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 in a shorter amount of time, showing that he remembered the task. However, because his explicit memory was gone, he could not remember having learned the task. As far as he was concerned, the future tasks were the first time he’d ever done the task, despite the fact that he was now better at it.
The search for a specific structure that is responsible for implicit memories has come up almost empty. It is now thought that since implicit memories are so much more simple and primitive than explicit memories, no specific structure is needed. Instead, these memories are stored in the synapses of a group of neurons. A phenomenon known as Long-Term Potentiation is thought to explain implicit memories. Long-Term Potentiation is a fancy way of saying that when two (or more) neurons fire at the same time, they tend to stick together more tightly and its then more likely that they’ll fire together in the future. When neurons are linked together in this way, it causes a physical change in the arrangement of neurons and thus is reasonable to credit this as a form of memory. It is this kind of memory that caused Pavlov’s dog to salivate at the sound of a bell, because the neurons triggered by the sound of the bell became linked to the neurons triggered by the smell of food. Since these two, formerly unrelated, groups of neurons are now linked together, both groups now fire to either stimulus, resulting in the salivation reflex at either the presentation of food or the sound of the bell.
Even more simple forms of this linking of neurons has been demonstrated in very primitive organisms, such as slugs and snails. Since these organisms have no well defined brain yet are still capable of carrying out implicit memories, it is likely that even in humans there is no specific brain structure that is responsible to these types of memories.