Memory and Reactivation

As I have mentioned before, my lab has a particular interest in mechanisms involved in the formation and alteration of memory. One of the specific areas of interest is reactivation, or the replay of waking behaviors while asleep. I want to distinguish between the reactivation I’m discussing here and the reactivation we discussed in the memory episode of the podcast because they are related, but distinct. In the Bizarre Science segment, we talked about experimentally controlled reactivation of a specific memory engram. This is similar in concept to the replay I’ll be discussing, but theirs was forced reactivation during wakefulness rather than naturally occurring.

Let’s imagine I’m remembering my route to the bus stop. First I walk out my front door, then I take a left onto the sidewalk, walk past the coffee shop and take a left at the stoplight. We’ll talk about this more in later episodes, but there are neuronal cells that will fire preferentially at specific locations—they form a literal mental map as I’m either physically walking that path to the bus top or remembering walking that path. This forms the foundation of studies exploring memory replay in rodents. One can record from these cells, called place cells, in real time as animals are walking a track. You can then give them a period of wakeful rest and see that these place cells are firing in a similar sequence to that of when the animal was walking. You can then give them a period to sleep, and you’ll find this reactivation is occurring, but at a faster rate! Some within the sleep community, including myself, believe this reactivation is a vital function of sleep. Where reduced awareness of stimuli from your surroundings allows for reactivation of your waking activities with little interference. This would also give your brain time to sort new information—forming associations between new and old memories while getting rid of what is unimportant.

This sounds all fine and great, but there is a downside to this. Not only are your true memories stored and preserved during sleep, but false memories can also be strengthened during sleep. Studies conducted using word list learning tasks in humans showed that sleep improved false memories just as well as true memories. These sorts of findings can have major societal impacts when considering how fundamental ways in which our brains function influence things like our accuracy in remembering the facial features, height, or clothing of a perpetrator we saw fleeing a crime scene. This is where we see neuroscience seeping into policy and law. For decades, eye witness reports were considered the golden standard until there was evidence to prove just how unreliable they can be. There are multiple steps during encoding where what actually happened can become separated from the memory formed, and how that memory can even be changed across periods of sleep!