Much of our understanding about human learning and memory comes from the study of patients with amnesia. Amnesia is a disorder of memory impairment. It differs from dementia (as seen in Alzheimer’s disease) in that dementia combines memory impairment with general cognitive dysfunction.
The most extensively studied amnesic patient was Henry Molaison (1926-2008), known throughout the 55 years of his life when he was a famous neurology patient as “H.M.” In 1953, when H.M. was 27 years old, an experimental surgery was performed on him in order to treat severe epilepsy. The surgeon removed the medial temporal lobes on both sides of H.M.’s brain. The medial temporal lobe contains the hippocampus and the amygdala.
The tragic result of this experimental surgery was that H.M. was
left with severe anterograde amnesia. “Anterograde” means
“going forward”: H.M. could remember things from before his
surgery, but he seemed totally unable to develop new memories. The
outcome of studies of H.M. following his surgery was that we now
know that the hippocampus is critical for the consolidation
of certain forms of memory.
What we think of when we think of memory—something that is learned and then later recalled—is really long-term memory. In the initial processing of information, we use what is called working memory. Working memory enables us to hold a limited amount of recent information in mind, allowing us to answer questions, follow instructions, or make computations.
A test of working memory is the digit span. The subject is asked to repeat back a list of single digit numbers. The longest list that can be repeated correctly is known as the digit span.
Bilateral removal of his hippocampus had no effect on H.M.’s working memory. H.M. had a normal digit span, could engage in conversation and was quite capable of following instructions when he performed psychological tests. He could remember the name of a new acquaintance as long as he rehearsed or repeated it in his mind. However, once he was distracted, he would immediately forget.
The region of the brain thought to be most concerned with working memory is the prefrontal cortex, the area in the frontal lobes that is anterior to the motor control regions. Other regions of the cerebral cortex may be active depending on the task, but the prefrontal cortex is necessary to focus attention.
Long-term memory is what we classically think of as memory. As a result of our experiences (learning), the brain changes so that we can store information for long periods of time, and then later retrieve that information when we need it. Long-term memory encompasses both recent memories (what happened a few minutes ago) and remote memories (what happened years ago).
Because H.M. could remember facts, events, and people from prior to his surgery, this means that the damage to his hippocampus did not affect long-term memory storage and retrieval. Long-term memory storage is thought to depend on different areas of the cerebral cortex. What was greatly affected in H.M. was the ability to create new long-term memories. We now know that activity in the hippocampus is necessary to transfer information to the cortex so as to create a long-term memory. This process is known as memory consolidation.
Studies of H.M. also revealed that long-term memory has different qualitative forms. There are two broad categories: declarative and procedural memory. Declarative memory (also known as explicit memory) is conscious memory for facts, places, names, and events. After his surgery, H.M. lost the ability to form new declarative memories.
Procedural memory (also known as nondeclarative or implicit memory) refers to the ability to learn motor skills. Procedural memory can be thought of as “knowing how”, while declarative memory can be thought of as “knowing what”.
The psychologist Brenda Milner was the first to show that procedural memory was intact in H.M. through the use of the mirror drawing task. He was required to trace a five-pointed star without directly viewing the star or his hand; he could only see the image and his hand reflected in a mirror. Just like control individuals, H.M.’s ability to make the tracing improved with repeated trials, even though he never had a memory of doing the task. Motor learning does not require the hippocampus, but instead depends upon activity in the cerebellum and basal ganglia.
For those who are interested in reading more about memory
studies and H.M., check out this biography:
Corkin, Suzanne (2013) Permanent Present Tense, The Unforgettable Life of the Amnesic Patient, H.M. Basic Books
This paper provides a nice clinical review about human memory
Budson, Andrew, E. (2005) Memory Dysfunction. New England Journal of Medicine 352: 692-9 link.
Off-campus access: open LINK TO PROXY SERVER. Next, paste the URL [http://www.nejm.org/doi/full/10.1056/NEJMra041071] in the "URL to proxy" box and click login.