We are used to finding homonymous hemianopias in the occipital lobe extending out fully to the periphery. In one eye the field is larger than in the fellow eye. That is because the temporal field is about twice the number of degrees versus the nasal field. The periphery of the temporal field beyond 60 degrees is monocular. That area is fed by vessels deep in the calcarine fissure and can either spare or cause an isolated peripheral defect in only one eye.
Figure 1 and 2 demonstrate a hemianopia with sparing of the temporal crescent.