Older adults with failing memories often state that they have difficulty remembering what happened to them yesterday, yet they can remember very well events that occurred when they were in high school or college. These impressions were probably noted from time immemorial, but they were given scientific credibility by Ribot, a nineteenth-century French neurologist, who proposed that following brain damage, recent memories are more likely to be lost than remote ones. Even relatively mild damage, such as a concussion sustained during sport or in a car accident, will cause similar effects. The reason given for this pattern of lost and preserved memories is that remote memories were consolidated (made relatively permanent) in the brain before the onset of damage (or deterioration, in the case of older adults), whereas the damage hindered the consolidation of more recent memories and left them vulnerable to decay or interference.
In the 1950s, Brenda Milner and her collaborators, William Scoville and Wilder Penfield, reported that damage to the hippocampus, a region on the inside surface of the medial temporal lobes, which lie next to your temples, leads to a profound amnesia that conformed to Ribot's Law. Henry Molaison (H.M.), the most celebrated of the neurosurgical patients, could acquire few, if any, memories of which he was consciously aware from the time of his surgery in 1953 to his death in 2008. Initial, casual observation indicated, however, that he remembered events that occurred prior to his surgery very well. Based on this evidence, and that of similar cases, investigators concluded that the hippocampus is a temporary memory structure that helps retain and retrieve memories until they are consolidated in other parts of the brain. Once consolidated, the hippocampus is no longer needed for their retention or recovery; these functions would now be mediated by other parts of the brain. This was the standard consolidation theory accepted by most neuroscientists and enshrined in most textbooks.
Fifty years after the report on H.M. was published, the standard consolidation theory reigned supreme. Lynn Nadel, Gordon Winocur, and I, with the help of our students and collaborators, dethroned this theory and replaced it with an alternative, which has evolved into the “Multiple Trace Theory and Trace Transformation.” According to this theory, every time you consciously experience a new event or think of an old one, the hippocampus automatically lays down a memory trace.