From PhysOrg.com :
People may permanently store memories in their brains, even if they cannot consciously recall them, according to a study by Duke University Medical Center researchers.It appears we don't lose the data, just the (metaphorical) URL. The data becomes low on the brain's search engine ranking, so we forget that we've seen it before.
"This finding provides insight into a fundamental neurological process and also may help us develop a tool for identifying so-called lost memories," said Roberto Cabeza, Ph.D., a member of the research team and associate professor of psychology and neuroscience in the Duke Center for Cognitive Neuroscience.
In the current study, Cabeza and his colleagues used a sophisticated imaging technique to detect brain activity in the medial temporal lobes (MTL) of test subjects exposed to "new" and "old" experiences. Located deep inside the brain, the MTL is known to play a role in a person's ability to determine whether something happened in the past.
When subjects viewed an old word, they exhibited heightened activity in the rear portion of the MTL, whether or not they correctly stated that the word was old, Cabeza said. "This indicates that the brain has the correct answer even if we don't consciously think we've seen the word before," he said.
So why would a person make a mistake when asked about an event's oldness, if his or her brain holds the correct answer?
The researchers found that when a subject correctly reported seeing a "new" word, the scanner indicated that there was heightened activity mainly in a front portion of the MTL, rather than in the rear portion, as happens with old words. But when a subject mistakenly classified as new a word that was actually old, activity increased in both parts of the MTL, Cabeza said. This may lead the MTL to give mixed messages, resulting in an incorrect conscious response, he said.
I conjecture that the amount of data required to retrieve the memory is far greater than the momory itself. It is a "relational database", with multitudinous links to comparatively small bits of data. After a time of disuse, the neural connections fade, the neurons get re-used for other applications, and although some remain, not enough when compared with dreams and other stored data to cross the threshold of recognition. We don't have a memory so much as a forgettery, with bits of data still taking up space long after the reason for them to be remembered is gone.