The researchers made participants study three different scenes (for example, a park, a lake, a kitchen) each superimposed with a different face. After a short break, the participants had to do a test. They would be presented with one of the three scenes from the study phase, and after a delay, all three faces seen in the study phase were superimposed on the test scene. All the participants had to do was identify which of the three faces was the one that initially corresponded with the test scene. During the test, two parameters were evaluated: the first one was whether the participant correctly identified which face corresponded with the test scene, and the second parameter was where the participant was looking on the screen, and how much time he or she spent looking at each of the three faces. This was done through a technique called eye-tracking, where a camera records the position of your eye-gaze.
What the researchers found is that the participant’s eyes spent significantly more time viewing the correct matching face (the “right answer”), even when the participant selected an incorrect face as the answer. So what this means is that even when the participants couldn’t consciously remember which of the three faces was the right one, their eyes lingered the longest on the correct face.
In addition to the eye-tracking study, the researchers were looking at brain signals from the participants and determined that the hippocampus, a brain region we already knew was involved in conscious memory retrieval, was also supporting memory even when the participants were making incorrect responses. In a way, this means that your hippocampus supports the expression of memories through your eye-movements, even when you’re not consciously remembering correctly.
The relevance of this study lies in the fact that in some circumstances, the movements of your eyes is a more truthful and reliable account of memories than your verbal accounts. I’m sure you can imagine instances when this could be helpful: for example, to study the memory processes of non-verbal beings, such as babies or chimpanzees (who doesn’t want to know what a chimpanzee is thinking?). However, eye-tracking could also be used to get information from someone who is attempting to withhold it (surely, a more acceptable alternative to waterboarding). So next time you have something to hide, invest in a good pair of sunglasses…
Reference: The eyes have it: hippocampal activity predicts expression of memory in eye movements. (2009) Hannula DE, Ranganath C. Neuron 63:592-599.