Study sheds light on how we form new memories

Jul 18 2006

A study conducted by researchers at Carnegie Mellon University and the University of Pittsburgh involving an amnesia-inducing drug has shed light on how we form new memories.

For a paper to be published in the July edition of the journal Psychological Science, researchers gave participants material to remember in two experimental sessions -- once after being injected with a saline placebo and once after an injection of midazolam, a drug used to relieve anxiety during surgical procedures that also causes short-term anterograde amnesia, the most common form of amnesia. Anterograde amnesia, which was portrayed in the film "Memento," impairs a person's ability to form new memories while leaving old ones unharmed.

The study revealed that the drug prevented people from linking a studied item to the experimental context. That linkage is necessary for a process known as recollection, in which people retrieve contextual details involved in the experience of studying the information. People sometimes recognize something as having been studied without using recollection (in this case, without remembering details of the study event) if the item seems sufficiently familiar -- a process called familiarity. Although the recollection process was affected by the drug, the familiarity process was not. This is the same pattern that is found with patients suffering from anterograde amnesia. They are unable to form new associations, severely limiting the accuracy of their recognition judgments.

"This helps us understand the general functions of memory. It helps us to relate, for example, the memory declines seen in old age to those seen in patients with hippocampal damage," said Lynne Reder, a professor of psychology at Carnegie Mellon and the study's lead author.

Using a double-blind, within-subject protocol, the scientists compared the participants' performance on the test after studying the material either under the influence of midazolam or after receiving an injection of a saline placebo. In both sessions, participants viewed words, photo graphs of faces and landscapes, and abstract pictures one at a time on a computer screen. Twenty minutes later, they were shown the words and images again, one at a time. Half of the images they had seen earlier, and half were new. They were then asked whether they recognized each one.

The researchers predicted that the more participants relied on recollection with saline, the more they would be hurt under the influence of midazolam. Their findings matched those predictions. Researchers found that the participants' memory while in the placebo condition was best for words, but the worst for abstract images. Midazolam impaired the recognition of words the most and did not affect recognition of abstract pictures.

The experiment further reinforced the thought that the ability to recollect depends on the ability to link the stimulus to a context. While the words were very concrete and therefore easy to link to the experimental context, the photographs were of unknown people and unknown places (not, for example, of Marilyn Monroe or the Eiffel Tower) and thus hard to distinctively label. The abstract images were also unfamiliar and not unitized into something that could be described with a single word (such as Picasso's "Guernica"). This meant that a person could not easily link the image with a context, regardless of drug condition.