Current Medical Imaging Reviews, 2005, 1, 319-329 319 1573-4056/05 $50.00+.00 ©2005 Bentham Science Publishers Ltd. Extended Concepts of Occipital Retinotopy Christopher W. Tyler*, Lora T. Likova, Chien-Chung Chen, Leonid L. Kontsevich, Mark M. Schira and Alex R. Wade Smith-Kettlewell Brain Imaging Center Eye Research Institute, 2318 Fillmore Street, San Francisco, CA 94115, USA Abstract: Retinotopic mapping is a key property of organization of occipital cortex, predominantly on the medial surface but increasingly being identified in lateral and ventral regions. The retinotopic organization of early visual areas V1-3 is well established, although anatomical landmarks can help to resolve ambiguities in poorly-defined functional maps. New morphing techniques are now available to define the metric mappings quantitatively within each retinotopic area. In the dorsal occipital regions, there is fair agreement that area V3A should be split into separate V3A and V3B maps, and that beyond them lies a further area, V7. We specify the eccentricity mapping of both V3B and V7 for the first time, showing how the latter is roughly parallel to the meridional mapping and offering formal accounts of such paradoxical behavior. In ventral occipital cortex, we support the analysis of Zeki and Bartels [1] and Wade et al. [2] that V4 maps the full hemifield, and show the existence of two more areas, a ventromedial map of the lower quadrant, emphasizing the upper vertical meridian, and an adjacent area with a dominant foveal representation. In lateral cortex, the motion area defined by a motion localizer shows pronounced retinotopy, particularly in the eccentricity parameter. A dorsolateral map between the motion area and V3B, which represents the lower quadrant with an emphasis the foveal part of the lower vertical meridian, may be a counterpart to the ventromedial map. INTRODUCTION Now that the entire medial surface of occipital cortex is established as being devoted to the primary visual projection areas V1-3 [3-9], efforts are switching to the lateral occipital cortex, which is less strongly retinotopic. Numerous functional MRI studies have demonstrated extended retinotopic activation in the lateral occipital lobe, as in the example shown in Fig. 1. The activation in this case is for a rotating wedge stimulus of a flickering checkerboard spanning a 45° sector of the visual field Fig. 2. The activation phases relative to visual field position are indicated by the colors in the icon (right field). In order to have the most comprehensive view of occipital retinotopy, it is important to have clear definitions for each retinotopic area. There is broad agreement on the definitions of retinotopic areas V1-3 as fanning out from the horizontal meridian of V1, lying along the fundus of the calcarine sulcus, through the dorsal and ventral quadrants of V2 and V3 projecting in mirror-symmetric fashion in the adjacent sulci [10]. The anatomical arrangement of the occipital lobe is presented in Fig. 3 in terms of flatmaps centered on the occipital pole [11]. This approach to cortical representation retains the full local connectivity of the occipital lobe by avoiding the cut along the horizontal meridian of V1 that is common in other flatmap representations. For our study, the retinotopic areas were defined with the rotating wedge and scaled expanding ring stimuli depicted in Fig. 2. One innovation is the use of a stable fixation grid of thin dark lines that was continually present during the *Address correspondence to this author at the Smith-Kettlewell Brain Imaging Center, Eye Research Institute, 2318 Fillmore Street, San Francisco, CA 94115, USA; Tel: (415) 345 2105; Fax: (415) 345 8455; E-mail: cwt@ski.org varying stimulus cycle. These thin dark lines do not generate any fMRI signal in themselves, but provide an invariant pattern to help the observer stabilize the fixation of the eyes at the center of the pattern throughout the long period of the scan. Without this fixation grid, small movements of the eyes can seriously perturb the accuracy of the mapping in the region of the foveal representation. To provide a complete representation of the occipital activation patterns with minimal distortion, we employ flatmaps centered near the occipital pole of each hemisphere (Fig. 3). The gyral and sulcal landmarks are shown in terms of the local cortical curvature as light-gray and dark-gray shading, respectively. It may be noted that this kind of occipital-pole flatmap offers an alternative view of the organization of occipital function with several advantages. It is easy to orient the flatmaps relative to the anatomical brain structure because they show the cortex as though the brain were unfolded as viewed from the back of the head. Unlike 3D rendered views of the brain Fig. 1, this view has the great advantage that it allows presentation of medial, ventral and lateral views of the occipital cortex all in a single image. The retinotopic boundaries obtained by standard methods in the cited studies generate the area definitions shown as colored outlines in Fig. 3 (see Methods). It can be seen that the border of area V1 as defined by retinotopic stimulation forms an arrowhead-shaped region lying along the lips of the calcarine sulcus (bright gray shading). This anatomical marker can be helpful in identifying the borders of V1 where the functional activation is less robust. The retinotopic areas V2 (green outlines) and V3 (blue outlines) are split into dorsal and ventral segments representing the lower and upper hemifields, respectively. They fall in the ladder-like structures of the two sulci adjacent to the calcarine, but the functional borders are rarely as well-defined as the calcarine anatomy.