Forgotten, But Not Gone: Sea Slug Brains Show Traces of Lost Memories
All these years it seemed impossible that you would ever remember the name of that cute guy or girl you sat next to in biology class but never said a word to.
But there’s hope.
Two Chicago-area researchers have uncovered what they think is the first piece of physical evidence showing that forgotten memories could still live on inside our brains.
Scientists have for years disagreed about what happens inside the brain when memories fade, with some arguing that the underlying memory trace decays and others saying that it merely becomes inaccessible.
One potential clue can be found inside a lab at Dominican University in suburban River Forest, where married professors Bob Calin-Jageman and Irina Calin-Jageman have worked with students for the past decade to study the molecular structures associated with long-term memory.
Last year, Bob, a psychology professor and director of Dominican’s neuroscience program, and Irina, a biology professor, turned to their favorite research subjects – sea slugs – to see if they could find traces of lapsed memories.
The animals are ideal for studying the process of learning and memory because they have only 20,000 neurons in their central nervous system (compared to 60-80 billion in human brains), making it easier for scientists to figure out how they learn. The slugs also have some of the largest neurons in the animal kingdom, making it easier to record the electrical activity that takes place inside the cells.
Working with students in Dominican’s Behavioral Science lab – a team that calls itself the “Slug Squad” – the researchers were able to identify molecular fragments of memories the slugs had formed during an experiment involving electric shocks and then forgotten after being left alone for several days.
The Calin-Jagemans say that the findings, published this month in the journal Learning & Memory, are the first to show that even as memories fade, genetic changes evoked by learning can persist in the brain, where they might trigger relearning.
“I think we’ve all had an experience where we’ve forgotten something, and then you sit down and have to redo something and it all comes back, or it comes back easier,” Bob said. “So in some way, we were expecting something to still be there [in the slug’s brain]. But we were surprised that we could still find it. It’s very, very subtle.”
During the experiment, the researchers subjected the sea slugs to electric shocks of 90 milliamps. (By comparison, a phone charger uses about 2,000 milliamps of electricity.) The uncomfortable experience instilled a fear memory in the animal such that it would curl into a ball every time it was touched thereafter.
However, after being left alone for several days, the memory appeared to wear off. When touched again, the slugs’ reaction wasn’t nearly as dramatic, with no sign of evidence that the painful electric shocks changed the way they behaved.
After the experiment, the Calin-Jagemans ground up the slugs in order to examine the 1,200-plus genes activated inside each of their brains as part of forming the protective response. Although nearly all of the genes associated with the memory were switched off, the researchers said they were both surprised and thrilled to find that 11 of the genes remained active, representing what could be the lingering fragment of a forgotten memory.
For Irina, the discovery was “the most exciting finding” of her career, she said.
So what does all this mean for the perpetually forgetful people in your life?
The Calin-Jagemans aren’t sure, and they acknowledge that the evolutionary connection between the sea slug and humans is distant.
But they hope their work helps other researchers who are able to study the process of forgetting in more complex animals.
“Forgetting is something we don’t really understand,” Irina said, noting that most research in neuropsychology has focused on how information gets into the brain, not how it fades.
Learning more about forgetting, she said, could lead to pharmaceutical breakthroughs or other methods to help slow down the process.
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