Research into the way memories are encoded, stored, and retrieved in the brain has come a long way since the 1960s, when scientists began experiments with H.M.—an epileptic patient who had large swathes of his brain, including the hippocampus, surgically removed, and was no longer able to form long-term memories.
Neuroscientists now widely accept that the hippocampus is crucial for turning short-term “memory traces”—temporary changes in the brain’s wiring that result from experience—into long-term memories, but the debate continues over how memories are retrieved and “updated” once they have been made, and how the act of recalling a memory affects its content. The current study sought to explore the effects of repetition on memory retrieval in order to better understand how the process works.
The results support the authors’ “Competitive Trace” theory, which hypothesizes that repeated instances of a learning event—such as seeing the same image multiple times—create a series of similar but not quite identical memory traces in the brain. Even though the image itself is held constant, other factors such as your attention, your emotional state, or the aspects of the image you're focusing on, will vary from one learning event to the next. So later, when you want to recall that image, all of the different memory traces have to compete with each other, and only the overlapping central features of each memory become strengthened in the brain. At the same time, all of the nonoverlapping information, the contextual detail, begins to fade.
The authors conclude that when we learn by repetition, we rely upon a sense of familiarity more than we do on an accurate recollection: “Our findings suggest that although the ability to generally recognize something is strengthened with multiple encounters, one’s ability to discriminate among similar items in memory decays... in contrast to past beliefs, repetition may reduce the fidelity of memory representations.”
So while repetition may be fine for remembering basic information or concepts, what about when it comes to remembering important scenes from our past, when the small details matter more than the overall gist? This study’s findings—if they hold true over longer periods of time, or for memories more complex than simple image recall—may have implications for the way we conceptualize memory in the classroom, on the witness stand, or in our day-to-day lives. For example, presenting evidence multiple times to those on trial could impact the detail and quality of testimonies.
Ultimately, this research reminds us that memory is not a static record of the past embedded in our neurons; it is a living, imperfect form of thinking that is constantly being rewritten, whether we intend it or not.