Topographic Mapping Of Human Visual Cortical Areas

Functional magnetic resonance imaging (fMRI) has been used to map the location, extent, and topographic organizations of several cortical areas in the human visual system. Signals used by fMRI are thought to represent local changes in blood flow that serve as an indirect marker for the activation of pools of neurons. Eccentricity and polar angle coordinates in several visual areas (V1, V2, V3, VP, and V3A) are usually mapped using phase-encoded, contrast-reversing checkerboard stimuli that were presented as a rotating hemifield (polar angle) or expanding annulus (eccentricity). To maximize attention and arousal and, thus, maximize activity in extrastriate cortex, subjects typically are required to fixate and report changes in the location of a small centrally located stimulus.

Spin echo imaging is used to acquire functional brain maps, whereas gradient-recalled images are generally used to acquire anatomical data that are used for the reconstruction of three-dimensional models of the brain, which then served as input to one of several brain-flattening algorithms to build unfolded cortical maps or inflated brain reconstruc tions. Figure 12A illustrates the representation of eccentricity in the visual cortex for three subjects in both three-dimensional and two-dimensional reconstructions of the occipital cortex. These cortical maps illustrate the representation of eccentricities ranging from 1.4° to 24° as color-coded bands that extend both dorsally and ventrally from the collateral sulcus (ventrally), to the calcarine sulcus, and then onto the exposed lateral surface of cortex. The representation of polar angle in occipital cortex from these three subjects is illustrated in Fig. 12B. Regions of cortex activated are color-coded yellow when the hemifield checkerboard is located within 5° of the superior vertical meridian, purple when within 5° of the horizontal meridian, and orange when within 5° of the inferior vertical meridian. Because the vertical and horizontal meridians form the borders of many visual cortical areas in macaque monkeys, these data formed the basis for identifying the borders between cortical areas in the human occipital cortex. A large purple region located within the calcarine sulcus corresponds to the horizontal meridian representation of area V1. This region is bordered dorsally by an orange region corresponding to the inferior vertical meridian representation at the border between areas V1-V2. In ventral cortex, V1

Figure 12 Mapping of visual topography in human visual cortex. (A) Representation of visual field eccentricity from 3° to 30°. Visual stimuli and their representational shades are represented on the left. The locations of activations for three subjects are illustrated on surface reconstructions of the occipital pole (top) and on unfolded cortical maps (bottom). (B) Representation of polar angle determined through activation from rotating checkerboard hemifield (left). (C) Combined visual field topography from three subjects illustrating the multiple representations of the horizontal and vertical meridia found in each subject (left), the localization of the 3-24 isoeccentricity contours (right), and their average overlap (middle). (D) Summary of the topography of human visual cortical areas V1, V2, V3, VP, V3A, and V4v. Area V1 is located in the fundus of the calcarine fissure and is bordered by the representations of the superior and inferior vertical meridia. Area V2 is located both dorsal and ventral to area V1. The anterior border of dorsal V2is formed by a representation of the horizontal meridian (HM2), and the anterior border of ventral V2 is formed by a second representation of the horizontal meridian (HM4). From DeYoe et al. (1996).

Figure 12 Mapping of visual topography in human visual cortex. (A) Representation of visual field eccentricity from 3° to 30°. Visual stimuli and their representational shades are represented on the left. The locations of activations for three subjects are illustrated on surface reconstructions of the occipital pole (top) and on unfolded cortical maps (bottom). (B) Representation of polar angle determined through activation from rotating checkerboard hemifield (left). (C) Combined visual field topography from three subjects illustrating the multiple representations of the horizontal and vertical meridia found in each subject (left), the localization of the 3-24 isoeccentricity contours (right), and their average overlap (middle). (D) Summary of the topography of human visual cortical areas V1, V2, V3, VP, V3A, and V4v. Area V1 is located in the fundus of the calcarine fissure and is bordered by the representations of the superior and inferior vertical meridia. Area V2 is located both dorsal and ventral to area V1. The anterior border of dorsal V2is formed by a representation of the horizontal meridian (HM2), and the anterior border of ventral V2 is formed by a second representation of the horizontal meridian (HM4). From DeYoe et al. (1996).

is bordered by a yellow band that corresponds to the representation of the superior vertical meridian at the ventral V1-V2 border. The width of V2 can be calculated by determining the location of the next representation of the horizontal meridian that corre sponds to the border between areas V2 and V3 in dorsal cortex and between V2 and VP in ventral cortex.

Figure 12C illustrates the average organization of eccentricity and polar angle from the three subjects illustrated in the two previous parts. Area V2

corresponds to a band of cortex that extends from the vertical meridian representation at the border of V1 to the horizontal meridian representations at the border with area V3 in dorsal cortex and VP in ventral cortex. According to this view, area V2 is approximately 1-1.5 cm wide in the region from 1.5° to 24° of eccentricity. This estimate is in agreement with the value of 1-1.5 cm for foveal V2 based on the extent of CO stripes observed in unfolded and flattened human occipital cortex. At more peripheral regions corresponding to intermediate eccentricities, the CO stripes of V2 extend from 1.8 to 3.4 cm. Thus, a potential discrepancy exists between the width of V2 derived from fMRI topographic mapping studies and the width of V2 reported from CO architecture at intermediate eccentricities.

The amount of cortical tissue devoted to the representation of a given portion of the visual field (mm2/deg2) is described by the cortical magnification function (CMF). This magnification function varies as a function of eccentricity, with larger values typically found for central visual fields. The calculated CMF for many topographically organized cortical areas is based upon measurements derived from fMRI mapping. The CMF derived for V2 is described by the function 25.19(0.09 + E)~153, where Eis eccentricity in degrees.

Areas V1, V2, and several other extrastriate cortical areas have been localized on a surface-based map of the entire cerebral cortex based on the visible man data set. This method provides a convenient method to represent topographic mapping data and functional activation data in a common format that is readily converted to the Talairach stereotaxic space. Figure 13A illustrates the locations of areas V1, V2, V3, VP, V3A, and V4v in the visible man based on topographic mapping data. The uncertainties of the extents of these visual areas are illustrated by question marks on the surface map. Figure 13B illustrates the locations of these areas on a surface model of the visible man's right hemisphere. The lateral view in Fig. 13B illustrates the remaining uncertainty of the foveal projects of these areas. The surface-based atlas also provides a convenient format for the illustration of activation foci from functional studies of color, motion, form, and face processing derived from a wide variety of sources. Corbetta and colleagues have performed a comprehensive study of perceptual processing in the human occipitotemporal cortex. Figure 13C illustrates the location of these activation foci for perceptual tasks involving color, motion, and form processing. Color-processing tasks activated several foci within ventral and dorsal V2; one focus was localized to dorsal V2 in a motion-processing task, whereas three foci were localized to V2 in a form-processing task. Tasks involving the processing of faces do not preferentially activate V2 but instead tend to engage areas in the inferior temporal cortex.

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