A 63-year-old woman went to an ophtalmologist because of episodes of decreased vision in her "right eye" and headaches. Past medical history was notable for diabetes, elevated blood cholesterol, and coronoary artery disease. About 5 or 6 weeks ago the patient began having episodes of sudden "blurry wavy" vision. She believed this was mostly in the right eye, but she did not try looking with one eye at the time. The episodes would last 15 to 20 minutes, occurring three to four times per week, and were accompanied by a severe left retro-orbital headache.

She was able to recognize faces during the episodes but had difficulty in reading. She denied any other symptoms. Two days ago an episode began that resulted in persistent decreased vision on the right.

Physical examination revealed an elevated blood pressure 180/78 and was otherwise normal.

Neurological examination was normal with the exception of visual testing that revealed a right homonymous hemianopia.

On the basis of the symptoms and signs shown in bold above, where is the lesion? what is the most likely diagnosis?