Sleep Loss Disrupts The Neural Signature Of Successful Learning Part 2

Materials and methods

Participants

Fifty-nine participants (32 females, mean ± standard deviation [SD] age = 20.10 ± 1.80) were recruited voluntarily and completed a preliminary session (see below). 

Standard deviation is a statistical measure of the dispersion of data distribution. Memory refers to the human brain's ability to memorize information. The two don't seem to have much correlation or connection, but there is a certain connection between them. Let's take a look at it together.

First of all, we need to know that standard deviation can reflect the degree of dispersion of data distribution. That is to say, if the standard deviation of a set of data is larger, then the degree of dispersion of this data set will also be larger, meaning that the difference between the data will be larger. big. Memory requires a series of processes such as the brain's perception, understanding, encoding, storage, and retrieval of information. Because memory requires the brain to process this information, if the information itself is relatively discrete, it may lead to unsatisfactory memory performance.

Secondly, standard deviation can also reflect the stability and reliability of data. If the standard deviation of the data is smaller, then the variance of this set of data will also be smaller, and the difference between the data will also be smaller. In this case, the reliability and stability of the data are higher. This should also have a certain connection with memory. If a person has a good memory, his information processing ability should be relatively stable, which means that he will be more accurate and reliable in processing information, so his memory will be relatively good.

In summary, although standard deviation and memory do not seem to have much correlation there is a certain connection between them. When processing information, if the data is too discrete, it will easily cause information to be lost and forgotten. If the data is more stable, the information processing will be more accurate and reliable, providing better protection for people's memory. Therefore, we should focus on exercising our memory and improving the stability and reliability of our memory by improving our cognitive and information-processing abilities.

Click Know to improve short-term memory

After the preliminary session, 10 participants were excluded for not meeting the performance criterion, and 1 participant was excluded for not meeting the study requirement of being a native English speaker. Among those individuals who met the performance criterion of the preliminary session, 18 participants withdrew due to being unable to commit to the main study schedule. 

Our final sample size was n = 30 participants (17 females, mean ± SD age, 20.10 ± 1.65), each of whom completed both the sleep and sleep deprivation conditions (order counterbalanced, see Fig. 1a). Following standard procedures in our laboratory (Ashton et al. 2019; Strachan et al. 2020; Harrington, Ashton, Ngo, et al. 2021; Harrington, Ashton, Sankarasubramanian, et al. 2021), participants were asked to refrain from caffeine and alcohol for 24 h and 48 h, respectively, before each study session. Participants reported no history of sleep or psychiatric disorders. 

Written informed consent was obtained from all participants in line with the requirements of the Research Ethics Committee of the Department of Psychology at the University of York. Participants received £100 compensation upon completion of the study.

Statistical power was calculated before data collection using an effect size of d = 0.56 from Ashton et al. (2020). This effect size was derived from a paired-sample t-test comparing forgetting after a night of sleep or total sleep deprivation. Based on this effect size, our preregistered sample of n = 30 participants provided 83.7% power (alpha = 0.05, 2-tailed).

Tasks and stimuli

Visuospatial task (see Fig. 1b) One hundred images of neutral scenes were taken from the International Affective Picture System (Lang et al. 2005) and the Nencki Affective Picture System (Marchewka et al. 2014). These were divided into 2 sets of 50 images for use in the sleep and sleep deprivation conditions (assignment of the image set to condition was counterbalanced). The visuospatial task was divided into 3 phases:

Training I: passive viewing

Each of the 50 images was presented in a randomly selected location on a grid background (exposure time = 3 s, interstimulus interval [ISI] = 1 s). Participants were instructed to try and memorize the image locations for a later test. The image presentation order was randomized.

Training II: active viewing

Each image appeared in the center of the grid and participants moved it to the location that they believed it had appeared at passive viewing. The image then reappeared in its correct location to serve as feedback. 

This continued until all images had been placed <4.8 cm (<150 pixels) from their correct location on 2 consecutive rounds of active viewing (images for which this criterion was met were dropped from subsequent active viewing rounds). The image presentation order was randomized.

Test

The test phase followed the same procedures as one round of active viewing with the exception that no feedback was provided. Three tests were completed (immediate, delayed, and follow-up).

Paired-associates task (see Fig. 1c)

Two hundred images of natural and man-made objects on a white background were taken from Konkle et al. (2010) and online resources. These were divided into 2 sets of 100 objects (50 natural and 50 man-made) for use in the sleep and sleep deprivation conditions (assignment of the object set to condition was counterbalanced). 

Three hundred adjectives (150 adjectives per condition, assignment counterbalanced) were taken from Cairney, Guttesen, et al. (2018). Within each condition, 100 adjectives were randomly selected as targets and the remaining 50 as foils.

Adjective familiarization

Each of the 100 target adjectives was presented for 3 s. Participants were instructed to rate how often they would use each adjective in conversation on a scale of 1–9 (1 = never, 5 = sometimes, and 9 = often) within an additional 4 s (ISI with fixation crosshair = 1.5 s ± 100 ms). Adjective presentation order was randomized.

Image familiarization

Each of the 100 images (50 natural and 50 manmade objects) was presented for 3 s. Participants were instructed to imagine themselves interacting with each object and then categorize it as being natural or man-made within an additional 4 s (ISI with fixation crosshair = 1.5 s ± 100 ms). The image presentation order was randomized.

Learning

On each trial, participants were presented with an adjective and image from each of the prior familiarization phases for 4.5 seconds. They were instructed to memorize the adjective-image pairing for a future test. To facilitate learning, participants were asked to create a story or mental image in their mind that involved the adjective and image interacting for the full duration of the trial and then to rate this association as realistic or bizarre within an additional 4 seconds. 

A longer ISI of 5s (±100 ms) was used to facilitate the analysis of EEG data acquired during adjective-image learning (this comprised a 2-s progression bar followed by 3 s of fixation). Adjectiveimage pairing order was randomized.

Test

Each of the 150 adjectives (100 from learning and 50 unseen foils) was presented for 3 s. Participants were first instructed to indicate whether the adjective was old or new within an additional 10 s. Feedback on accuracy (correct/incorrect) was then provided for 1 s. 

For correct old responses, participants were presented with 4 images (all of which had been seen at learning) and were asked to indicate which image was paired with the adjective within 10 s. 

Participants then rated how confident they were in their response on a scale of 1 (not confident) to 4 (very confident) within 10 s. For incorrect old responses or new responses, participants moved immediately onto the next trial (ISI with fixation crosshair = 1.5 s ± 100 ms). Adjective presentation order was randomized.

Psychomotor vigilance task

The psychomotor vigilance task (PVT) was obtained from Khitrov et al. (2014) (bhsai.org/downloads/pc-pvt). Participants were instructed to respond when a digital counter appeared on the screen (ISI = 2–10 s). Participants received feedback on their response times and the task lasted for 3 min.

Procedure

Preliminary session

Participants completed a preliminary memory assessment before entering the main study. They learned 180 semantically related word pairs (e.g. "Horizon– Sun") and were immediately tested with a cued recall procedure. Participants scoring between 50% and 95% were invited back for the main experiment. This ensured that participants were unlikely to perform at floor or ceiling in the visuospatial and paired-associates tests of the main study.

Session 1: Evening

Participants arrived between 8:30 PM and 10 PM. In the sleep condition (earlier arrivals), participants were immediately wired up for overnight EEG monitoring. Participants began the study by completing the Stanford Sleepiness Scale (SSS; Hoddes et al. 1973), PVT, and then the training and immediate test phases of the visuospatial task.

Overnight interval

In the sleep condition, participants went to bed at ∼11 PM and were woken up at ∼7 AM (thus achieving ∼8 hours of EEG-monitored sleep). In the sleep deprivation condition, participants remained awake across the entire night under the supervision of a researcher. During the sleep deprivation period, participants were provided with refreshments and were permitted to play games, watch movies, or complete coursework. 

Because our sample was mostly made up of university students and a significant number of daytime study hours would be lost as a result of overnight sleep deprivation, we chose to allow participants to complete coursework to facilitate recruitment. 

Importantly, all of the permitted activities were deemed suitable because they were not conceptually linked to the materials that participants had learned the previous evening (i.e. object-location associations) or would learn in the following morning (i.e. adjective-image pairings).

Session 2: Morning

Participants in the sleep deprivation condition were wired up for EEG monitoring (this was not necessary for the sleep condition as electrodes had already been attached the previous night). Participants then completed another round of the SSS and PVT and another (delayed) visuospatial test. 

Afterward, participants carried out the familiarization phases of the paired associates task before completing the paired associates' learning phase with EEG monitoring. Participants were not given any explicit instruction on what to do (e.g. when to go to sleep) during the 48-h interval that preceded session 3.

Session 3: Follow-up

Participants returned 48 hours after session 2 (thereby allowing for recovery sleep in the sleep deprivation condition) and completed a final round of the SSS and the PVT. They then carried out the paired-associates test and a final (follow-up) visuospatial test.

Equipment

Experimental tasks

All tasks were executed on a Windows PC and participant responses were recorded with a keyboard or mouse. The visuospatial task was implemented in Presentation version 14.1 (Neurobehavioral Systems, Inc.) and the paired associates task was implemented in Psychtoolbox 3.0.13 (Brainard 1997; Pelli 1997; Kleiner et al. 2007) and MATLAB 2019a (The MathWorks, Inc.).

Electroencephalography

EEG recordings were administered with 2 Embla N7000 systems and 1 Embla NDx system with REMLogic 3.4 software. The Embla NDx was acquired when upgrading our sleep laboratory from a 2- to 3-bedroom facility (the N7000 was no longer available for purchase). 

For all but 3 participants, the same EEG system was used in the sleep and sleep deprivation conditions. Gold-plated electrodes were attached to the scalp according to the international 10-20 system at frontal (F3 and F4), central (C3 and C4), parietal (P3 and P4), and occipital (O1 and O2) locations and were referenced to the linked mastoids. 

Left and right electrooculography electrodes were attached, as were electromyography electrodes at the mentalis and submental bilaterally, and a ground electrode was attached to the forehead. An additional reference electrode was placed at Cz for the NDx system. We ensured that all electrodes had a connection impedance of <5 kΩ immediately before any EEG data were collected (i.e. for participants in the sleep condition, impedances were checked before sleep and again in the morning before the learning task). 

Any electrodes that fell above this threshold were replaced and rechecked. All online signals were digitally sampled at 200 Hz (N7000) or 256 Hz (NDx, downsampled to 200 Hz during preprocessing).

Actigraphy

Participants wore wristwatch actigraphy devices (Actiwatch 2, Philips Respironics, United States) throughout the study so that we could monitor their sleep when they were outside of the laboratory

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