A 65-year-old woman is brought to the emergency department due to abnormal behavior. The patient was in her usual state of health until 3 hours ago, when she suddenly became confused and kept asking repetitive questions. She has had no headache, abnormal motor activity, or focal weakness or numbness. Medical history is significant for hypertension and migraine. Blood pressure is 136/84 mm Hg and pulse is 80/min. On physical examination, the patient is awake and alert but keeps asking, "What happened?" She can state her date of birth and address correctly, recognizes her family members, and is aware of current events. The patient recalls 0 of 3 objects after several minutes and cannot retain the physician's name despite many repetitions. Speech is fluent without dysarthria or aphasia. Cranial nerve examination shows no abnormalities. Motor strength, reflexes, and sensation are normal throughout. Blood cell count and serum chemistry panel are within normal limits. Urine drug screen is negative. CT scan of the head shows no hemorrhage. Two hours later, the patient spontaneously returns to normal. Temporary dysfunction of which of the following neural structures best explains this patient's symptoms?
