What happens when a memory disappears? Once upon a time, I could actually recall the details of organic chemistry. But then I took the class final and promptly forgot every piece of information related to the chemical properties of the carbon atom. This raises the obvious question: What does it mean to forget? Do we actually delete our memories, like an unwanted computer file expunged from the hard drive? Or does the memory always remain, a persistent neuronal ensemble that we just forget how to find?
A new paper in Neuron by scientists at UC-Irvine and Princeton suggests the latter alternative. The forgotten memory doesn’t disappear – we just can’t remember where we put it. The experiment went like this: a few dozen undergrads were put in an fMRI machine and shown a series of different words. They were then asked to perform various cognitive tasks, such as thinking about how the word is used, or pronouncing the word backwards.
A short while later, the students were shown the same words a second time and were asked to recall any details associated with the words. Once again, the fMRI machine captured their brain activity by measuring shifts in cortical blood flow. Here’s where things got interesting: When the students demonstrated a strong recollection of the task details, their brain produced a pattern of activity that was nearly identical to that produced during the task itself. In other words, the memory was a facsimile of experience. However, when the students failed to recall the task details the brain scanner still revealed a recognizable pattern of activity. (The pattern was weaker, but persistent.) Although the students had no memory of the task, their brain clearly did.
One of the interesting implications of this experiment has to do with why memory declines with age. The conventional assumption is that memory loss occurs because our memories vanish, because cells die and the hippocampus gets tired. But what if memory loss is actually triggered by the steady degradation of the frontal cortex, a brain area associated with memory retrieval? (The frontal cortex starts to lose cell density at about the same time we start to lose our memory – in our mid-thirties.) This suggests that our memories are still there, waiting to be found, like a misfiled piece of paper. The struggle of aging, then, isn’t simply a matter of holding on to the past – the brain has a seemingly infinite hard drive. Instead, the challenge is remembering where all of our memories are.
For more on the subject, check out this article on tip-of-the-tongue moments.