Procedure

To study unconscious, incidental vocabulary learning during sleep, we had to introduce the vocabulary learning task in a way that would prevent participants from consciously trying to attend to the sleep-played words. We decided not to mention that words would be presented during sleep; instead, we introduced the study as an investigation of “how sleep contributes to the ability to correctly guess the meaning of foreign words from a language one has never encountered before.” To implicitly prepare participants for vocabulary learning, they were shown examples of foreign words. They were told that after a nap they would be asked to guess the meaning of similar foreign words and would then have to learn the correct German translations of these words.

Participants were asked to keep a regular sleep schedule before the experimentation period. The night before the experiment, subjects were restricted to 4 h of sleep, to increase their propensity to sleep during the experiment. Sleep diaries and direct contact confirmed compliance with this protocol. On the day of the experiment, participants arrived at the sleep laboratory between noon and 1 pm, gave written informed consent and were outfitted with EEG electrodes and with in-ear headphones.

We then administered an auditory word-identification task to determine the optimal individual acoustic stimulation intensity for presentation during sleep. The identification task required participants to identify spoken German number-words “one” through “four” that were presented with varying intensity while constant Brownian noise was playing. The same Brownian noise would be playing during sleep to reduce the acoustic salience of the presented words. The intensity of played number-words varied randomly from clearly audible (signal-to-noise ratio SNR = 0.25) to clearly inaudible (SNR = 0.001). The optimal individual word identification threshold was defined as the lowest SNR at which participants could recognize at least 50% of words. This threshold was chosen as the target SNR (SNR target ) for word presentation during sleep (median SNR target = 0.02). At this intensity, words in the Brownian noise were still consciously noticeable and comprehensible but were unobtrusive.

72 Iber C.

Ancoli-Israel S.

Chesson A.

Quan S.F. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications. target ) within about 1 min. Once intensity was at SNR target , presentation of pairs of pseudowords and real German words was started. Each pair was repeated four times in sequence, changing the order of presentation each time (pseudoword – German word; German word – pseudoword; pseudoword – German word; German word – pseudoword). Stimulus onset asynchrony (SOA) for the words in a pair was 1075 ms, which corresponds to the expected peak-to-peak interval of slow-waves. Rhythmic auditory stimulation with this interval can entrain slow-waves, which has been shown to benefit memory consolidation [ 35 Ngo H.-V.V.

Martinetz T.

Born J.

Mölle M. Auditory closed-loop stimulation of the sleep slow oscillation enhances memory. Participants were then asked to take a nap in an electrically and acoustically shielded room. Afternoon naps started between 12:40 and 14:50 (M = 13:51, SD = 39.2 min) and lasted about 90 min (M = 93.5 min, SD = 33.1; See Table S1 for an overview of the time spent in different sleep stages and see Figure S2 for a detailed overview of sleep progress and stimulus presentation for each subject). Brownian noise was quietly presented at < 50 dB(A) SPL and moved to 65-74 dB(A) SPL during late intermediate sleep (N2), i.e., as soon as slow-wave sleep was imminent. At this point, visible delta activity had not yet reached the criterion for slow-wave sleep, a peak-to-peak amplitude > 75 μV []. Concurrently, rhythmic presentation of randomly paired spoken numbers “one” through “four” was initiated to habituate participants to verbal stimulation during sleep. Initial stimulus intensity was SNR = 0.001 (inaudible) and was increased slowly to the previously defined target intensity (SNR) within about 1 min. Once intensity was at SNR, presentation of pairs of pseudowords and real German words was started. Each pair was repeated four times in sequence, changing the order of presentation each time (pseudoword – German word; German word – pseudoword; pseudoword – German word; German word – pseudoword). Stimulus onset asynchrony (SOA) for the words in a pair was 1075 ms, which corresponds to the expected peak-to-peak interval of slow-waves. Rhythmic auditory stimulation with this interval can entrain slow-waves, which has been shown to benefit memory consolidation []. The onset interval between word pairs was 4300 ms. Presentation of the complete set of word pairs took approximately 15 min. If the continuously monitored EEG showed signs of arousal or if no slow-waves were visible for at least 30 s, stimulus presentation was stopped. Once a participant returned to slow-wave sleep, stimulation was resumed starting with the habituation procedure as described above. Only subjects with at least 20 sleep-played word pairs were included in the data analysis.

Participants were given some time to recover from their naps before continuing with the experiment. The time interval between waking and the start of the implicit memory test was 43 ± 22 min (M ± SD, see Figure S2 ). In the fMRI subgroup, participants were escorted to the MR center. Participants in the behavioral subgroup took the implicit memory test in the EEG laboratory. The time between the last sleep-played word and the first trial of the implicit memory test was significantly longer for the fMRI subgroup (1 h 31 min) than the behavioral subgroup (52 min; t(39) = 5.28, p < 0.001). Differences in this time interval between the two groups were not related to overall memory performance nor to performance split by the number of times the second word of a pair coincided with a slow-wave peak (all p > 0.172).

The implicit memory test was explained as a test that “measures the degree to which people intuitively understand the meaning of unknown words in a foreign language,” because at this point subjects were still unaware that words had been presented to them while asleep. Participants were instructed that they will be presented with pseudowords, which denote objects that are physically either larger or smaller than a shoebox; their task was to decide whether each “object” would fit into a shoebox. Responses were recorded by button press: up-key for “large object, does not fit into a shoebox,” down-key for “small object, does fit into a shoebox.” Pseudowords were simultaneously presented acoustically (fMRI subgroup: headphones; behavioral subgroup: loudspeakers) and visually (written on a screen). Responses to previously sleep-played pseudowords were scored as correct if they matched the size of the sleep-played real German word.

A total of 96 pseudowords were presented at awake-testing; 48 of them had not been played during sleep. The order of the 96 pseudowords was randomized for each participant. In the fMRI subgroup, we included a baseline condition in which 48 number words were presented, either the word “one” or the word “two.” The task was to indicate, on each trial, which was smaller (“one”) or, larger (“two”). We did not use this baseline condition in our data analysis. The presentation order on-screen was: (1) fixation (250 ms) – (2) pseudoword (self-paced timing but max. 6 s response time in the behavioral subgroup; fixed and jittered 5-7 s response time in the fMRI subgroup) – (2) blank (1 s). This procedure conforms to a rapid event-related fMRI design.

At the end of the experiment, participants were asked whether they had noticed anything unusual during their naps. When participants denied this question, we followed up by asking whether they had noticed acoustic presentations of words during sleep. No participant reported having noticed words being presented during sleep. At the end of the experiment, participants were fully debriefed and financially compensated.