The Brain (1962)

Starring Anne Heywood, Peter Van Eyck, Cecil Parker, Bernard Lee, Jeremy Spenser
Directed by Freddie Francis
(actor & director credits courtesy IMDB.com)

A scientist steals the brain of a dying financier for study and works to keep it alive, but the brain is so ruthless and powerful, it takes over his own mind.

According to Wikipedia, this was the third film adaptation of Curt Siodmak's novel "Donovan's Brain," after 1944's The Lady And The Monster and 1953's Donovan's Brain, and takes a somewhat different tack on the premise, focusing less on the novel's horror story, instead crafting a murder mystery as to who killed the financier in a plane crash.  Although well-done, it's a bit of a stretch that Holt's family and associates would answer any of Corrie's questions as he investigates the murder, driven by Holt's brain for answers.  For me, the 1953 adaptation is the best of the films, (and an earlier radio adaptation starring Orson Welles is very good as well).  While this version's mystery story is intriguing, it misses out by not focusing on the conflict between Corey/Corrie and the brain as it grows more powerful.