A 45-year-old man with AIDS is admitted to the hospital with a productive cough, fever, and night sweats. An X-ray film of the chest shows an ill-defined area of consolidation at the periphery of the right middle lobe and mediastinal lymphadenopathy. A sputum culture grows acid-fast bacilli. The patient develops severe headache and neck rigidity. Which of the following brain areas is most likely affected by this patient's infection?