In a study of fruit flies (Drosophila melanogaster), a team of researchers from the Scripps Research Institute Florida has shown the physiological mechanism by which a memory is formed and then subsequently forgotten. The team found that a single dopamine neuron can drive both the learning and forgetting process. The findings appear in the journal Cell Reports.

“We believe this system is set up to remove memories that are unimportant and not necessarily supposed to last a long time,” said Dr. Jacob Berry, first author of the study.

“I find it elegant that all of this is done with the same neuron. Our paper highlights exactly how this is achieved.”

To study memory in flies, the insects are conditioned to associate a particular odor with an electric shock.

Once they’ve been trained, Dr. Berry and colleagues observe that they subsequently avoid that odor, which confirms that the memory has been made.

By monitoring the activity of neurons in the brain before and after the conditioning process, they can get an inside look at the physiological underpinnings of memory formation.

In an earlier study, the team showed that there are specific dopaminergic circuits that are involved in both the formation of memory and the removal of memories. In the current study, the scientists used imaging techniques to look at the process in more detail.

They discovered that when a behavioral memory is degraded, the cellular changes made during the learning process are reversed by the same dopamine neuron that helped form the changes in the first place.

They also found that when this dopamine neuron is recruited to form a new memory, it also works to degrade older memories.

“Whenever you learn something new, you’re simultaneously forming a new memory while potentially interfering with or erasing old ones. It’s a very important balancing act that prevents you from becoming overloaded,” Dr. Berry said.

“For decades now, neuroscientists studying learning and memory have focused on how the brain acquires information and how that information is made to be stable memory, a process called memory consolidation,” said Professor Ron Davis, senior author of the study.

“Only recently have neuroscientists grasped the importance of active forgetting and begun to unravel the processes that causes the brain to forget.”

“This learning-and-forgetting process helps to explain retroactive interference, a common observation in psychology,” Dr. Berry added.

“Retroactive interference describes the situation when more recent information gets in the way of trying to recall older information — for example, calling your former boss by your current boss’ name.”

Although the study was done in fruit flies, the team expects that the findings will apply to higher organisms, including humans.

“Evolution worked out a lot of important processes like this pretty early on, so there’s a lot of relevance to studying these synaptic pathways in simpler organisms,” Dr. Berry said.

_____

Jacob A. Berry et al. Dopamine Neurons Mediate Learning and Forgetting through Bidirectional Modulation of a Memory Trace. Cell Reports, published online October 16, 2018; doi: 10.1016/j.celrep.2018.09.051