Memories can be divided into two types: implicit and explicit. Only the explicit memories require conscious attention when they are formed and produce conscious memories when they are retrieved. Implicit memories are formed without our awareness and either not retrieved in the normal sense or retrieved as guesses. Implicit memories may come in various types: a procedural memory like the skill of riding a bicycle; a perceptual memory that allows us to guess at something we have perceived when we are primed with a small part of the perception given in the same sensory modality as the original encounter; and finally, emotional conditioning.
Perceptual implicit memory and explicit memory differ in a number of ways:
1. They are different types of events in the brain. Implicit storage occurs 200-450 msec. after an event with a negative-going potential in the centroparietal region. Explicit storage occurs 900-1200 msec. after an event with a positive-going potential in the right frontal region.
2. Damage to the hippocampus interferes with explicit but not implicit memory.
3. Explicit memory retrieval is accompanied by feelings of remembering, recognition or familiarity and these feeling are absent when implicit memories are retrieved.
4. Implicit memory is not affected by the depth of processing of the original event whereas explicit memory is stronger if the original event is attended to in more depth.
5. Implicit memories can not be experienced or described in language. Explicit memories are experienced consciously, can be reported and are holistic with additional aspects that were not part of the retrieval specification.
6. Implicit memories using priming can be more accurate then explicit memories. However implicit memories are more prone to the illusion-of-truth type of errors.
Explicit memory is semantic or episodic. The picture appears clear that in order to have a conscious memory, the original experience must have been part of consciousness. The formation of these memories probably is an important function of consciousness.