BMe Research Grant
BMe Research Grant - 2016
“Sleep on it!” – suggests the idiomatic term, which is often used as a solution to any problem we face. Everybody has experienced the miracle of a good night’s sleep after a day spent learning. In the morning we see everything clearer, things start to make sense and the newly learnt mess of information is slowly converging into a coherent whole. But this is not a miracle – at least not in a biblical way – this effect is due to the scientific wonders of the processes taking place in our brain during sleep. These processes with the development of science are becoming more and more observable and these observations help us explore the relationship between cognitive functions and sleep. Thanks to these progressions, research concerning the relationship between sleep and memory is experiencing a unique renaissance and our research takes a leading part in this movement.
The research took place in the sleep laboratory of the Budapest University of Technology and Economics, Department of Cognitive Science. In our lab we are looking for answers concerning the neural mechanisms during sleep (nocturnal and afternoon nap) – from memory consolidation through emotional consolidation to the different sleep characteristics of clinical groups.
At first glance, sleep seems like a passive state, characterized by reduced responsiveness, relative inactivity, lack of consciousness and a lying posture with closed eyes. But not everything is as it seems. The results of sleep deprivation studies tell us that the loss of only one night's can impair the immune system, cause increased heart rate variability, and with many more physiological and psychological effect it causes cognitive impairment and decreased memory performance. In the field of sleep and memory research, the idea, that sleep plays an active role in the off-line consolidation of memories is getting more and more pronounced . The acquirement of new information is an incredibly fast process (takes milliseconds), which leads to the creation of a new memory element. But this initial state of a memory is very instable and vulnerable, it can be easily manipulated by external stimuli (etc. interference) or even worse, it can disappear. For new memories to get stronger, and to be transferred to the long term memory system, they need to go through the so called memory consolidation processes . During memory consolidation the initial memory becomes stabilized and more resistant to interference, which can be caused by other (external or internal) competing and disturbing stimuli . Theories supporting the active role of sleep claim that not merely reduced interference takes place in the background, but they suppose active sleep specific processes potentiating these off-line consolidation processes . One of these processes is the spontaneous reactivation of memories during sleep, along which the relevant memories are “replayed” and strengthened. The future relevance of memories can be manipulated experimentally (etc. instruction, evoked intention to forget, reward) .These manipulations give us the opportunity to compare the consolidation of relevant and irrelevant memories.
The results of earlier studies [7,8,10] support the idea, that sleep – nocturnal or afternoon nap – compared to the same amount of wake selectively enhance memories with future relevance. This selective enhancement is strongly correlated with sleep specific electroencephalographic (EEG) oscillations (slow wave activity [1–4Hz], sleep spindles [13–16Hz]) [8,10]. In contrast in Abel and Bäuml’s  study – with more than 300 participants – this difference between the retention of relevant and irrelevant memories disappeared after a good night’s sleep. The aim of our research was the exploration of the relationship between sleep and memory consolidation and the examination of differences in the consolidation of relevant and irrelevant memories. We aimed to manipulate the future relevance of newly acquired memory elements and with this manipulation to investigate the effects of daytime nap on memory consolidation. Our main questions were: Does sleep play an active role in memory consolidation? Is there a difference between the consolidation of relevant and irrelevant memories?
If we want to manipulate the future relevance of a newly acquired information, the directed forgetting paradigm is a favorable method to use  (Figure 1.). In this paradigm the participants have to learn two lists of words. Before the presentation of the first list the experimenter asks the participant to memorize as many words as he/she can (“remember” instruction). Thereafter comes the presentation of the first list on a computer. From this time forth the participants are divided into two subgroups generated by the second instruction. The “remember” group (control condition) receives the same instruction as before and after the second “remember” instruction comes the presentation of the second list. In contrast, in the “forget group” after the presentation of the first list, a simulated system crush appears, which gives us believable reason to ask the participant to forget the first list and only memorize the subsequent second list. The “forget” instruction is followed by the presentation of the second list. This learning phase is followed by – after a subsequent amount of delay – the recall phase. In the recall phase, irrespective of the instruction, both lists are tested in the presented order and the experimenter records the answers.
Figure 1. The basic procedure of the directed forgetting paradigm. Control condition creates the “remember” group and the Forget instruction creates the “forget” group.
In this research 10–10 words/list were used, the words were presented for 3 seconds and a two-hour delay was inserted between the study and recall phases. Half of the participants spent this time with free activity outside the lab, the other half had a chance for an afternoon nap in the sleep lab. Neural activity during these naps were monitored by EEG.
Figure 2. The procedure of our research. The experimental “steps” in the awake group is presented above and in the nap group below the arrow.
Figure 3. The locations of the electrodes used in our research.
The experiments took place between 12:00 and 16:00 at the BME Department of Cognitive Science. In this research 113 participants took part, at the age between 18 – 35.
Our results indicated that the so called directed forgetting effect – the „forget” group’s decreased recall performance for the first list and better for the second list (compared to the „remember” group) – still emerged after a two-hour delay (Figure 4.). Furthermore, this effect was more pronounced in the sleep group (in contrast with Abel and Bäuml’s  results, where sleep eliminated this effect). Nevertheless, this effect was mainly due to better recall performance of the second list (the list with future relevance).
Figure 4. The average word recall after a two-hour delay spent either awake (left side) or sleeping (right side). The bars are showing the mean word recalls for List 1 and List 2 in the two groups and the black lines indicate the confidence-intervals.
After examining the architecture of the afternoon naps, we realized that only half of our participants reached REM (rapid eye movement) state – the other half only slept in nonREM state – during the two-hour delay. Comparing the recall performance of those who reached REM (REM group) – as well as nonREM – and those who only reached nonREM (NREM group) interesting patterns were noticed. In the REM group the recall of the first and second list were consistent with our expectations – the „forget” group (N=13) remembered less words from the first and significantly more from the second list, than the „remember” group (N=14). Whereas participants from the NREM group showed no significant difference between the „forget” (N=16) and „remember” (N=15) group’s performance in the recall of the two lists (Figure 5.). These results imply that besides nonREM, the REM phase also has a significant role in the differentiation of relevant and irrelevant stimuli.
Figure 5. The different patterns of recall performance in the REM group (With REM group) on the left side and the NREM group (Without REM group) on the right side. The bars are showing the mean word recalls for List 1 and List 2 in the two groups and the black lines indicate the confidence-intervals. At the bottom of the figure the t and p values of the two groups are presented.
This implication is further supported by our other results. We only found significant correlation between the duration of the REM phase and the recall performance of the second (relevant) list in the „forget” group. This means that the duration of the REM phase has a positive relationship with better retention of futurely relevant words. Furthermore, in line with earlier studies [7,8] we investigated the role of sleep spindles in the consolidation of relevant and irrelevant memories. Our sleep spindle analysis shed light on an interesting correspondence. Significant correlations were only found in the “forget” group, however, in this case the retention of both the first and the second list was strongly and positively correlated with fast spindle amplitude.
All in all, our results show that besides nonREM phase and sleep spindles, REM phase also has an important role in the differentiation and strengthening of relevant memories. These results fit perfectly in the frames of “Active system consolidation theory”  which implies that nonREM and REM phase have equally significant roles to play in the off-line consolidation processes. While during nonREM – with the help of the sleep spindles – relevant and irrelevant memory elements are “tagged”, during REM these are further differentiated, and the ones with the “relevant tag” are strengthened.
This is the first research (according to our knowledge) to investigate the differences in the consolidation of relevant and irrelevant memories with the help of precise electro-physiological measurements. Furthermore, this high amount of participants (113) is extremely rare in the filed of sleep research. In conclusion, our results are unique and outstanding in their own field and they could play a determinative role in the further investigation of the interaction between sleep and memory. The publication of these results are in process. In the future we plan to investigate the consolidation of emotional memories during sleep, and with deeper understanding of these processes, the development of new, innovative and more beneficial therapeutic methods for the clinical population.
Posters, presentations, publications:
http://www.webmd.com/sleep-disorders/sleep-deprivation-effects-on-memory Sleep deprivation effects on memory.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3768102/ Review article: About Sleep’s Role in Memory.
http://www.brainfacts.org/sensing-thinking-behaving/sleep/articles/2015/the-secret-to-memory-a-good-nights-sleep/ The secret to memory is a good night’s sleep.
http://www.bbc.com/news/health-27695144 BBC: Sleep’s memory role discovered.