Chadwick et al., Current Biology, 20 (2010)

Memory trace. The brains of volunteers lit up a certain way depending on which movie clip they were recalling.

Pardon, Your Thoughts Are Showing

Your thoughts leave a trace—and it’s visible. Researchers have successfully identified the memory a person is recalling by analyzing their brain activity. The result offers new insights into how and where the brain records memories and may help scientists understand memory impairments caused by injuries, aging, and neurological conditions, such as a stroke.

Cognitive neuroscientist Eleanor Maguire and her colleagues at University College London are no strangers to mind reading. Their recipe of functional magnetic resonance imaging (fMRI) and a specific computer algorithm has gleaned the secrets of the hippocampus, a brain region that tracks where a person is and also plays a role in memory and learning. The technological tag team works like this: fMRI measures the brain's blood flow—associated with neuron activity—on the scale of voxels, three-dimensional "pixels" that each include roughly 10,000 neurons. The algorithm then interprets the changes voxel by voxel to learn the brain's patterns of activity over time. Last year, Maguire used the method to pinpoint where a person was "standing" in a virtual-reality room.

Now, Maguire's team has turned from spatial orientation to a more complex function of the hippocampus: so-called episodic memory of specific experiences, such as seeing the ocean for the first time. To test whether they could capture episodic memories, the researchers needed 10 volunteers to share the same ones. To do this, they showed them three 7-second movies and asked them to memorize what they saw. Each movie showed a different actress doing a simple task, such as rummaging in a handbag and then dropping an envelope in a mailbox. The volunteers were prompted to remember each movie while the fMRI scanned their brains so that the computer algorithm could match the pattern of activity to each memory. Then they were allowed to recall whichever movie they chose.

The computer nailed it. Each time the volunteers recalled one of the movies, the computer algorithm could determine which movie they were picturing with accuracy "considerably higher than would be expected by chance," notes Maguire. The findings confirm that the hippocampus is heavily involved in episodic memory.

"This is really an interesting result, ... it's the closest we've come to reading specific memories," says Lynn Nadel, a cognitive neuroscientist at the University of Arizona in Tucson. However, he cautions against concluding that the volunteers use the same neurons again and again to recall the same memory, because fMRI can't spot particular neurons in the act of signaling each other. The fMRI resolution limits researchers to seeing in voxels, so "you don't know that exactly the same neurons or circuits are at work every time," he explains.

Maguire, whose team reports its results online today in Current Biology, is refining her method. She's also hoping to study what happens in the hippocampus over longer periods or when the brain ages or is injured. "What happens to these memories over time is an important issue that remains to be resolved," she says.

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