Long Term Memory

Dec 18, 2008 by Celastrus Shop

Long term memory has a potentially limitless capacity. Or it could be that there is a limit to it, and we just don’t live long enough to discover it. Some researchers have suggested that we never actually forget information; instead, the process of active recall degrades over time, meaning that older adults have more trouble bringing old information to mind.

This is but one of the mysteries surrounding long term memory. Like all brain functions, modern science has been busy working out the technicalities of how we create, store, and recall memories. It seems long term memory is stored as meaning; or more precisely, as a string of visual and verbal associations which encode meaning in multiple neural networks within the brain. Long term memory has a much larger capacity and higher level of organization than short term memory. By contrast, short term memory can store about seven items maximum before it must either transfer them to long term memory or erase the items to make room for new content.

Known as the “dual store” memory model, in this system short term memories can form temporary connections to long term memory once they enter the brain. However, most items in your short term memory will be dismissed after about 20-30 seconds if you don’t make some effort to remember them. The process of encoding short term memories into your long term memory system is commonly called consolidation, or in neurobiological terms, “long-term potentiation”. Fortunately, research suggests that we have considerable influence over which memories are consolidated into our long term memories: repeating and reviewing something you want to learn, and associating it with meaningful content that already exists in your long term memory, seems to determine to a large extent which items you will remember.

Through this process, what was a temporary strengthening of neural connections is repeated and “set” long enough that the actual structure of the neurons is physically changed. Once the neural connection is made more solid, long term memory can last from anywhere from thirty seconds to decades and beyond. Once established as long term, each memory is subject to fading, for assorted reasons, in the natural forgetting process. As such, multiple recalls of a memory may be necessary for long term memories to last to their full potential. This also depends on the depth at which the memory is processed (how strong the neural connection becomes, through meaning and association).

To put this all simply: long term memories are stored at a rate and depth dependent on the amount of meaning associated with the memory, and will last potentially all our lives, depending on how often and meaningfully we recall them. This recall can be deliberate or happen naturally, and is often dependent on our individually perceived importance of the material.

Our long term memory most likely stores information in neural networks, which psychologists call schemas. Schemas are basically mental models of the world that the brain creates to store and organize information. Current scientific thinking posits that these schemas interlink into intricate knowledge structures that we access (consciously or unconsciously) every time we learn something new or recall an old, previously learned piece of information. In other words, schemas help us learn by offering a framework of meaning; the brain can insert a new piece of information into this framework by linking it to related items already encoded in long term memory.

As you might expect for such a vast network, long term memory can be separated into several distinct categories. The most obvious kind of memory– what you probably think of when you read the word “memory”— is explicit, or declarative memory: these are the memories we have conscious access to, and come in two subcategories, episodic and semantic memory. Episodic or autobiographical memory refers to your knowledge of past events in your personal life. Semantic memory refers to knowledge of factual information, like vocabulary terms or the names of state capitals.

The other well-defined type of long term memory is implicit or procedural memory. This is knowledge that you’ve retained implicitly but may not be able to access consciously, and is often related to skills you have acquired. Remember the old saying, “One you learn how to ride a bicycle, you never forget?” It’s actually true! Riding a bike is one of many acquired skills that involves learning how to move your body in a certain way. Once you master bike riding (or swimming, etc.), the information is encoded into the cerebellum— the part of the brain responsible for processing movement— as implicit memory. Learning how to manipulate objects in the physical world, such as handling a pen or driving a vehicle, is also encoded in implicit memory. We usually access implicit memory at a subconscious level once we have learned how to perform the action.

A couple relatively unexplored domains of long term memory are prospective memory and emotional memory. Prospective memory is the memory of upcoming events, or actions that you plan to do at a future date. While most memory is retrograde and records events that have already happened, prospective memory is essential for foresight and our ability to plan ahead.

Finally, emotional memory may not be a distinct category so much as a quality that can color any kind of long term memory. It is mostly being studied in relation to explicit memory. Psychologists define emotional memory as any consciously available memory that nevertheless evokes a strong emotional or involuntary physiological reaction (positive or negative) when it is recalled.

All these aspects might be linked together into a larger schema that stores a complicated long term memory: for instance, when you remember the first time you saw one of your favorite movies, you might recall the story of the film itself (semantic memory), where you saw it and with whom (episodic), and the emotions that the story and the context evoked in you (emotional). It is this connection between networks of meaning that makes long term memory so powerful and available to your faculties of recall.