"You can plan out those events and think about the sequences of actions you'll do," says Laura Colgin. "And all of that happens on a faster time scale when you're imagining it than when you actually go and do those things." (Credit: iStockphoto)

Alzheimer's disease

How the brain plays back memories in fast forward

A newly discovered mechanism in the brain may explain how we can recall nearly all of what happened on a recent afternoon—or make a thorough plan for how to spend tomorrow—in a fraction of the time that it takes to actually live out the experience.

The findings could advance research into schizophrenia, autism spectrum disorders, Alzheimer’s disease, and other disorders where real experiences and ones that exist only in the mind can become distorted.

The mechanism compresses information needed for memory retrieval, imagination, or planning and encodes it on a brain wave frequency that’s separate from the one used for recording real-time experiences.

Brain cells share different kinds of information with one another using a variety of different brain waves, analogous to the way radio stations broadcast on different frequencies. Researchers found that one of these frequencies allows us to play back memories—or envision future activities—in fast forward.

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“The reason we’re excited about it is that we think this mechanism can help explain how you can imagine a sequence of events you’re about to do in a time-compressed manner,” says Laura Colgin, assistant professor of neuroscience at the University of Texas at Austin.

“You can plan out those events and think about the sequences of actions you’ll do. And all of that happens on a faster time scale when you’re imagining it than when you actually go and do those things.”

As reported in the study, published in the journal Neuron, fast gamma rhythms in the brain encode memories about things that are happening right now; these waves come rapidly one after another as the brain processes high-resolution information in real time.

The scientists learned that slow gamma rhythms—used to retrieve memories of the past, as well as imagine and plan for the future—store more information on their longer waves, contributing to the fast-forward effect as the mind processes many data points with each wave.

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Mental compression turns out to be similar to what happens in a computer when you compress a file. Just like digital compression, when you replay a mental memory or imagine an upcoming sequence of events, these thoughts will have less of the rich detail found in the source material. The finding has implications for medicine as well as for criminal justice and other areas where memory reliability can be at issue.

The research could also explain why people with schizophrenia who are experiencing disrupted gamma rhythms have a hard time distinguishing between imagined and real experiences, Colgin says.

“Maybe they are transmitting their own imagined thoughts on the wrong frequency, the one usually reserved for things that are really happening. That could have terrible consequences.”

The researchers next plan to use animals with neurological disorders similar to autism spectrum disorders and Alzheimer’s disease in humans to better understand what role this mechanism plays and explore ways to counteract it.

The Esther A. and Joseph Klingenstein Fund, the Alfred P. Sloan Foundation, the National Institute of Mental Health, and the Office of Naval Research funded the work.

Source: University of Texas at Austin

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