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Recalling temporary memories

Working memory lets you temporarily hold information in your mind so you can quickly act on it. m-imagephotography/iStock/Thinkstock

Sometimes you need to remember information for only a short period of time―like when dialing a phone number. This type of memory is called working memory. It lets your brain hold onto information as you’re using it, then discard it if it’s no longer needed or store it in your long-term memory.

To investigate the underlying neural mechanisms for working memory, a team led by Dr. Bradley Postle at the University of Wisconsin-Madison carried out a series of brain imaging and stimulation studies in student volunteers. The research was funded in part by NIH’s National Institute of Mental Health (NIMH). Results were published in Science on December 2, 2016.

The researchers measured the brain activity of the young adults by using fMRI or electroencephalography (EEG) while they performed working memory tasks. The volunteers were shown 2 items that they might need to recall later on. These included either a face, a word, or the direction that dots moved on a screen. The scientists determined the pattern of the volunteers’ brain activity as they focused on each item. Using this information, the scientists could “see” when the subjects had the image in their minds.

The participants were then asked to mentally focus on only one of the images to compare and match it with new images. As the subjects focused on the one image, its unique pattern of activity continued to exist in the brain scans. The brain activity patterns representing the other, temporarily unneeded, item disappeared―similar to what's observed when people are asked to forget information.

Using a technique called transcranial magnetic stimulation, or TMS, the researchers attempted to stimulate the memory of the temporarily unneeded item. They delivered magnetic stimulation to the brain regions that the item had uniquely activated when subjects had been thinking of it. This stimulation brought the subjects’ attention back to the unneeded item and interfered with their performance on the recognition test for the item they were asked to focus on.

Most models of the brain assume that brain activity patterns need to continue in order to hold something in your working memory, Postle explains. “But we’re watching people remember things almost perfectly without showing any of the [continuous] activity that would come with [brain cells] firing. The fact that you’re able to bring it back at all in this example proves it’s not gone. It’s just that we can't see evidence for its active retention in the brain,” he says.

More research is needed to investigate how memories can be stored and retrieved in the absence of sustained brain activity patterns and how the brain determines what to discard.

—by Tianna Hicklin, Ph.D.