Researchers at the Donnelly Centre for Cellular and Biomolecular Research have made new insights into why some things can’t be learned — or rather, can’t be remembered.

In a study published in Scientific Reports, Daniel Merritt, a PhD student in Dr. Derek van der Kooy’s lab, and his colleagues found that memory blockages in Caenorhabditis elegans, or nematode worms, are likely due to issues with memory recall, as opposed to memory storage. Merritt’s publication is the first to confirm that memory blocking occurs in worms and refutes the running theory that memory blockage is rooted only in memory storage.

Memory blocking: what is it?

The phenomenon of memory blocking was first characterized by American psychologist Dr. Leon Kamin in the 1960s.

Kamin devised a set of experiments in which he played a tone before giving a rat an electric shock. After repeated tone-shock pairings, the rat learned to freeze at the sound of the tone. Kamin then played a tone and flashed a light before shocking the rat.

Kamin was intrigued to find that when he later tested the rat’s reaction to the light alone, it didn’t freeze. It was as if the rat had never learned this newer light-shock association. It was ‘blocked’ from memory.

Kamin’s findings made an enormous impact on the way researchers viewed and studied memory.

“People [came] to believe that in order for memories to form, there needs to be something surprising about what is happening to the animal,” explained Merritt in an interview with The Varsity. According to this theory, the rats in Kamin’s experiments had already learned that the tone predicted the shock and therefore didn’t form a memory of the light’s association to it.   

But Merritt and his team’s research points to another explanation: while a new memory is formed, it just can’t be recalled.

The researchers trained worms to avoid ammonium chloride — a substance they normally crawl toward — by starving them in its presence. They then starved these same worms in the presence of both ammonium chloride and benzaldehyde, a compound that has an almond scent. Similar to Kamin’s rats, the worms didn’t learn to avoid benzaldehyde.

Switching the order of the chemicals yielded the same results. Worms that were trained first with benzaldehyde and later simultaneously with benzaldehyde and ammonium chloride didn’t learn to avoid ammonium chloride.

Blocking the minds of worms

The researchers used fluorescent tags to observe the molecular motions in the worms’ neurons during the memory-blocking tasks.

When a worm formed a memory of benzaldehyde, a protein kinase enzyme called EGL-4, which is important to cell signalling, moved toward the nucleus of the neuron. When a tag was attached to EGL-4, its movement could be visualized under a microscope.

Even in the blocking condition, when the worms behaved as if they hadn’t learned about benzaldehyde, the researchers saw EGL-4 moving toward the nucleus.

“We’ve shown that [the worm] does learn, it learns perfectly well, it just can’t remember afterward,” said Merritt. “So the memory is formed, it [just] seems to be inaccessible — like a sort of amnesia.”

The researchers hope to pinpoint the exact molecular mechanisms behind these irretrievable memories.

Memory blocking is difficult to study in humans for various reasons. However, where the human brain has an estimated 86 billion neurons, adult hermaphrodite C. elegans worms have just 302. These neurons are connected to each other in nearly identical ways across worms.

“I think worms are the perfect animal to [study] because, again, you can identify particular cells, [and the] particular molecular changes that are occurring in them,” explained Merritt.

“I think that memory is this really interesting thing. It’s ephemeral; it seems hazy, nearly intractable at times — very difficult to get a grip on,” said Meritt. “But at the same time it forms the narrative thread that unites our experiences and integrates the times in our lives to form us as people.”