Wednesday, October 11, 2017

Sleep and False Memories


When you remember a past event, you are not just playing back a video or audio file of a previous encounter.  Instead, memories are reconstructed.  That means that many sources of information can be combined to influence what you remember about the past.
Most of the time, of course, that is a good thing.  When you are having a discussion about World War II, for example, it does not matter if the information about the war that you talk about came from a single lecture you attended or from years of classes and books you read.  What is important is just that the information is organized around the topic of discussion.
Of course, the specific events and the order of those events do matter a lot in eyewitness situations.  However, quite a bit of research demonstrates that eyewitness accounts are also reconstructed, and the means that information encountered after the initial event can influence later memory.
Does the amount of sleep you get affect how likely it is that you will mix together different sources of information when thinking about an eyewitness event?  This question was explored in a paper by Steven Frenda, Lawrence Patihis, Elizabeth Loftus, Holly Lewis, and Kimberly Fenn in the September, 2014 issue of Psychological Science.
They used a typical misinformation procedure in this study.  First, participants saw a sequence of photographs of two crimes (a car break-in and a thief stealing a woman’s wallet). At some point after seeing the photo sequences, participants read text stories that described the events in the photographs.  However, three of the facts in the story differed from what was shown in the photos.  For example, a photo might show the thief putting the stolen wallet in a jacket pocket, while the story might describe him putting it in his pants pocket.  About 20 minutes after reading the text, participants then got a test about the event.  The critical questions on this test focused on the misinformation parts of the event.  The key measure is whether participants recall what happened in the photograph or whether they use the information from the text of the story to answer the question.  For each question on the test, participants were also asked the reason for their response.  The strictest measure of a false memory is when participants choose the information they read in the story, but tell state that they saw it in the picture.
To explore the influence of sleep deprivation, some participants were kept awake for a full night, while others were allowed to sleep as much as they wanted.   Half the participants saw the pictures in the morning followed by the stories with the misinformation about 40 minutes later.  The other half the participants saw the pictures followed by the misinformation in the morning.
When participants see the pictures in the morning and then the misinformation soon after, sleep deprivation influences their tendency to have false memories.  The sleep deprived participants remember more of the misinformation than the rested participants.
When participants see the pictures the evening before seeing the misinformation, though, sleep deprivation has no reliable impact on false memories.  The likelihood of incorporating information from the stories in their recall is low for all participants who saw the photos the night before.
What is going on here?
Think about how people could respond accurately on this test.  When they encounter the initial event, they have to remember both what they saw as well as when they saw it.  That is, they have to keep track of the source of the memory.  That way, when they read about the event later, they can separate what they saw from what they read.
People who are sleep deprived seem to have more trouble than those who are rested keeping track of the source of the information they get.  They are able to remember facts about the events, but they are more likely to combine together different sources of information.
This result suggests that we might want to be careful about how much we trust the details of memories of events that happened when we were sleep deprived.  The lack of sleep may make it difficult for us to remember the source of the information we have encountered.

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