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. 2021 Jun 11;4:722. doi: 10.1038/s42003-021-02234-7

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© The Author(s) 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Spectrograms of example segments of tree shrew (a) and rat (d) sleep, with the corresponding delta power, red (0.5–4 Hz) and theta-alpha power, blue, (4–12 Hz) shown below. Black arrows indicate periods of increased theta-alpha power accompanying NREM/REM transitions. Selected transitions for tree shrew and rats are shown in (b) and (e) respectively. Top level panels show delta vs. theta-alpha power for the sleep segments highlighted in (a) and (d). Circled epochs 1–2 are expanded in the surrounding numbered panels, showing the corresponding filtered LFP traces. Note that in the example rat delta and theta-alpha power decrease together while in the tree shrew an initial decline in delta power is accompanied by a transient increase at the theta-alpha band, see arrows in (a). Indeed this transient increase in theta-alpha power during transitions was significant for the population of tree shrews (n = 4) as shown in (c), while theta-alpha significantly decreased during transitions in rats (n = 5) as shown in (f), paired t-tests, *p < 0.05, **p < 0.01, ***p < 0.001. Error bars reflect the standard error of the mean. Color bars reflect Fourier power in arbitrary units.