A 60 year old man presents to a physician because of difficulty in reading and coming down stairs, which he attributes to an inability to "look down." Physical examination reveals that the patient looks around by moving his head rather than his eyes and also shows a distinctive axial rigidity of neck, trunk, and proximal limb muscles. He shows povey of movement and dysahric speech. Mentally, the patient responds very slowly but has better memory and intellect than are initially apparent. Which of the following pathologic findings of the brain would most likely be present?