A 62-year-old man is brought to the ED after sudden onset of double vision and difficulty walking. He reports that his right eyelid “dropped” and bright lights are bothering him. On exam, the right eye is "down and out" with ptosis and a dilated, poorly reactive pupil. Motor strength is 5/5 in all extremities. Pinprick and vibration sense are intact. MRI shows a small infarct in the ventral midbrain involving structures adjacent to the cerebral peduncle.

Which of the following additional anatomic findings on axial section best localizes the lesion to the rostral midbrain level?