Explaining how the brain creates false memories
October 3, 2005
It often doesn’t take much to make an eyewitness to a crime change her or his story. While Mafia hardball tactics for intimidating witnesses make the headlines, just seeing or hearing a different version of the “facts” can be enough. One key is remembering the context for an event. If we can successfully recall that we personally witnessed one version of the story as it occurred last Thursday, then we’re more likely to realize that it’s different from the article we read in the newspaper the next day. If we don’t recall the context of either the original version or the revised version, our chances for making a mistake increase.
So what’s going on in our brain when we remember both the context and the event itself, and what’s different when we don’t? Yoko Okado and Craig Stark designed a study to examine the problem. They showed participants eight different slide shows telling stories on topics ranging from students talking in the hallway outside of class to the theft of a woman’s wallet.
Each slide show had 50 pictures and was shown twice. During the second showing, 12 of the pictures were surreptitiously changed in an effort to create false memories.
Two days later, participants were tested on their memory for those 12 critical slides.
Sure enough, a significant portion of the time, participants responded with false memories: They believed they had seen the same slide in both presentations (the man stole the wallet and hid behind a tree), when in fact they had seen two different slides (in the first slide show, the man hid behind the door, but in the second show, he hid behind the tree).
This result matched earlier research finding that when people watch a movie and then are presented with a written account that doesn’t match the movie, they will often falsely “remember” that the written story agrees with the movie...
(snip)
...Other fMRI research had previously revealed that when accurate memories are formed, they correspond to increased activity in particular parts of the brain: the medial temporal lobe and the prefrontal cortex. Okado and Stark’s data matched this finding: when participants had accurate memories, these parts of the brain were more active during the first slide show. When they had false memories, these parts of the brain were more active during the second, “false” slide show.
What’s more, Okado and Stark observed something else: a trend toward increased activity in other areas of the brain (parts of the hippocampus and parahippocampal cortex) when accurate memories were not being formed. Okado and Stark argue that this activity may be related to the inability to correctly recall the context for an item. Other studies have found that the left parahippocampal cortex is among the regions responsible for recalling the source of a memory. Since the region is less active when the critical second slide is presented during creation of a false memory, participants may not be forming the necessary contextual information they need to recall that the misleading slide was not, in fact, present during the original slide show. Similarly, when true memories were formed — when the misleading picture in second slide show was presented, but the participant still accurately remembered the correct first slide — these areas were again more active, suggesting that people had correctly recalled the context of the misleading second slide.
So apparently one of the key moments in determining whether a memory will be true or false occurs right as that memory is formed. During that critical instant, our entire conception of the past — right or wrong — is shaped.
So what does this say about Mafia “hardball” attempts to influence witnesses? Ironically, they may be doing exactly the wrong thing. If a witness is intimidated by physical force to change her original story, isn’t she more likely to remember the context? It might be more effective to use subtler techniques to get witnesses to change their tune.
Full article at: http://cognitivedaily.com/?p=109
Okado, Y., & Stark, C.E.L. (2005).
Neural activity during encoding predicts false memories created by misinformation.
Learning & Memory 12, 3-11.
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