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. Author manuscript; available in PMC: 2014 Feb 21.
Published in final edited form as: J Theor Biol. 2012 Dec 5;319:10.1016/j.jtbi.2012.11.029. doi: 10.1016/j.jtbi.2012.11.029

Figure 1.

Figure 1

Map of the three-well potential surface, U = x2 (x − 1)2 + y2 (y − 1)2 + xD for different values of D: (A) D = 0.2, (B) D = 0, (C) D = −0.2. This surface is the conservative part of the potential given in Eq. (3) (i.e., omitting frictional terms), with only a horizontal force, −D, acting. Shading shows the value of U, with darker shading corresponding to lower values. Lines in (B) indicate how the space is zoned into REM sleep, NREM sleep, and Wake. The xD term provides a horizontal force (arrows), pushing the particle towards Sleep (REM sleep or NREM sleep) in (A) and Wake in (C). For D = 0, all three potential wells are equally stable.