[1] The opposing effect, where vision in half of the center of the visual field is lost, is known as macular splitting.
[2] The favored explanation for why the center visual field is preserved after large hemispheric lesions is that the macular regions of the cortex have a double vascular supply from the middle cerebral artery (MCA) and the posterior cerebral artery (PCA).
If there is damage to one vascular pathway, like in the case of a MCA or PCA stroke, there is still another blood supply that the macular portions of the visual cortex can rely on.
[1] Patients with macular sparing often retain their ability to perform high resolution visual acuity tasks.
Patients with macular splitting fare much more poorly on such tasks, particularly if they lose vision in their right visual field.