Article Figures & Data
Figures
- Figure 1.
Tonotopic maps in auditory cortex. A, Sound stimuli were pure tone bursts presented in cycled progressions from low frequencies to high: 88 to 8000 Hz in half-octave steps. Each 28 s progression from low to high (red-to-blue color scale) was followed by a 4 s stimulus pause. Sound stimuli were designed to induce a traveling wave of response across cortical tonotopic maps: fMRI responses peak sooner in map regions preferring low frequencies and progressively later in regions preferring higher frequencies. Linear cross-correlation analysis was used to determine the temporal delay that best fit the observed fMRI response time course of each voxel and to assign a corresponding best frequency. B, Analyses were performed in each individual subject's (n = 10) volumetric space. C, Resulting color-coded frequency maps were projected onto each subject's cortical surface meshes. Surfaces were minimally inflated to expose the auditory cortex on the temporal plane. D, In 20/20 hemispheres, two primary mirror-symmetric tonotopic maps (high-to-low-low-to-high) were observed, and three sample right hemispheres are shown with a voxelwise threshold of p < 0.05 (FDR corrected). The posterior (high-to-low) and anterior (low-to-high) maps contain the regions hA1 and hR, respectively, and the low-frequency union between the two maps is the hA1–R border. Dotted lines indicate how surface patches containing the two maps were defined for the next step of analysis. E, Group averaged tonotopic maps across all 10 subjects after cortex-based alignment indicates the consistency of tonotopic map location relative to HG. Correlation threshold R > 0.15.
- Figure 2.
Spatial layout of PAC relative to HG. Surface patches containing the two primary mirror-symmetric tonotopic maps (“high-low-low-high, hA1 and hR) were selected from the cortical surface meshes (n = 20 hemispheres) and are plotted here with the borders of HG indicated (solid lines: anterior border = FTS; posterior border = HS). In 9/20 hemispheres, HG was a single gyrus with a smooth crown. In 8/20 hemispheres (partial duplications *), an SI was present on the gyral crown (dotted lines) splitting HG into two divisions that remained connected by a common medial stem. In 3/20 hemispheres (complete duplications **), a dividing sulcus was present that reached all the way down to the medial base of HG so that the two divisions did not remain connected by a common medial stem (also indicated with dotted lines). Note that in the case of complete duplications, there is a difference in the standard nomenclature and there are considered to be two Heschl's sulci (HS1 and HS2): the dividing sulcus (dotted line) is HS1 and the posterior border (solid line) is HS2. In some cases, the posterior end of the functional maps extended onto less prominent gyri of the planum temporale, which are also indicated by dotted lines (outside the posterior border of HG) when present. These plots reveal a continuous anatomical–functional relationship across the anatomical variants of HG, as described in Results. As shown in the lower left inset, gyral/sulcal borders were drawn corresponding to cortical surface transitions between convexity and concavity, as described in Material and Methods.
- Figure 3.
Results of separate scans run in five of the same subjects in which the tonotopic mapping stimuli were presented in reversed order (high frequencies-to-low, rather than low-to-high). A consistent anatomical–functional relationship is observed.
- Figure 4.
Heschl's gyrus variants are part of a continuum, rather than distinct subtypes. Top row, Diagrams of hA1 (blue) and hR (orange) locations on cross-sections of HG. hA1 is located on the posterior side or division of HG on single and duplicated gyri, respectively. hR is likewise on the anterior side or division of HG. L and H depict the location of low and high frequencies on the tonotopic maps. Middle and bottom rows, Actual hA1 and hR locations in axial and sagittal anatomical views from three sample subjects, as identified based on the functional tonotopy data. The regions were selected on the cortical surface meshes (as shown in Fig. 1) and projected into volumetric anatomical space. The hA1–R border between was defined along the gradient reversal (the low-frequency representation) at the center of the two maps.
- Figure 5.
Tonotopy relative to curvature of HG. Curvature index versus preferred frequency values of all surface voxels within the two primary tonotopic maps, across all subject's hemispheres with a single HG (A), partial duplication (B), and complete duplication (C). Positive curvature values indicate concavity (sulcal), and negative values indicate convexity (gyral). Systematically, low frequencies tend to be represented on a gyrus (HG) and high frequencies within adjacent sulci. Error bars indicate SEM. D, Diagram of a single gyrus showing how a fold between mirror symmetric maps brings equivalent topographic points on the two maps closer together in space. E, Actual tonotopy data on HG from a sample subject for comparison, sagittal slice view.
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