Memory is essential for learning. Learning and memory are complicated processes of the brain in which information from our experiences (i.e., our senses) can be stored and then retrieved later. Memory and learning are important evolutionarily but also for everyday functioning. When these processes fail, as in neurodegenerative diseases, simple tasks become very difficult.


Much of our understanding about learning comes from studies on individuals with brain lesions. The most famous case was a man named H.M. with intractable epilepsy who underwent surgery to remove both of his medial temporal lobes. After the surgery, H.M. had significant memory deficits. He could not form new memories. He could not remember what happened the day prior, could not remember anyone he met, but was able to recall remote events in his life. Remarkably, H.M. retained his ability to improve his performance on procedural tasks despite being completely unaware that he had learned it previously. What H.M. taught us was that forming new declarative memories require structures in the medial temporal lobes (hippocampus and related structures), but over time memories become stored in other areas of the brain and are independent of the medial temporal lobe structures.


In humans, storing information involves two systems: declarative (also called explicit or conscious memory) and non-declarative (also called implicit or unconscious memory).


Declarative memory is memory that can be “declared” by language and is within the realm of our consciousness. Declarative memory is further divided into Episodic memory (i.e. memory for events and experiences) and Semantic memory (i.e. memory of facts).


Non-declarative or implicit memory or “procedural” memory is within the realm of the unconscious. That is, when we learn to ride a bike or play an instrument, it is very difficult to explain in words “how” we do it. In fact, focusing on every detail of how we are riding a bike may make riding a bike more difficult.  Non-declarative memory does not require us to be aware that we are “remembering” what it is we are trying to remember. Some people call this “muscle memory.” Non-declarative memory is further divided into Skills and Habits, Priming, Classic Conditioning, and Non-associative learning (also called habituation).



Memory can be classified by time


Learning and Memory can be thought of as processes that occur in stages over time:


(1) Acquisition is the first stage where information is acquired through our primary sensory organs and “registered.” This first stage has also been called registration and occurs within fractions of seconds. The primary structure involved is the prefrontal cortex


(2) After information is registered, the information then moves into the next processing stage called Short Term Memory (also called working memory or immediate memory). Information can be retained in this form of short term memory for seconds or minutes and the primary structures involved include the Prefrontal Cortex and Hippocampal Formation (within the temporal lobe). Short term memory is easily disrupted by distraction and can be enhanced with rehearsal. For example, when told a phone number, if someone diverts your attention while trying to remember the number, you will likely forget it. However, repeating the number over and over in your head will improve your ability to remember it. 


(3) If we were only able to remember things for a few minutes, we would have a very difficult time surviving. We would injure ourselves or repeat the same mistakes over and over again. Therefore, the ability to store information for longer than a few minutes is essential to an organism’s ability to survive. This is where Long Term Memory comes into play. Long term memory occurs when short term memory is “consolidated” into a more permanent form that may last for days to years. The primary structures involved include the Association Cortices and Hippocampus and requires changes in the synaptic connections between neurons (e.g., Long Term Potentiation/Long Term Depression). Long-Term Potentiation (LTP) is a process that occurs between neurons whereby their connection become “stronger” and/or more efficient. This requires NMDA/AMPA/Ca++ mediated changes that are not discussed here. 


(4) The final stage is Retrieval, the process in which the stored information is brought to awareness when needed. 


It is important to note that this process is not necessarily linear. This is a dynamic process involving multiple stages occurring simultaneously as new information is added to old information in ways that are still poorly understood. 



Below we review the various components of memory as well as the mechanisms responsible for remembering things.

Declarative Memory, Nondeclarative Memory, and the Neurobiology of Memory


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