An 80-year-old man presented to the general medicine outpatient clinic with complaints of short term memory loss and reduced mobility. His wife reported a seven month history of poor concentration and described episodes in which her husband would be in a world of his own followed by periods where he could be completely normal. More recently he had had disturbing visual hallucinations. On examination, he had a marked dysarthria and perseveration, and bilateral rigidity and bradykinesia but no rest tremor. Investigations revealed normal blood tests. Based on these findings, what is the most likely diagnosis?