The Marcus Gunn pupil in experimental tract lesions.

Isolated surgical lesions were made in three Macaca mulatta monkeys. Two had right optic tract section. One control underwent section of the right half of the chiasm. Preoperative evaluation revealed no fundus, pupil, or VER abnormalities. Postoperatively, a classic Marcus Gunn pupil was found contralateral to the isolated optic tract lesions. The control had normal pupils. The nature of the lesions was documented histopathologically, the pupillary findings, photographically. Possible explanations are explored. Contrary to traditional teaching, lesions occurring posterior to the chiasm may indeed produce the Marcus Gunn pupillary phenomenon.