This finding by Marvin Chun, professor in the Department of Psychology, and his team contrasts with the belief that all explicit (conscious) memory, and implicit (unconscious) memory, has distinct neural bases. The belief that the two types of memory are distinct has been illustrated by examples, including amnesiac patients with damage to the hippocampus and associated brain structures who have severely impaired explicit memory but intact implicit memory.
Instead of looking at how the storage of the two types of memory differs, Chun, Nicholas Turk-Browne, and Do-Yoon Yi focused on the common elements between them. Sixteen men and women viewed 120 photographs and answered which photos were taken indoors or outdoors. Each image was then shown once again. The study used functional magnetic resonance imaging (fMRI) to record brain activity during the test.
Fifteen minutes later the subjects were given a third recognition test, this one unexpected, which included the original 120 photos plus 60 new photos. The subjects' response was again recorded by fMRI. By coding the brain imaging data according to whether the items were subsequently remembered or forgotten, the researchers were able to examine the neural signatures of memory formation.
"Remembered photographs were characterized by strong activation of medial temporal brain regions in response to their first presentation, but reduced activation in the same regions when the photos were repeated," Chun said. This reduction known as repetition attenuation is a well-known signature of implicit memory. The brain signal is not as strong when an image is viewed, remembered, and then viewed for a second time.
"Importantly, only the explicitly remembered photographs produced this weaker signal, demonstrating for the first time that explicit and implicit memory are strongly linked during the encoding of new information in the brain," he said.
Neuron 49: 917-927 (March 2006)