Several limitations confound conventional MRI in correlating lesion location with visual field defects. These limitations also apply to CT imaging of occipital lesions.8- 11 First, because visual processing in the striate cortex is organized in ocular dominance columns that have a cross-sectional width of 0.4 mm,16 5-mm cuts that are widely used in major MRI centers may not provide sufficient resolution to determine the exact extent and location of the lesion identified by perimetry. However, because the rostral-caudal extent of lesions in our 14 patients differed more than 5 mm from a revised map,6 which was based on MR images of 3 patients whose slice thickness was not specified,6 limitation of slice thickness does not explain the disparate result. Second, the determination of the extent of the lesion is complicated by areas of edema surrounding the lesion, which may be difficult to differentiate from the actual area of infarction. However, all our patients were imaged many months or years (mean duration of lesions, 14 months) after the onset of visual field loss, when edema should have completely resolved. Third, tissue damage may disrupt function but be undetected by imaging. For example, patients 3 through 7 and 10 had homonymous field defects without sparing of the temporal crescent, indicating that the most anterior portion of the occipital pole should be involved. However, on MRI, no signal change was detected in the anterior medial occipital cortex, suggesting that tissue damage may extend beyond visible MRI change; alternatively, the monocular temporal crescent of field (representing the nasal retina of the contralateral eye) may not extend to the extreme anterior part of the occipital cortex, although this seems unlikely. Fourth, areas of signal change in MRI do not specify destruction of all neural elements, which may be spared in an area of visible damage. This pertains particularly to neoplasms and hemorrhages, which we therefore excluded from our study. Fifth, because we used a 2-dimensional image to estimate the size of a 3-dimensional lesion, the accuracy of measurements depends on the orientation of the cuts. However, there is no ideal head orientation for studying the calcarine fissure because the incline of the calcarine fissure relative to other brain and skull reference coordinates is variably oriented among individuals,17 and it also varies with minor changes in head position. Finally, and most important, the exact boundaries of the striate cortex cannot be determined by MR images, and there is a natural and substantial variation across individuals in the exact dimension and location of the striate cortex.14 Moreover, no compensation for cortical infolding or curvature of the calcarine fissure can be made using MR images, which allows only estimation of the extent of the lesion.