The eyes remember

Eyewitness testimony is notoriously flaky, but new research suggests that eye movements can accurately reveal what a person remembers, even if the person isn’t aware of the memory.

In a memory test, participants’ eye movements picked the right answer even when the participant failed to, Deborah Hannula and Charan Ranganath, both of the University of California, Davis, report in the Sept. 10 Neuron. The eye movements corresponded to activity in the hippocampus, an important learning and memory center in the brain. The results suggest that eye movements can reveal unconscious memories activated in the hippocampus, the authors say.

Some neuroscientists have believed that the hippocampus is involved only in conscious or “declarative” memories, such as of  people, facts or events. People who have damage to the hippocampus aren’t able to form or recall declarative memories. These amnesiac patients can, however, learn new skills that require unconscious “procedural” memories, such as riding a bike — leading to the belief that an intact hippocampus is not needed for unconscious recall.

But the new study suggests that the hippocampus actually is involved in memories of relationships that a person does not consciously recollect. Hannula and Ranganath showed volunteers in a functional MRI scanner pictures of faces paired with an outdoor scene. After presenting about 50 such pairs, the researchers showed the landscape picture followed by three faces. Participants were then asked to choose which face had previously been matched with the landscape.

When the scene was shown, activity in the hippocampus increased, followed 500 to 750 milliseconds later by eye movements directed toward one of the three faces. When the hippocampus was more active, the eyes lingered on the correct face. Less hippocampus activity occurred when the eyes dwelled on an incorrect face.

Even when participants ultimately selected an incorrect choice, the hippocampus activity accurately predicted whether the eyes focused on the correct face. However, communication between the hippocampus and the prefrontal cortex —the brain’s executive control region — was reduced compared with trials in which the volunteers made the right choice. That result could mean that the hippocampus and prefrontal cortex must communicate properly for a person to remember correctly.

This finding is a clue that something beyond the hippocampus is needed to make a memory conscious, Ranganath says. Even if you don’t remember learning a relationship between two objects, “your hippocampus and eyes might have some of that information left over,” he says.

It’s still an open question whether the volunteers were conscious of the right choice at the time their eyes lingered on the correct matching face, say Dharshan Kumaran and Anthony Wagner, both cognitive neuroscientists at Stanford University in Stanford, Calif., in a commentary in the same issue of Neuron. The volunteers may have been immediately aware of the right choice but then second-guessed themselves in the brief time between seeing the pictures and making a selection.

“There are undoubtedly instances in which first impressions will lead to the correct answer,” Wagner says. “Our attention gets sucked in to what is the right answer, but when we linger on the choices we sometimes get tricked and ultimately embrace the wrong answer.”

Regardless of whether they reveal unconscious or conscious memories, eye movements could be used to help scientists understand how much patients with disorders such as dementia or schizophrenia remember. People with such disorders may remember more than they are able to say, Hannula says. Eye movements might also help researchers learn about memory in animals and children who are too young to talk.

One day, eye movements may be used to help assess the accuracy of eyewitness testimony or reveal memories people aren’t aware of or would rather not admit having, Ranganath says.

“It is, at the very least, another measurement of memory,” Kumaran says.