An important alternative classification of long-term memory used by some researchers is based on the temporal direction of the memories.
Retrospective memory is where the content to be remembered (people, words, events, etc) is in the past, i.e. the recollection of past episodes. It includes semantic, episodic and autobiographical memory, and declarative memory in general, although it can be either explicit or implicit.
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|MRI studies have shown that the same parts of the brain are used when remembering the past as when imagining a similar event in the future, which shows that past memories are also accessed and drawn on when projections are made into the future.|
This is sometimes referred to as “mental time travel” as it allows us to project ourselves at will either backwards or forwards in time within our personal lives.
Prospective memory is where the content is to be remembered in the future and may be defined as “remembering to remember” or remembering to perform an intended action. It may be either event-based or time-based, often triggered by a cue, such as going to the doctor (action) at 4 pm (cue), or remembering to post a letter (action) after seeing a mailbox (cue).
Clearly, though, retrospective and prospective memory are not entirely independent entities, and certain aspects of retrospective memory are usually required for prospective memory. Thus, there have been case studies where an impaired retrospective memory has caused a definite impact on prospective memory. However, there have also been studies where patients with an impaired prospective memory had an intact retrospective memory, suggesting that to some extent the two types of memory involve separate processes.