well memory is still a poorly understood phenomenon, esp in the area of Long term memory. There are a number of processes that can help to explain short term memory, such as facilitation between synapses. But it remains for the most part a mystery. Part of this has to do with Penfields experiments at McGill where using a very small electric stimulus was able to provoke vivid memories in the patient. Yet a large lesion to the brain does not knock out memories of say 1975 to 1980. (drugs might), but leads usually to fuzzier recall across the boards; think of a holographic image here. The film can stand up to significant damage, and unlike a photo negative, you don't lose all of a piece of the image, more like a piece of all of the image.