We forget, a lot. It’s always been assumed that we forget either because new information is coming in and ‘overwrites’ (replaces) older memories, or because memory just sort of degrades. There’s some kind of selection at work, of course, because some things we forget more readily than others. A new study by a team from Tsinghua University (Beijing, China) and Cold Spring Harbor Laboratory (New York, USA) reveals that memories are actively removed and at the molecular level ‘overwriting’ and ‘degrading’ are one and the same.
The research work was performed with drosophila melanogaster (fruit flies), which were first trained with two odors and one followed by a pain stimulus. Over varying periods of time, the flies were tested for memory retention. A second experiment used two new odors to confuse the flies. A third experiment reversed the odors followed by a stimulus. In all cases, when the flies forgot, their brain cells revealed the presence of a molecular pathway including the small protein called Rac. The amount of Rac varied, and it was discovered that for certain memories the action of Rac was being blocked – thereby increasing the length of retention for a memory.
The flies forgot what they learned after some period of time in a process driven by Rac. Rac switches on when flies simply forget with the passage of time, they report. It just switches on faster when the insects either get distracted by new information or “confused” by conflicting experiences. When Rac was blocked, new memories decayed more slowly, extending their life from a few hours to more than a day. When Rac levels were artificially increased in fly neurons, the insects’ new memories were erased more rapidly.
The findings open up a whole new avenue of study in neuroscience of the process of forgetting, said researcher Yi Zhong. Ironically, this line of exploration may turn out to reveal much about how memories are made.
“We still don’t really understand the substrate of memory in terms of what is formed and what is erased,” Zhong said. “The study of forgetting may be a better way to identify the material basis of memory.”
Programmers will be familiar with the concept of actively removing memory; it’s called a ‘garbage collection’ service in computer memory, where no longer used bytes are cleared. Very often there are special routines to determine what memory to clear and when. Something like this seems to be happening with organic memory, although at this point the exact mechanisms and ‘programming’ behind it is unknown. This is one of those discoveries that provides a new ‘paradigm’ (model) to consider for memory – one that is more complex in that it opens the possibility for organic clock timing, selection protocols, and feedback pathways all involved with ‘forgetting.’ A much more complex picture than previously thought.