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Scientific Understanding of Consciousness |
Nonconsolidated Hippocampus-Dependent Memories
Short-term memories do not require the hippocampus. Long-term memories require the hippocampus for consolidation but not for recall. Memories longer than the short-term memories require the hippocampus for storage in consolidation but not for recall. Epilepsy patient HM who had his hippocampi removed demonstrated that usual conversation can be maintained and long-term memories recalled without the hippocampus. Nonconsolidated hippocampus dependent memories comprise a large portion of our memory storehouse spanning a time. From a minute ago to perhaps a few days ago: Where in the parking lot did you park your car?; did you talk with your friend, John, yesterday?; did you remember to mail the package at the post office this morning? Items such as these will be forgotten months later but typically can be recalled for a few hours or days. Just try to remember your hotel room number two weeks after you have returned home. However, it would be very inconvenient not to remember your room number in order to retrieve your key from the desk clerk at the hotel. If any of these nonconsolidated hippocampus-dependent memories tend to have long-term significance, the neural circuitry will actively rehearse during sleep and the memory will be consolidated as a long-term memory. Otherwise, the synapses involved will be reused for other memories, and those of lesser significance will fade. I don't know a specific name for these nonconsolidated hippocampus-dependent memories, but they fall somewhere between short-term memory and long-term memory.
Immediate memory refers to the information that occupies our current stream of thought. Immediate memory can be extended to last minutes or more by rehearsal. (Squire & Kandel; Memory, 131) The extended transient phase of memory, which can last as long as an hour or even more, constitutes we call short-term memory. (Squire & Kandel; Memory, 131)
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