CogBlog – A Cognitive Psychology Blog

Have you ever walked into a room and subsequently forgotten why you entered the room in the first place? Or have you ever misplaced a valuable item, say your iPhone or favorite sweatshirt, and tried to retrace your steps by visiting all of the places you think you last had it, only to come up empty-handed? If these sound like common occurrences, there is something easy you can do to help reduce the number of these painful experiences!

The above situations illustrate failures of a specific type of memory. Broadly, this memory is referred to as episodic memory, or memory of specific events that an individual has experienced and can explicitly recall (e.g. remembering putting your phone someplace). As a subcategory of episodic memory, these situations are examples of context-dependent memory at work. Successful context-dependent memory is seen when there is an improved recall of an event when the context cues present at the time of the event are also there at the time of memory retrieval. For example, let’s say that while watching t.v. on the couch, you remember to feed your dog.  On the way over to the dog food, you pass the kitchen and decide to make yourself something to eat first. After you’ve eaten your meal, you forget your primary reason for getting off the couch, and decide to return to the couch to resume watching t.v. Only moments after plopping down on the couch do you remember your poor, hungry dog!Watching t.v. on the couch served as an effective context clue because only after returning to the same place where you had your initial thought was your memory activated. Similarly, if you’re retracing your steps to recover a lost item, and in doing so  another item or place successfully jogs your memory, then you’ve demonstrated context-dependent memory.

So, what’s this magical antidote that has the power to enhance context-dependent memory, in turn making you feel less like your senile great-grandfather? A simple, yet powerful thing called sleep. Before further explanation, let’s outline the basics of the sleep cycle. Sleep is divided into two basic states: REM (rapid eye movement) sleep and NREM (non-rapid eye movement) sleep, which is then split into four distance stages. People typically begin their sleep by cycling through each of the four stages of NREM sleep, then entering a short period of REM sleep where dreaming usually takes place. There have been a number of recent studies showing a positive correlation between NREM sleep and consolidation of episodic memories (2007), however the specific features of the memories and exactly when they are processed or reinforced during NREM sleep remained largely unknown.

In 2011, Helm et. al. published a study providing many more important details on the link between NREM sleep and episodic memory. They cleverly devised an experiment that would distinguish between the two main subcategories of episodic memory: context memory (as described above) and item memory. Item memory refers to knowledge of what events happened, while context memory deals more with contextual features of the remembered event, such as when, where, or how it happened.

The study involved 27 healthy 18-23 year-olds. To control the experiment and reduce any effects of outside factors, all participants kept a regular sleep schedule and didn’t consume any caffeine or alcohol three days prior to the study. All subjects participated in an initial learning session at 12:00 PM, where they were asked to study two separate lists of 50 single word nouns, one list at a time. Both lists were presented with a unique poster (either of “The Simpsons” characters, or of two children and a surreal background), and the participants were asked to somehow associate each word on the list with the picture. These posters serve as context clues for later recognition. The whole group was then randomly split in half and assigned to either a Nap-group or a No Nap-group. In the six hours between initial learning and the recognition test, the Nap-group were allowed a 120 minute nap, while the No-Nap group had to stay awake, doing their typical daily activities.

After the six hours, the subjects were brought back to see how well they could recall the words. Facing a different direction than in their initial learning and with the original wall posters removed, the subjects were presented with a mixture of the 100 words studied before and 100 new single word nouns. They could respond either “old” if they thought the word being shown was on the lists studied at 12:00, or “new” if they hadn’t yet seen the word. If they responded “old”, they were shown digital images of both original wall posters to refresh their memories, and then asked to which list and associated context the word belonged.

If participants were able to better identify from which list they saw a particular word, that’s evidence of context memory at work because the cues at the time of learning and at the time of recall match up and function to enhance memory. Simply identifying a word as “old” or “new” tests the individual’s item memory because there were no context clues present at the time of recall. The subjects were oriented differently in the testing room and the posters were taken away at this time to effectively get rid of any context clues to ensure that that data only represented item memory.

The results showed that there were no differences between the Nap-group and No Nap-group in item memory, yet there existed significant between-group differences in context memory. Furthermore, the amount of NREM stage 2 sleep in the Nap-group had a positive correlation with context memory performance. There was no such relation found regarding any other stage of sleep or total amount of sleep. Interestingly, significant context-memory enhancement was only identified for list 2.

The results more clearly show us how sleep affects episodic memory, suggesting that sleep can specifically improve context-dependent memory, yet doesn’t seem to have an effect on item memory. However, this depends on what type of sleep you’re getting, as stage 2 sleep, one of the four stages that make up NREM sleep, appears to be by far the most important in improving such memory. Also, the fact that significantly better context memory was only seen for list 2 words suggests that recency of learning seems to have an effect on establishing any relation between sleep and context memory, as these words were read closer to nap time. It would be very interesting to give the subjects even more word lists to see if those lists learned closer to the sleep period would still be remembered better.

In conclusion, more sleep can result in a sharper, less forgetful you! Especially for college students, who are virtually always busy, it can be very easy to forget about a meeting, lose track of a personal item, or forget to send an email to check in about that summer job. The context cues are everywhere, and sleep can help us to more effectively use them!

If you’d like to read the full article click here 

 References

Helm, E. V., Gujar, N., Nishida, M., Walker, M. P., & Robertson, E. (2011). Sleep-Dependent Facilitation of Episodic  Memory Details. PLoS ONE, 6(11), e27421.

Marshall, L., & Born, J. (2007). The Contribution Of Sleep To Hippocampus-dependent Memory Consolidation. Trends in Cognitive Sciences, 11(10), 442-450.