Visual Field Topography

Traditionally, visual field topography has been a primary source of information used to identify and map different visual areas in animals. Early work showed that a number of cortical visual areas contain a complete representation of the visual field, though each representation is split along the vertical meridian so that half of the field is represented in each hemisphere. Generally, then, the topographic arrangement of photoreceptors in the retina is maintained in the central connections. This results in an orderly arrangement of cells responsive to different locations in the field of view. In the cortex, neighboring positions in the visual field tend to be represented by groups of neurons that are adjacent but laterally displaced within the cortical gray matter. Neurons representing the vertical midline of the visual field are represented in both hemispheres and are functionally linked by interhemi-spheric, callosal connections. The end result is that the topography of the visual field is preserved in the visual cortex. [Because the visual field is directly imaged on the retina by the optics of the eye, visual field topography translates into retinal topography (retino-topy) but is upside down and backward due to the optical properties of the cornea and lens of the eye. Also, as will be described later, visual space is systematically distorted in the cortical representation.]

1. Charting Human Visual Areas by Retinotopy

Functional neuroimaging has been used to chart the retinotopic organization of visual areas in the human occipital lobe. Figures 7A-7C illustrate the results of such an experiment that used functional MRI. Subjects viewed a checkered hemifield that flickered at 8 Hz and slowly rotated about the center of gaze. During an fMRI scan series lasting approximately 4 min, the hemifield made five complete rotations, thereby sweeping through each angular position five times. Each location in the visual cortex went through five cycles of alternate activation and quiescence. However, cortical locations representing different angular positions in the visual field were stimulated at different times in the rotational cycle. Measurement of the temporal phase of the cyclic activation thereby

Figure 6 Summary of visual area topography for a variety of simian and prosimian species. Areas with the same name in different species are generally thought to be homologous. Other areas with apparent topographic similarity may or may not be functionally identical. Area DL is likely to be at least partially similar to macaque V4. DM may be homologous to macaque V3 and/or V3A. Note that this figure depicts left hemispheres unlike other figures in this article. (Adapted from Zilles and Clarke, 1997.)

Figure 6 Summary of visual area topography for a variety of simian and prosimian species. Areas with the same name in different species are generally thought to be homologous. Other areas with apparent topographic similarity may or may not be functionally identical. Area DL is likely to be at least partially similar to macaque V4. DM may be homologous to macaque V3 and/or V3A. Note that this figure depicts left hemispheres unlike other figures in this article. (Adapted from Zilles and Clarke, 1997.)

identified the angular position represented at that cortical location. (See upper legend in Fig. 7.)

The polar angle representations depicted in Figs. 7A-7C are displayed on 3-dimensional models of each subject's occipital lobe and on corresponding maps of the unfolded and flattened cortical surface (Figs. 7A'-7C'). To facilitate the identification of cortical areas representing different visual field quadrants, the maps are pseudo-colored with a coarse scale to show brain regions that were maximally responsive when the midpoint of the checkered hemifield was within 45° of the horizontal meridian (white), the superior vertical meridian (dark gray), or the inferior vertical meridian (light gray). To produce these flat maps, the 3-dimensional cortical surface model was slit along the lateral occipital sulcus and was then unfolded using a computer algorithm. The resulting flat maps show bands of activation corresponding to alternating representations of quadrants at the horizontal and vertical meridia. On the medial wall of the hemisphere, these bands are oriented roughly parallel to the calcarine sulcus (cf. Figs. 7B and 7B'). Comparison across the three cases illustrated in Figs. 7A-7C provides an indication of the overall consistency across subjects as well as the degree of variation within each subject.

Visual field mapping can also be accomplished in a complementary manner using an expanding checkered annulus to provide information about the representation of visual field eccentricity (distance from the center of gaze). Figures 7D-7F illustrate the resulting patterns of activation.

The right side of Fig. 7 depicts the extraction of contours representing isoeccentricity and isopolar angle bands and their compilation into a final composite map showing the layout of visual space in the occipital lobe. Note that there are actually several reiterated representations of visual space, as indicated by the alternation of vertical and horizontal meridian representations (VM vs HM) labeled along the right margins of the meridia and composite maps.

From maps of the meridian representations, it is possible to estimate the locations of the borders between different visual areas. To do this, it is assumed that the borders are found at the representations of the horizontal and vertical meridia, as they are in subhuman primates. On the basis of this assumption, the layout of cortical visual areas in the human occipital cortex is illustrated on the large cortical flat map in the center of Fig. 8. To create this map, the 3-dimensional brain model (top) was slit (yellow dashed line) along the calcarine sulcus rather than along the lateral occipital surface. As a result, the vertical meridian representations (marked by black arrowheads) are oriented approximately vertically in Fig. 8 rather than horizontally as in Fig. 7.

Consistent with previous human studies, area V1 occupies the calcarine fissure but can extend onto the surface of the cuneus and lingual gyrus. The representation of the horizontal meridian contained within the calcarine fissure separates the upper and lower fields of V1 (split along the yellow dotted line in the large flat map of Fig. 8). It does not mark a border between different visual areas. As in the macaque, the borders between V1 and the two halves of V2 (V2d, V2v) are marked by vertical meridian representations for the inferior and superior halves of the visual field (VMi, VMs). The inferior vertical meridian representation is located along the upper lip of the calcarine sulcus, whereas the superior vertical meridian is located along the bottom lip. V2d and V2v together provide a second complete representation of the visual hemifield, but it is split in half with the inferior field represented dorsally and the superior field represented ventrally.

In the dorsal cortex of the cuneus, the border between V2d and V3 is marked by another representation of the horizontal meridian (HM). In complementary fashion, the border between V2v and VP is also delineated by a ventrally placed horizontal meridian representation (HM). Areas V3 and VP each contain a map of a quadrant of the visual field. Together these partial maps make up a third complete hemifield representation. In the macaque, these two quarter-field maps contain neurons with somewhat different visual response properties and anatomical connections. This suggests that they are functionally distinct, and, as a result, they have been given different names by some investigators. Others consider the formation of a complete hemifield representation as a more parsimonious way to summarize the data, even if some internal heterogeneity must be accepted. In this framework, the two quarter-field representations are termed V3d and V3v.

The anterior border of V3 is an inferior vertical meridian representation (VMi). In the macaque, this would form the border with area V3A, at least for visual field eccentricities within 10° of the fovea. Accordingly, this area has been termed V3A in humans. Some investigators have argued that parts or all of macaque V3 and V3A should be considered a single dorsomedial area (DM), such as that seen in owl monkeys (see Figs. 6D and 6E). V3A in macaque monkeys contains both an inferior and a superior quadrant representation. In one study, human V3A

Stimulus

Right Visual Field

Mean Polar Angle

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