In humans, the act of forgetting may seem like it’s the same thing as clicking the ‘delete’ option on your computer. Luckily, most digital files aren’t really deleted forever, as computers often have backup features or ways to recover deleted items. As it turns out, memory in the brain may work the same way. 

A study co-led by a U of T researcher found that forgetting seems to involve a special nervous system state where memories can be reactivated, so it is not a total erasure of memories. In this article, The Varsity has broken down the basics of the study’s findings and their consequences for memory and forgetting. 

A novel state of nervous system functioning 

Due to the structural and functional similarity of its nervous system to that of humans, researchers chose the roundworm Caenorhabditis elegans, a common model organism for research on the nervous system, to model the neurological mechanism of memory. 

The researchers first established an aversive memory — a memory of something the a worm would want to avoid — in C. elegans worms. They were trained over a span of four hours to become familiar with the odour of a disease-causing bacteria called Pseudomonas aeruginosa

Immediately after the exposure, the worms avoided coming into contact with the bacterial odourants. They’d even travel a relatively long distance to avoid the unpleasant smell. However, the researchers found that an hour after exposure to the bacterial odour, the worms no longer appeared to remember their initial dislike of the scent. 

Reactivation of established memories 

The researchers then decided to investigate the possibility of reactivating the forgotten memories in the roundworms.

After the designated one-hour window reserved for forgetting, the researchers attempted to remind the roundworms of their aversive memories by deliberately exposing them to the odorous bacteria for three minutes. After this brief exposure, C. elegans subjects exhibited significant effort to avoid the bacterial smell, when compared to worms that had ‘forgotten’ about their initial exposure to the bacteria. 

To confirm the change in the roundworms’ behaviour, researchers used another method to test the worms’ ability to reactivate the aversive behaviour. The researchers alternately exposed the worms to odourants from the original strain of bacteria and then to odourants from a separate, benign bacteria called OP50, five times. They tested one marker of these roundworms’ memory retention and found that it was significantly higher than in untrained control groups of worms.

The study’s significance 

Scientists cannot test current hypotheses on the process of forgetting without first understanding these cognitive processes on a deep level. In this study, researchers manipulated the trained olfactory response of C. elegans to demonstrate the existence of a unique memory state of the nervous system that can be reactivated by environmental cues such as re-exposing the roundworms to odorous bacteria. 

This novel state of cognitive functioning prompts behaviour that’s fundamentally different from C. elegans who were never given aversive memories and ones that were regularly conditioned to remember the bacterial odour. 

Although the roundworms that were not exposed to the bacteria and the roundworms that seem to forget about their former exposure resided in the same laboratory conditions, and were not regularly exposed to threats, the worms who were formerly exposed to the bacteria were able to easily relearn aversive memories that researchers had established. The discovery of this novel state of memory explains how trained animals can exhibit behaviours we might think they’ve forgotten, when they encounter conditions that are similar enough to reactivate their memories of past learning experiences.