This protocol describes methods used to examine neural mechanisms underlying sleep-dependent memory consolidation during naps in early childhood. It includes procedures for examining the effect of sleep on behavioral memory performance, as well as the application and recording of both polysomnography and actigraphy.
This protocol allows for the measurement of sleep-dependent memory consolidation in children and provides a practical approach for examining the benefit of napping on declarative memory in typically developing children. Results from this technique will have implications for early childhood education, as they will provide empirically based recommendations for nap lengths in preschool classrooms. The main advantage of this method is that it can be used to assess both sleep distribution and sleep architecture, which provide data regarding brain states that support memory consolidation.
Although these methods were developed for examining memory consolidation during naps in preschool-age children, they can be adapted for use with any age group and for overnight sleep bouts. Proper electrode site measurement and preparation are critical for achieving low impedances and high-quality data, which can be difficult in preschool children. Demonstrating the procedure with Tamara Allard will be Arcadia Ewell and Benjamin Weinberg, research assistants in the laboratory.
After explaining the procedure to the child and parent using age-appropriate materials, have the child engage in a quiet activity. Using a flexible tape measure and china marker, measure the distance from the inion to nasion, and place a mark at the halfway point. Measure the distance from the preauricular notch on one ear to the preauricular notch on the other ear, and place a mark at the halfway point.
The intersection of these two marks is the reference point, or CZ.Measure 10%of the inion to nasion distance up from the inion and 10%of the preauricular notch to preauricular notch measurement from this point on either side. Then make the O three marks on one side and the O four mark on the other. Measure out 20%of the preauricular notch to preauricular notch measurement from the reference point on either side of the head, and make the C three and C four marks on each side.
Measure 20%of the inion to nasion distance up from the reference point, and measure out 20%of the preauricular notch to preauricular notch measurement from this point on either side. Then make the F three and F four marks on each side of this point. When all of the marks have been placed, use an alcohol swab to clean the first electrode location before using a slightly abrasive gel to exfoliate the skin.
Remove any residual cleaning material, and fill an electrode with electrode cream. For electrodes being placed where hair is present, apply an additional drop of electrode cream to a gauze square, and place the gauze on the back of the electrode. Place the electroencephalography electrodes on each marked location on the scalp, on each mastoid, and in the center of the forehead.
Place the right electrooculography electrode adjacent and slightly superior to the outside of the right eye, and place the left electrooculography electrode to the outside and slightly inferior to the left eye. Place one electromyography electrode on the right cheek, just above the smile line, and have the child say the word milk out loud while feeling for the location at which the muscle contractions in the neck and chin are maximal, to place a second electromyograph electrode on the left side, just above where the chin meets the neck, adjacent to the esophagus. When all of the electrodes have been placed, attach the electrodes to the recording device, and initiate the recording to ensure that all of the electrodes pass the impedance test.
To administer the encoding phase of the visuospatial memory task, set out an age-appropriate number of to-be-remembered stimuli, and direct the child to identify each image by name. Instruct the child to remember the location of each image within the grid, and replace the cards with blank images. Then present the images one at a time, and ask the child to identify the previous location of the image within the grid, informing the child whether or not the correct location was indicated.
After the child has reached a threshold of 75%correct image identification, administer the immediate recall phase. Present the images again one at a time, and ask the child to recall the corresponding location without providing visual or verbal feedback and probing each item only one time. At the conclusion of the immediate recall phase, have the child use the restroom before initiating the child's typical pre-nap routine.
Allow the child to nap in their typical nap location. Then conduct the delayed recall phase for the visuospatial memory task as previously demonstrated for the immediate recall phase approximately 15 to 30 minutes after the child wakes to avoid sleep inertia. At the end of the delayed recall assessment, use water-based spray to remove the electrodes.
For electrodes applied with tape, use a cotton pad soaked with baby oil to saturate the tape before gently pulling the tape up from the corners. In this representative experiment, children's recall accuracy on the visuospatial memory task after a nap was better than their recall accuracy after a similar period during which they remained awake, signifying a nap benefit. Moreover, those who spent the prior day in the wake condition did not recover memories during overnight sleep.
Of interest, the nap benefit was only significant in children who napped habitually. Moreover, performance on the visuospatial memory task was related to the data obtained from the PSG recordings during the nap. Specifically, the fewer sleep spindles recorded during the non-rapid eye movement stage two sleep was related to a better immediate recall accuracy.
Importantly, there was a positive correlation between the change in recall after nap and sleep spindle density during non-rapid eye movement stage two sleep. No other measure of sleep physiology was determined to be related to memory performance. Although this paper focuses on PSG, a complete understanding of sleep-dependent memory consolidation will rely on proper administration of complementary techniques, such as actigraphy and parent report, to determine sleep cycles.
To compare data obtained from the nap and wake promotion conditions, it is important to keep the conditions as similar as possible in terms of their duration and interfering, distracting content. This technique can enhance our understanding of the relationship between sleep and memory during early childhood, a period of developmental significance as children transition from biphasic to monophasic sleep patterns.
