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WINDOWS TO THE PAST: Memory failing you? Your eyes suggest otherwise

9.18.2009

By Andy Fell

Image showing a study participant's eye movement in UC Davis study.

Researchers showed the study participants a picture, then showed them a face superimposed on the same scene (left). Then the researchers showed three different faces on top of the scene (main image), and asked the participants to pick the face that they first saw superimposed on the scene. Green circles illustrate one participant's fixations (i.e., gaze is being maintain on a particular location) and the lines illustrate saccades (i.e., changes in gaze point) from one fixation to the next as the participant examines the display. The size of each circle represents the amount of time spent fixating a particular location - a larger circle means that more time was spent in that location. In this case, the participant looks disproportionately at the face on the left - the face that was previously paired with the scene. The brain image on the lower left shows activity in the hippocampus, as measured by functional MRI. ((Debbie Hannula/UC Davis))

Your eye movements can show that the elements of a memory are in place even when you cannot consciously recall it or when you get it wrong, according to a new study from the UC Davis Center for Neuroscience. The findings, published Sept. 10 in the journal Neuron, could have several practical applications.

It has been known for some time that a part of the brain called the hippocampus is necessary for the conscious recall of memories, but Debbie Hannula, a postdoctoral researcher at UC Davis, and Charan Ranganath, associate professor of psychology, addressed the controversial question of whether the hippocampus can support memories even when people are unaware of them.

Hannula and Ranganath tracked the eye movements and scanned brain activity of volunteers using a functional MRI (fMRI) machine while they performed a simple memory test. The volunteers were briefly shown a scene, then a face superimposed on the scene. To test their recall, they were shown the same scene again, but with three faces on it, and asked which one they associated with the scene.

Previous work conducted by Hannula and her colleagues showed that the eyes move to the correct face before the volunteer is aware of recalling the memory. In the new experiments, the hippocampus lit up with activity seconds before the eyes fixated on the correct face, even when participants failed to explicitly recall it.

“The signal in the hippocampus was closely tied to how long people spent looking at the right face,” Ranganath said. “So even when your conscious memory is wrong, the eyes (and the hippocampus) may have it.” Ranganath said.

The fMRI images also showed more activity in another part of the brain, the prefrontal cortex, when memories were correctly recalled. The scans showed that communication between the hippocampus and the prefrontal cortex was increased when people accurately recalled the face.

“We think that conscious memory comes from communication between the hippocampus, prefrontal cortex, and other parts of the brain,” Ranganath said. “But the hippocampus may be able to support memories, even when you can’t recall them.”

The eye-movement technique could have a range of uses. For example, it could be used to look for evidence of memories in people who are unable or unwilling to consciously recall them, to study

hippocampal function in patients or young children, and in studies aimed at developing new drugs to improve memory function, Ranganath said.


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