A 54 year old white male presents with gradual onset of mild dementia, ataxic gait, and stale myoclonus. An MRI scan is normal, and an examination of his cerebrospinal fluid reveals no abnormalities, but the patient's EEG is remarkable for recurrent bursts of high-voltage slow waves. Over the next 6 months, the patient's dementia rapidly worsens, accompanied by general hypeonicity and profound dysahria. The patient dies sholy thereafter. Which of the following is the mostly likely neuropathological finding on autopsy?