HM’s brain

Born in 1926, Henry Molaison had been hit by a cyclist when he was seven and from the age of 10 then started to have epileptic seizures that subsequently started to worsen as he neared adulthood. By the time he was 27, these seizures were so crippling that he underwent surgery for a bilateral medial temporal lobe resection. This involved cutting out significant portions of Henry’s brain in the temporal lobe area to try and control the seizures. However, the surgery resulted in severe anterograde amnesia, a type of amnesia that leads to deficits in encoding new information into the brain.

Henry’s legacy in terms of our knowledge about localisation of function in the brain and memory is highly significant. Firstly, given that his short-term memory was normal, this demonstrated that the short- and long-term memory systems in the brain must to some extent be separate otherwise Henry’s brain damage would also have affected short-term memory processing. Henry’s brain damage specifically targeted episodic memory and he was therefore unable to form new memories of any event experienced after the surgery and this continued to the end of his life. However, he was able to form new procedural long-term memories. Procedural memories are those memories which are automatic such as knowing how to drive. This type of knowledge does not start out as automatic because clearly skills such as driving must be learned. These skills develop over time, however, and activities such as driving become easier and more automatic if we practice them regularly. Despite his extensive brain damage, Henry could form new procedural memories on activities such as a pursuit rotor task in which a participant tracks a moving object on a screen with a cursor. This task requires precision and must be practiced regularly to gain expertise in the task. Henry was able to show that he could develop these skills even though he could not remember previous practice sessions due to his episodic memory deficit. Such testing with Henry and other amnesic patients lead memory researchers to understand more about how memory processing is carried out in the brain and in particular to understand that skill memory does not require the use of medial temporal lobe systems to work effectively.

If ever there was doubt that HM is a very long case study, then this article confirms it. HM’s brain is now preserved and continues to excite interest to the present day.

This article was inspired by Laura Swash.