Does Your Brain Learn New Words During Sleep?

Language Learning

Hippocampus Activation During Language Learning

Site map Located underneath the cerebral cortex, the hippocampus is crucial for the formation of new memories

For the MSc in Cognitive Neuroscience program at the University of York, I chose to study how sleep affects word learning, for my Empirical Project. At the time, I was more interested in sleep research than the language aspect of the study, but after some background reading, I began to realize how interesting this area of study is. What is particularly fascinating to me is that in the past we could only measure behavioral activity, but now we can directly monitor what is happening during the brain when someone is learning new words. For instance, in the picture above, you can see activation of the hippocampus, which is associated with the initial stages of vocabulary acquisition (with activity highest early in training: Gluck, Myers & Meeter, 2005). I would like to share with you what I learned and also point you to some further reading, should you be interested.

All of the words that humans know in adulthood were once unfamiliar to them. New word learning involves a process called lexicalisation, in which novel words become part of a person’s mental lexicon. In order to understand the fundamental procedure of word learning, many studies have examined the type of memory trace that has been created and whether the information has been stored in long-term memory. One way in which researchers can closely follow what happens during language acquisition is to introduce nonwords (made up words) to subjects and study the mental processing that occurs when they are learnt and become wordlike, with an associated meaning and memory trace.

A vast amount of literature links sleep with memory and learning. For instance, people who show the greatest increase in the level of REM sleep show the greatest increase in skill in Morse code (Mandai, Guerrien, Sockeel, Dujardin, & Leconte, 1989).

In another example, Fenn, Nusbaum and Margoliash (2003) had their participants listen to monosyllabic consonant-vowel-consonant words then had them type the words. Their recognition performance was high right after training but then got worse throughout the day. However, it completely recovered after a night’s sleep and the memories lasted up to six months.

While there is some disagreement about which sleep stages are fundamental for processing memories, all evidence suggests that some form of sleep is crucial to memory consolidation.

So what happens if you train participants on a set of nonwords and then measure their brain activation after a night's sleep? Do we learn language in our sleep?

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Brain Activity

Visual Word Form Area Activation During Language Learning

Visual Word Form Area

Participants were trained on visually presented novel words on two consecutive days. In the scanner, they viewed trained novel words, real words and unfamiliar novel words.

Using fMRI, we can see a very large region of activation was found around a known activation peak of an area of the brain called the "Visual Word Form Area", (at x= -44, y= -58, z= -15 (Vigneau, Jobard, Mazoyer & Tzourio-Mazoyer, 2005) along the fusiform gyrus) which has been shown to selectively process written words, in most cases regardless of variations in font size, case, and position in the visual field (McCandliss, Cohen & Dehaene, 2003). The red pattern of activation seen above was produced on a standard grayscale brain using MRICRON software, comparing all word conditions to baseline (Rorden, 2007).

Trained Nonwords vs Untrained Nonwords

The fMRI data revealed very little difference between the two trained novel word conditions. This indicates that some form of memory consolidation occurred that was not dependent on sleep. However, if we compare the brain activation for learned nonwords (red) compared to unfamiliar nonwords(green) above, we can see that trained nonwords have unmistakably become processed differently than untrained nonwords. In fact, they have come to be processed more like real words, with activation in areas such as the superior temporal gyrus, which has been shown to be activated in semantic decision tasks (Binder et al., 1997). Furthermore, the activation in the learned nonword condition was similar to our real word condition.

In sum, our data suggests that the trained novel words were treated somewhere in between real words and untrained novel words. The pattern of brain activation we found suggests that novel words became slightly wordlike in the way they were processed. Some form of consolidation has occurred without sleep, but we predict that with more time and increased exposures, the activity in the trained nonword conditions would become more similar to the real word condition and that sleep is crucial for the lexicalisation process.

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References and Further Reading

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  • https://sleepdata.org/about
  • This is a good recent article by Business Insider about why sleep is important for memory

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