Question: What mechanisms are involved in implicit and explicit memory, and what is the importance of the hippocampus in explicit memory?

Carolyn McGettigan answered on 25 Mar 2010:

Explicit memory is that involved in conscious recall for facts about the world (semantic memory) and events (episodic). Implicit is memorised information that isn’t necessarily accessed consciously, for example how to ride a bike or how to walk. I say not necessarily because I suppose you could direct your conscious attention to how you do these things (e.g. thinking ‘how do i climb stairs?’ while actually doing so is usually quite a peculiar experience). Other examples of implicit memory are in things like priming, for example where performance on a task like deciding if a string of letters is a word or a made-up word can be faster if it is followed by rapid, subliminal presentation of an item that is related to the target (e.g. deciding that ‘play’ is a word is faster if it is preceded by ‘play’ than if preceded by ‘crow’, even if the participant hasn’t seen the ‘prime’ word).

It seems that explicit and implicit memory can be separated functionally to some extent. There is the famous example of the patient HM, whose medial temporal lobes were damaged in surgery and who lost function of his hippocampus and surrounding cortex. He lost the ability to remember event from his past and to form new long-term memories for events. However, he could still perform learned tasks and acquire new skills. He could also do short-term memory tasks like immediate repetition of word lists. This case suggested that the medial temporal lobes, including the hippocampus, are crucial in explicit memory, but that implicit memory and short-term memory can be supported by other parts of the brain. I think that the hippocampus is still probably involved in all sorts of memory tasks – at some level, you could say the brain might need to encode everything in conscious experience for potential explicit recall in the future. I’ve seen the hippocampus involved in a short-term memory task in fMRI, where I got people to listen to made-up words, like ‘trepidoskam’, and repeat them aloud after 6 seconds. This is a classic short-term memory task, and I found that people who were better at repeating the made-up words were those people who activated their left hippocampus more strongly.