Anatomy of the visual word form area: adjacent cortical circuits and long-range white matter connections

Abstract

Circuitry in ventral occipital-temporal cortex is essential for seeing words. We analyze the circuitry within a specific ventral-occipital region, the visual word form area (VWFA). The VWFA is immediately adjacent to the retinotopically organized VO-1 and VO-2 visual field maps and lies medial and inferior to visual field maps within motion selective human cortex. Three distinct white matter fascicles pass within close proximity to the VWFA: (1) the inferior longitudinal fasciculus, (2) the inferior frontal occipital fasciculus, and (3) the vertical occipital fasciculus. The vertical occipital fasciculus terminates in or adjacent to the functionally defined VWFA voxels in every individual. The vertical occipital fasciculus projects dorsally to language and reading related cortex. The combination of functional responses from cortex and anatomical measures in the white matter provides an overview of how the written word is encoded and communicated along the ventral occipital-temporal circuitry for seeing words.

Figure 1. The location of the VWFA in relation to visual field maps

The image at the left is a ventral view of the cortex in a single subject. The cortex is inflated; the shading indicates the positions of gyri (light) and sulci (dark). Visual field maps [V1, V2, V3, hV4, VO-1, VO-2] were defined by retinotopic mapping, and the VWFA was defined by a standard localizer. The ventral maps, VWFA and right VWFA (rVWFA) are shown as colored regions on the right hemisphere and colored borders on the left hemisphere. Small faces indicate face-selective regions of interest (mFus, pFus, IOG, right hemisphere) based on the mean Talaraich coordinates of activations in 11 subjects. The overlaid orange-red heatmap on the left hemisphere are significant fMRI responses (p<10⁻⁴) to a word positioned 3 degrees to the right of fixation (see inset at upper right), contrasted with a word 3 degrees to the left of fixation. Significant activations occur within several visual maps and the VWFA. The activations are at positions that are expected based on the location of the word in the visual field. For simplicity, only the largest cluster of the VWFA region of interest is shown in the left hemisphere.

Figure 2. Inter-subject differences and within-subject stability of VWFA localization

(A) The VWFA localizer (words > phase-scrambled words, p<10⁻³) produces areas of significant activation in VOT cortex in left and right hemispheres. The activations are shown for two typical subjects on a single coronal slice (left) and on an inflated cortical surface (right). Blue outlines indicate the boundaries of visual field maps. The black outlines indicate the boundaries of the VWFA and rVWFA regions of interest as defined in the main text. Dotted horizontal lines indicate approximate locations of coronal slices. The MNI y-coordinate (anterior-posterior axis) is indicated next to the coronal slices. VOT cortex is shown from a ventral view (left hemisphere is on right side). Inset at top indicates the magnified region of this ventral view of VOT cortex. (B) The VWFA localizer was repeated four times in the same individual (last scan 16 months after first scan). The VWFA (black boundaries) was defined as significant (p<10⁻³) activity outside visual field maps (blue boundaries). Only the left hemisphere is shown for simplicity. The measured location of the VWFA is stable over time.

Figure 3. Mean diffusion weighted imaging signal intensity in the ventral temporal lobe

(A) Cortical Surface rendering showing the mean signal intensity measured in the cortex with a spin-echo DWI sequence. T2* weighted gradient-echo measurements of the temporal lobe show substantial signal loss immediately anterior to the VWFA, however spin-echo measurements do not. (B) Axial slices through the white matter adjacent to the VWFA showing the mean DWI signal intensity. There is not signal loss in the white matter. Imaging artifacts are unlikely to substantially impact fiber tractography in the white matter near the VWFA.

Figure 4. Average white matter tracts connecting to the VWFA

The top panel shows a heat-map of the number of subjects (N=27) with VWFA white matter connections passing through each voxel. Three distinct fiber tracts are consistently identified across subjects: The inferior longitudinal fasciculus (ILF, see sagittal slice), the vertical occipital fasciculus (VOF, see coronal slice) and the inferior frontal occipital fasciculus (IFOF, see axial slice). The location of the occipital temporal sulcus (OTS) is marked on the coronal plane. The bottom panel shows the cortical endpoints of these tracts displayed on a 3-dimensional rendering of a subject's cortical surface.

Figure 5. Major white matter tracts passing near the VWFA

The ILF (orange) and the IFOF (blue) are large fascicles connecting much of the occipital lobe including ventral occipital temporal (VOT) cortex to anterior brain regions. The vertical occipital fasciculus (green) connects the occipitotemporal sulcus (and other VOT regions) including voxels identified as the VWFA to the lateral portion of the occipital parietal junction. The tracts are shown for a single representative subject. The bottom panel shows the endpoints of these three tracts on a 3-D rendering of the same subject's cortical surface.

Figure 6. Axial slices showing the VWFA, nearby white matter fascicles, and visual field maps

The visual word form area (VWFA) is shown in red. For this individual, cortical voxels were identified corresponding to the following field maps: V1, V2, V3, V4, Lateral-Occipital 1 and 2 (LO- 1 / LO-2), Temporal-Occipital 1 and 2, (TO-1 / TO-2), Ventral-Occipital 1 and 2 (VO-1 / VO-2). White matter voxels in peri-VWFA white matter fascicles were identified from diffusion-weighted images: Inferior Longitudinal Fasciculus (ILF), Inferior Frontal Occipital Fasciculus (IFOF), and the Vertical Occipital Fasciclus (VOF).

Figure 6. Axial slices showing the VWFA, nearby white matter fascicles, and visual field maps

The visual word form area (VWFA) is shown in red. For this individual, cortical voxels were identified corresponding to the following field maps: V1, V2, V3, V4, Lateral-Occipital 1 and 2 (LO- 1 / LO-2), Temporal-Occipital 1 and 2, (TO-1 / TO-2), Ventral-Occipital 1 and 2 (VO-1 / VO-2). White matter voxels in peri-VWFA white matter fascicles were identified from diffusion-weighted images: Inferior Longitudinal Fasciculus (ILF), Inferior Frontal Occipital Fasciculus (IFOF), and the Vertical Occipital Fasciclus (VOF).