Memories that Share a Common Structure are Linked Together

Post by Amanda McFarlan

What's the science? 

The memories that we form in our everyday lives often share a common structure. For example, many songs are made up of different notes, but follow a similar structure. Additionally, memories are highly susceptible to interference while they are being formed which can disrupt the process of consolidation and therefore, memory retention. This week in Current Biology, Mutanen and colleagues proposed that sharing a similar structure might link memories together and determine how they are stored and modified. They investigated whether sharing a common structure among memories affected their retention and susceptibility to interference. 

How did they do it?

The authors created two memory tasks: a sequence of actions and a sequence of words. The sequential order of actions and words in the memory tasks either followed the same ‘structure’ (each action corresponded to one semantic category of words - e.g. a thumbs-up action always corresponded to a type of vegetable; see figure) or did not follow the same structure. 

The authors performed two experiments, each with multiple stages. In Experiment 1, the steps were: 1) Participants learned a sequence of actions 2) 6 hours later, they learned a sequence of words that either did or did not follow the same structure as the previously learned sequence of actions, 3) Participants were tested on their word recall and then immediately learned a new sequence of words, designed to interfere with memory consolidation, before being asked to recall the first list of words again. 

In Experiment 2: 1) Participants learned a word sequence, 2) 6 hours later, they learned a sequence of actions that either did or did not follow the same structure as the previously learned word sequence, 3) Participants were tested on their ‘skill level’ (measured by their response time) for the sequence of actions, 4) Participants immediately learned a new sequence of actions (to interfere with memory consolidation), and 5) Their skill level was tested again for the first sequence of actions. The authors also tested participants without any prior learning on word recall and measured skill level before and after interference. Finally, the authors investigated how well participants could retain their initial learning by retesting their word recall or measuring their skill level 12 hours after initial learning. 

What did they find?

In Experiment 1, the authors found that total word recall decreased following interference (i.e. learning new words) during word sequence learning. However, this decrease in word recall was not as robust when the sequence of words and the previously learned sequence of actions followed the same structure versus when they did not. 

In Experiment 2, the authors found that interference during action sequence learning resulted in a reduction in skill level only when the sequence of actions and the previously learned sequence of words did not follow the same structure, while there was no change when they did follow it. The authors also showed that the effect of interference on performance was much greater when participants had not previously learned a sequence of actions or a sequence of words, respectively. Together, these findings suggest that new memories can be protected from interference if they share a common structure with a recently acquired memory. 

Next, the authors revealed that there was a significant improvement in skill level 12 hours after initial learning when the sequence of actions and words had different structures. They also found a significant decrease in serial word recall (the precise order of the words in the sequence) 12 hours after initial learning when the motor and word memory tasks had the same structure, but not when the memory tasks had different structures. Together, these findings suggest that new memories can affect the retention of recently acquired memories when they share the same common structure.

What’s the impact?

This is the first study to show that memories that share a common structure are linked together. The authors found that, when an earlier memory and a newly formed memory shared a common structure, the newly formed memory was protected from interference while the retention of the earlier memory was disrupted. Together, these findings provide insight into how similarly structured memories are stored and modified.

Mutanen et al. A Common Task Structure Links Together the Fate of Different Types of Memories. Current Biology (2020). Access the original scientific publication here.