QUADRANTIC VISUAL-FIELD DEFECTS - A HALLMARK OF LESIONS IN EXTRASTRIATE (V2/V3) CORTEX

We report 2 patients with homonymous quadrantic visual field defects. The first patient experienced scintillations in the left lower quadrant, leading to the discovery of an astrocytoma in the cuneus of the right occipital lobe. Postoperatively she had a left lower quadrantanopia that precisely respected the horizontal meridian. The second patient presented with a left lower quadrantanopia, sparing the central 10-degrees of vision that also respected the horizontal meridian. An astrocytoma was resected from the right upper peristriate cortex. We must explain how a lesion in extrastriate cortex produced a homonymous field defect with a sharp horizontal edge in these 2 patients. Areas V2 and V3 are each divided along the horizontal meridian into separate halves flanking striate cortex. Consequently, the upper and lower quadrants in extrastriate cortex are physically isolated on opposite sides of striate cortex. We propose that a lesion involving V2/V3 may be sufficient to create a visual field defect. Although the lesion may have irregular margins, if it crosses the representation of the horizontal meridian in extrastriate cortex, it will produce a quadrantic visual field defect with a sharp horizontal border because of the split layout of the upper and lower quadrants in V2/V3.