Competing Memories

Neuroscientists have discovered a way to prevent interference between competing memory systems in the brain, allowing different kinds of memories to be stored
almost simultaneously – and recalled more accurately at a later time.

Recall: it's never as total as you think.

The method, as explained in the journal Nature, makes use of a technology called transcranial magnetic stimulation (TMS), which uses electromagnetic current to generate synaptic activity in the brain.

A team led by Harvard neuroscientists Edwin Robertson, MD and Daniel Cohen, MD studied a group of 120 volunteers, and discovered that by inhibiting certain memory-coordinating brain regions through TMS, it’s possible to reduce competition between storage of different memory types – such as word lists and motor tasks – allowing both sets of memories to be stored with equal clarity:

When learned in quick succession, declarative and motor skill tasks interfere with one another and subsequent recall is impaired. Depending on the order of the tasks, we were able to prevent memory interference in humans … Our observations suggest that distinct mechanisms support the communication between different types of memory processing.

But I should probably back up for a second, because not everyone knows about the several competing, overlapping systems our brains use to store memories. So let’s run over the essentials.

As any multitasker knows, the more types of tasks you’re working on at once, the harder it is to mentally keep track of everything you’ve accomplished (and/or learned). A major reason for this is that our connectomes shuffle different kinds of information around via different – and sometimes redundant – memory pathways.

Two of the most thoroughly studied pathways are those of declarative memory, which you can think of as “memory for facts and ideas,” and procedural memory, which might be called “memory for how to do things,” such as riding a bike or swimming.1

Declarative memories form quickly and easily – and they’re just as easy to forget. Formation of procedural memories, on the other hand, depends much more on stimulus associations and feedback loops from your environment – so these memories take longer to form, but they’re also much harder to forget. For example, long after you’ve forgotten the exact circumstances of the day you first learned to ride a bike (declarative), your connectome has no trouble recalling how to ride a bike (procedural) – even if it’s been years since you last rode one.

Now, for this study, the volunteers performed two tasks back-to-back: memorizing a list of words (declarative), and tapping their fingers in a coordinated pattern (procedural) – actually, half the subjects learned the tasks in the order I just listed, while the other half reversed the order.

The next day, the volunteers were tested on how well they recalled how to perform both the tasks – and as you can probably guess, they didn’t remember either of them particularly well. Robertson and Cohen chalk this up to interference between the declarative and procedural memory systems, which don’t play nicely together.

But the coolest part was still to come. A while later, the scientists had the volunteers learn the tasks again – but this time, they used TMS to apply electromagnetic current to a few carefully-chosen brain regions after the subjects practiced each task.

The subjects showed the most improvement when the competing memory pathway was disrupted by TMS after each task. To put it more precisely, stimulating the dorsolateral prefrontal cortex (DLPFC), which is involved in declarative memory formation, after the finger-tapping task helped the volunteers remember their word lists better; and stimulating the primary motor cortex (M1), which is important in the formation of procedural memories, after the word-recall task helped them remember their finger-tapping skills better.

“Our observations suggest that distinct mechanisms support the communication between different types of memory processing,” Robertson [says]. “This provides a more dynamic and flexible account of memory organization than was previously believed. We’ve demonstrated that the interference between memories is actively mediated by brain areas and so may serve an important function that has previously been overlooked.”

In short, the less other types of memories are allowed to compete with the most recent one, the more clearly new memories can be laid down. This is just one more incredible way our connectomes have evolved to help us keep track of what’s most relevant – even if we don’t always agree with their decisions.


1. Some neuroscientists think there may be several additional memory pathways – for instance, an amygdala-dependent pathway for storing “emotional memories,” such as those that trauma victims experience as flashbacks – but debate continues about whether these are actually distinct memory systems, or just different aspects of a few main pathways (or whether that question is even a meaningful one). If you’d like to learn more about these ideas, I recommend starting with this paper and this one.

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