by Brad Norman

Created on: April 30, 2009   Last Updated: May 02, 2009

Among the brain's many functions, one of the most famous and interesting is memory. In its most basic definition, memory is simply a long term change caused by a stimulus. Though most people refer to memory as the ability to recall facts or names, there are actually several forms of memory, each thought to be governed by different areas of the brain.

The two most broad categories of memory are "explicit memory" (also called declarative memory) and "implicit memory". Explicit memory is perhaps the more commonly thought of type of memory and includes the ability to recall facts, phone numbers and events that have happened. Implicit memory, on the other hand, is an unconscious form of memory. This involves the less efforted tasks, such as skills you have learned or conditioned responses. It's things you remember how to do without having to think about them, such as riding a bike or walking. You don't have to consciously recall the steps involved in the activity, you simply just do it.

These two types of memories are separate and controlled by different areas in the brain. Due to this fact, it is actually possible to lose one type of memory while leaving the other in tact. This was seen in the famous patient, nicknamed H.M.

H.M. had a large portion of his medial temporal lobe removed in a surgery that was intended to help control convulsions he had been experiencing. This lobectomy removed important brain structures such as the hippocampus, amygdala (ah-MIG-da-LAH), and some of his cortex. The effects of removing these structures drastically impacted his memory. He could no longer form new explicit memories. That is, someone could introduce them-self to H.M., but as soon as they left the room he would forget having ever met them. Interestingly enough, H.M. maintained his ability to recall memories that were formed prior to his surgery.

This led scientists to conclude the the hippocampus and amygdala are the structures responsible for forming explicit memories, but these structures do not appear to be involved in the storage or retrieval of memories. It's analogical to the gas input hole on your car. It's where the memories (gas) goes in, but it's stored somewhere else (the gas tank) and retrieved somewhere else (the engine).

Also, H.M. still had the ability to form new implicit memories (memories involving unconscious skills). This was demonstrated by teaching H.M. a fairly difficult task. When asked to do the task on future trials, H.M. was able to complete the task

• 1
• 2
• Next Page >>