of the two primary maps (the hA1-R border) was consistently centered on the full Heschl's structure: on the gyral crown of single HGs and within the sulcal divide of duplicated HGs. The anatomicalfunctional variants of PAC appear to be part of a continuum, rather than distinct subtypes. These findings significantly revise HG as a marker for human PAC and suggest that tonotopic maps may have shaped HG during human evolution. Tonotopic mappings were based on only 16 min of fMRI data acquisition, so these methods can be used as an initial mapping step in future experiments designed to probe the function of specific auditory fields.

Tonotopic maps in human auditory cortex using arterial spin labeling

“…The overall layout and the spatial arrangement of the tonotopic gradients described for the CBF tonotopy is in good (qualitative) agreement with those in the BOLD tonotopy (in the corresponding panel of Fig. 3A, the two reversed gradients forming the high‐low‐high frequency pattern are indicated by white double arrows and the additional low frequency clusters by black single arrows), and similar to maps shown in previously published BOLD signal studies (Da Costa et al, 2011; Formisano et al, 2003; Humphries et al, 2010; Moerel et al, 2012). However, in the CBF tonotopy, extreme low or high preferred frequency values are less represented than in the BOLD tonotopy.…”

“…The CBF tonotopy was in good agreement with the BOLD tonotopy obtained by the BOLD time course extracted from the same ASL signal. The BOLD tonotopy obtained from the ASL sequence, in turn, agreed very well with those of previous studies employing GE‐EPI BOLD sequences (Da Costa et al, 2011; Formisano et al, 2003; Humphries et al, 2010; Moerel et al, 2012; Saenz and Langers, 2014). Although acquired simultaneously and therefore susceptible to the same correlated artifacts (e.g., physical noise, motion, …), ideally, the BOLD signal and CBF represent physically independent modulation of the ASL signal.…”

“…For visualization, the maps were projected on the inflated cortical surfaces. The color‐coded single‐subject maps were transformed into the common CBA space and averaged to obtain a group tonotopic map (Da Costa et al, 2011; De Martino et al, 2013; Formisano et al, 2003; Herdener et al, 2013; Humphries et al, 2010; Langers et al, 2007; Moerel et al, 2012). …”

“…However, the exact extent of the PAC remains ambiguous: Some studies consider the main high‐low‐high frequency gradients to be entirely included in the PAC (Da costa et al, 2011), while other studies attribute part of them to auditory belt regions (Humphries et al, 2010; Talavage et al, 2004). Moreover, three main theories about the orientation of the PAC coexist: The classical interpretation locates the PAC along HG and finds its foundations in cytoarchitectonic studies.…”

“…The oblique interpretation is a more recent theory proposing an oblique orientation of PAC with respect to HG (Baumann et al, 2013). Finally, additional tonotopic gradients outside of the HG are often observed, especially at high spatial resolution and single‐subject level (Da Costa et al, 2011; Herdener et al, 2013; Moerel et al, 2013). …”