In the late 19th century German psychologist Hermann Ebbinghaus published an influential book exploring memory and learning. Ebbinghaus was the first to chronicle a learning phenomenon which became known as the "spacing effect."
The spacing effect suggests information is more effectively encoded into long-term memory when learning sessions are interspersed with large breaks. Over 100 years of research has backed up this observation, yet it is still unclear exactly how memory is strengthened by spacing out learning sessions.
To better understand how the spacing effect works, a new study looked at the brains of mice tested with an everyday memory task. The animals had to find a piece of chocolate in a maze. They had three opportunities to hunt for the prize, with the chocolate in the same location each time.
The researchers experimented with different time spans between each of the three chocolate hunts. Interestingly, in the short term, longer breaks between prize hunts seemed to hinder the animals’ ability to remember where the chocolate was.
"Mice that were trained with the longer intervals between learning phases were not able to remember the position of the chocolate as quickly," says Annet Glas, a neurobiologist working on the study from the Max Planck Institute. "But on the next day, the longer the pauses, the better was the mice's memory."
Zooming in on neuron activity in the dorsal medial prefrontal cortex, a brain region fundamental to learning processes, the researchers expected to see consecutive learning phases reactivating the same neural pathways.
"If three learning phases follow each other very quickly, we intuitively expected the same neurons to be activated," says Pieter Goltstein, another researcher working on the project. "After all, it is the same experiment with the same information. However, after a long break, it would be conceivable that the brain interprets the following learning phase as a new event and processes it with different neurons."
But exactly the opposite was found. Only with longer breaks between the learning phases were similar neuron activity patterns detected. Short consecutive learning phases seemed to present with different clusters of neuron activity.
Goltstein says this seems to indicate breaks between learning phases can strengthen long-term memory pathways. And this mechanism plays a role in the oft-observed spacing effect.
In the mice, the optimal spacing between learning phases was found to be 30 or 60 minutes. Only these intervals improved longer-term memory retrieval the next day. Shorter or longer timeframes between learning phases offered no particular benefits to memory retention the next day.
“Overall, our data show that trial spacing increases the strength of connectivity within the [neuron] ensemble, supposedly making memory more robust and increasing the probability of memory retrieval,” the researchers conclude in the newly published study. “Our findings provide the first direct description of how activity of the same neuronal population during memory encoding and retrieval mediates the spacing effect, a phenomenon originally described over a century ago.”
The new study was published in the journal Current Biology.
Source: Max Planck Institute
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