Short association fibres form topographic sheets in the human V1–V2 processing stream

Figures

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  Fig. 1.. Localisation of the explored V1–V2 cortexin vivousing fMRI retinotopic mapping. Smoothed (a) polar angle and (b) eccentricity maps of the right hemisphere of a representative participant. Our fMRI experiment could stimulate the entire visual field in the polar angle and approximately 6.9° in the eccentricity direction. (c) The entire V1–V2 border was manually delineated based on the retinotopic maps. (d) The explored region of interest (ROI) corresponded to a dorsal sub-region in the right hemisphere in the representation of the left lower visual field. The approximate location of the ROI, which was localised using anatomical landmarks, is highlighted in (c) and (d).

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  Fig. 2.. Localisation of the region of interest (ROI) using anatomical landmarks. The ROI was located in a dorsal sub-region in the right hemisphere in the cuneus of the occipital lobe, the latter highlighted in (a). It corresponded to the inferior cuneal gyrus covering the (i) V1–V2 border, mapped using (a–d) functional MRIin vivoas shown in a representative participant and (f, g) V1-specific stria of Gennari (SoG) in thepost mortemspecimen. The inferior cuneal gyrus could be localised using (ii) superior sagittal sulcus (SSGS), (iii) inferior sagittal sulcus (ISGS), and (iv) calcarine sulcus (Iaria & Petrides, 2007). These are shown on (a) sagittal and (b) near-coronal surfacesin vivo. The gyrus was located above the calcarine sulcus and below the SSGS with the V1–V2 border running almost parallel to the two. The ISGS mapped to the crown of the gyrus and appeared as a shallow sulcus along the ROI in most participants, as shown in (e) a coronal plane in the location of the asterisk (*) in (b, c). (f, g) Localisation of the ROI in thepost-mortemspecimen using the same landmarks. A: anterior, P: posterior, I: inferior, S: superior, L: left, R: right.

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  Fig. 3.. Geometries of the SAF mapped in the localised V1–V2 gyrusin vivowere consistent across participants, characterised by a large bending angle. Here, SAF are shown for five representative participants (rows) (See results for other participants inSupplementary Figs. S7, S8 and S9). (a) Surface (left) and volume (right) views of a representative coronal plane along the gyrus, looking from the posterior end. In diffusion space, (b) the mirror view of SAF is shown looking from the anterior end, corresponding roughly to the location of the plane in (a), marked by the asterisk (*) in (a, c), and (c) SAF mapped throughout the gyrus, pointing to a sheet geometry. The corresponding fibre distribution maps and sulcal landmarks are shown inSupplementary Figure S10. A: anterior, P: posterior, I: inferior, S: superior, L: left, R: right.

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  Fig. 4.. SAF connecting the V1–V2 gyrus exhibited a topographic sheet geometryin vivo, demonstrated for a single participant. (a) A 2D surface fit to the SAF streamline dimensions corresponding to the length and extent of the fibres in a representative participant helped to appreciate the sheet geometry. Here, the colour coding represented eccentricity from retinotopic mapping. The surface thickness represented the thickness of the SAF layer, estimated as variance over all streamline coordinates in the respective dimension. (b) Eccentricity projected onto the SAF streamlines confirmed topographic order in the respective anatomical direction (white arrow). (c, d) Topography in the eccentricity direction (grey arrows) is shown at three locations along the SAF streamlines (dotted arrows). For this participant, SAF could be mapped only up to 5° eccentricity (up to 6.9° mapped by fMRI) using tractography possibly because of locally unresolved crossing fibres in the superficial white matter.

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  Fig. 5.. Retinotopic topography was preserved in the V1–V2 SAF sheet in the eccentricity directionin vivo. Three representative participants are shown (rows) (See results for other participants inSupplementary Figs. S12 and S13). Eccentricity on the V1 cortex was projected onto each corresponding SAF streamline viewed on (a) sagittal, (b) coronal, and (c) axial slices. Arrows in (c) show the direction of increasing eccentricity, aligned with the extent of the SAF sheet roughly in the anterior-posterior direction. (d) 3D sagittal view of the SAF showing (e) three coordinate locations along the streamlines at which the projected eccentricity is shown to better appreciate along-track topography. See alsoSupplementary Video V.1.

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  Fig. 6.. Retinotopic topography was preserved in the V1–V2 SAF sheet also in the polar angle directionin vivobut to a lesser extent compared to eccentricity. Three representative participants are shown (rows) (See results for other participants inSupplementary Figs. S14 and S15). Polar angle on the V1 cortex was mapped onto each corresponding SAF streamline viewed on (a) sagittal, (b) coronal, and (c) axial slices. Arrows in (c) show the direction of increasing polar angle, aligned with the thickness of the SAF sheet roughly in the left-right direction. (d) 3D sagittal view of the SAF showing (e) three coordinate locations along the streamlines at which the projected polar angle is shown to better appreciate along-track topography. See alsoSupplementary Video V.2.

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  Fig. 7.. V1–V2 SAF geometry and topography mapped using 0.2 mm isotropic spatial resolutionpost mortemDWI. (a) Fractional anisotropy (FA) and (b) mean diffusivity (MD). Highly myelinated V1-specific Stria of Gennari (SoG) visible on MD. (c) Manually delineated V1 and V2 masks with SoG as landmark were used to map (d) the V1–V2 SAF. For the marked area in (d), dense and multi-layer superficial white matter fibres are shown in (I) DTI primary eigenvector and (II) multi-shell multi-tissue constrained spherical deconvolution (MSMT-CSD) fibre distribution maps. To demonstrate SAF topography, streamlines were clustered based on their location on the V1 cortical border using separate sets of manually created labels in the (e) polar angle and (f) eccentricity directions, giving different sets of output streamlines. V1–V2 SAF were topographically organised along both, as shown in planar and 3D views.

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  Fig. 8.. High-qualitypost mortemDWI corroborated the V1–V2 SAF geometries mappedin vivo. SAF shown on representative coronal slices in (a) a single participant whose gyral geometry resembled thepost mortemspecimen very closely and (b) the humanpost mortemspecimen. The sizes of the V1–V2 gyruspost mortemandin vivowere 2 cm and 1.4 cm along the inferior-superior axis, respectively, and their approximate distances from the occipital pole were 2.5 cm and 1.8 cm. Fibre orientation maps (dashed-line boxes) were similar in the superficial white matter, showing robustness to increased partial volume effects from adjacent tissuein vivo. (c) Historical map of SAF created based on histological fibre mapping (Sachs, 1892) also shows the V1–V2 SAF where the longer SAF were closer to a V-shape and the short U-shaped SAF were closest to the cortical boundary (highlighted in yellow by the current authors).