n. loss of vision in one fourth, or one quadrant, of the visual field. Homonymous quadranopia is the loss of vision in both (upper or lower) quadrants of the same half (right or left) of the visual field of each eye (e.g., left upper quadranopia); it is caused by postchiasmatic brain injury (see postchiasmatic visual deficit). Also called quadranopia; quadrantanopsia; quadrantic hemianopia.