Friday, March 21, 2008

A couple of things about memory


Children's Memory May Be More Reliable Than Adults' In Court Cases
Researchers Valerie Reyna, human development professor, and Chuck Brainerd, human development and law school professor--both from Cornell University--argue that like the two-headed Roman god Janus, memory is of two minds--that is, memories are captured and recorded separately and differently in two distinct parts of the mind.

They say children depend more heavily on a part of the mind that records, "what actually happened," while adults depend more on another part of the mind that records, "the meaning of what happened." As a result, they say, adults are more susceptible to false memories, which can be extremely problematic in court cases.

The implications of these results for legal testimony is not what I find especially interesting here. In fact, there are reasons why the testimony of children has sometimes been found to be less reliable than that of adults. Namely, in some cases, the techniques used to interview the children (before trial) have been improperly coercive or suggestive of particular interpretations.

What does seem interesting is the hypothesis that in adults memories of the same event tend to be stored in two distinct forms: literal details of "what happened", and interpretive judgments about the "meaning" of an event. But that in children it is primarily the actual details that are stored.
Reyna and Brainerd's Fuzzy Trace Theory hypothesizes that people store two types of experience records or memories: verbatim traces and gist traces.

Verbatim traces are memories of what actually happened. Gist traces are based on a person's understanding of what happened, or what the event meant to him or her. Gist traces stimulate false memories because they store impressions of what an event meant, which can be inconsistent with what actually happened.

The researchers have experimental evidence to support their conclusions. Some of this is noted in earlier accounts, such as this:

Children Less Prone To False Memories, Implications For Eyewitness Testimony, Study Shows
In a study published in the May issue of Psychological Science, Brainerd and Reyna presented a list of words for groups of first, fifth and ninth graders. Many of the words from this "study list" were related to each other (by belonging to certain categories such as animals, furniture, men's names) while others were unrelated "filler" words.

After a short break, the students were presented with a new "test list" composed of study list words, new words belonging to the aforementioned categories (animals, furniture, etc.), and distracter words that were new and entirely unrelated to the categories or the study list. Their task was to identify whether they had previously heard a word or not.

As predicted, if the test list provided a new word with a closely related meaning (a "semantic relation") to a word from the study list, older children were more likely to assert that they had heard it before. Simply put, the older children had more false memories in this case than younger children.

One can speculate about what's going on here. As people mature through childhood, they are constantly learning about the interrelationship of isolated details and events. (For instance, "Dad acts more scary after he's been drinking beer.") In addition, the accumulation of details makes more literal forms of memory cumbersome (and liable to confusion), so people learn to make abstractions and interpretations that summarize details and make storage easier by associating similar details in more general categories. However, this kind of fuzzy storage (or "fuzzy traces" as Brainerd and Reyna call it) can misrepresent the facts. (For instance, "Dad was drunk when he hit me" – which might not actually be true.)

Second, and not directly related to this, there are two quite indepentdent studies that show something about the relationship between memory and experience of stress.

The first item concerns observation of squirrels:

Correct Levels Of Stress Hormones Boost Learning, Squirrel Study Suggests
Tests on the influence that a stress-related hormone has on learning in ground squirrels could have an impact on understanding how it influences human learning, according to a University of Chicago researcher.

Jill Mateo, Assistant Professor in Comparative Human Development, has found that when they perform normal survival tasks, ground squirrels learn more quickly if they have a modest amount of cortisol, a hormone produced in response to stress, than those with either high or low levels of cortisol.

In humans, cortisol production is also related to stress and is known to have an impact on learning, but that impact is not well understood, Mateo said.

This should sound familiar to anyone who's been through even a few moderately difficult college courses. Namely, if the work in a particular course isn't difficult enough to cause at least a little stress, retention of the details may not be very complete. Without some stress, the material just doesn't seem "important" enough, even if it's new to the student, to compel the student's attention to the details and the complexity. But of course, if the material is difficult enough to cause excessive stress, anxiety can get in the way of successfully organizing the material in the student's mind.

The second study looked at the actual neurobiology of learning under conditions of acute stress:

Short-term Stress Can Affect Learning And Memory
Short-term stress lasting as little as a few hours can impair brain-cell communication in areas associated with learning and memory, University of California, Irvine researchers have found.

It has been known that severe stress lasting weeks or months can impair cell communication in the brain's learning and memory region, but this study provides the first evidence that short-term stress has the same effect.

As it turns out, another stress-related hormone besides cortisol is involved, corticotropin releasing hormone (CRH), and the latter is more significant under conditions of acute stress:
In their study, Baram and her UC Irvine colleagues identified a novel process by which stress caused these effects. They found that rather than involving the widely known stress hormone cortisol, which circulates throughout the body, acute stress activated selective molecules called corticotropin releasing hormones, which disrupted the process by which the brain collects and stores memories.

Learning and memory take place at synapses, which are junctions through which brain cells communicate. These synapses reside on specialized branchlike protrusions on neurons called dendritic spines.

In rat and mouse studies, Baram's group saw that the release of CRH in the hippocampus, the brain's primary learning and memory center, led to the rapid disintegration of these dendritic spines, which in turn limited the ability of synapses to collect and store memories.

The researchers discovered that blocking the CRH molecules' interaction with their receptor molecules eliminated stress damage to dendritic spines in the hippocampal cells involved with learning and memory.

The role of cortisol, in learning under conditions of moderate stress, remains somewhat less clear. In addition to the squirrel study, anecdotal experience with so-called "flashbulb memories" supports the idea that some degree of stress can assist the formation of memories. The Wikipedia article states, without references, "Some biologists believe that the hormone cortisol, which is released in response to stressful incidents, cooperate with epinephrine (adrenaline) to cause the formation of flashbulb memories by the brain, functioning to help remembering things to avoid in the future." The squirrel study suggests cortisol actually has some role in memory formation, rather than being just a coincidental byproduct of stress. (See also the article on Emotion and memory.

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