Subject with clear topographical separation of spindles.

<p>Amplitude-frequency plot of an example subject from the group that exhibits a clear topographical separation of low and high frequency spindles: in the frontal channel, <b>(A)</b> Fpz, most spindles do not exceed 12 Hz, with nearly no activity over 14 Hz; in the parietal channel, <b>(D)</b> Pz, most spindles fall between 13 and 16 Hz, with only a few appearing below 12 Hz.</p

Fitted shape parameter across subjects.

<p>Values of the shape parameter of fitted gamma distribution for each of the scalp locations, assembled subject-wise; unreliable fits (as assessed by the KS plot) were removed; bars denote 95% confidence bounds of fitted parameter. There is no consistent effect of location on the shape parameter that would be exhibited across subjects and that would point to two distinct statistical processes governing frontal and parietal spindles.</p

Example overview of statistical spindle properties.

<p>Statistical properties of spindles in subject 3: in grey, histograms of inter-spindle intervals for four scalp locations (<b>(A)</b> Fpz, <b>(B)</b> Fz, <b>(C)</b> Cz, and <b>(D)</b> Pz; inter-spindle interval taken as center-to-center separation in time of automatically detected spindles of stage 2 sleep; bin width of histograms: 1s); superimposed in solid lines, gamma distributions fit to the distribution of inter-spindle intervals. In all four locations, the maximum likelihood distribution fit yields a gamma distribution with shape parameter close to one, suggesting a Poisson process.</p

Fitted shape parameter across locations.

<p>Values of the shape parameter of fitted gamma distribution for each subject; each panel represents a different scalp location (<b>(A)</b> Fpz, <b>(B)</b> Fz, <b>(C)</b> Cz, and <b>(D)</b> Pz); unreliable fits (as assessed by the KS plot) were removed; bars denote 95% confidence bounds of fitted parameter. Shape parameter reflects the statistical nature of the generation process and, as can be seen above, most channels and most subjects exhibit hallmarks of a Poisson random process (shape parameter not different than 1).</p

Example of gamma fitting evaluation.

<p>Quality assessment of gamma distribution fits for subject 3 (the fits from Fig. 2) across locations (<b>(A)</b> Fpz, <b>(B)</b> Fz, <b>(C)</b> Cz, and <b>(D)</b> Pz): Kolmogorov-Smirnov plots for each of the four fits of gamma distribution to the spindle interval distribution; dotted lines represent 95% confidence bounds. In all four panels the KS plots lie entirely within the confidence bounds pointing to statistically acceptable agreement between the model and the data.</p

Subject without a clear topographical distinction of spindles.

<p>Amplitude-frequency plot of an example subject from the group that exhibits no clear topographical trends: the distribution is similar across scalp locations, with a dominance of low-frequency spindles and a very faint presence of high-frequency spindles over all channels.</p

SSVEP responses to circle and square stimuli; organization of the plot as in <b>Figure 2</b>.

<p>SSVEP responses to circle and square stimuli; organization of the plot as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112099#pone-0112099-g002" target="_blank"><b>Figure 2</b></a>.</p

SSVEP responses to stimuli of different sizes; organization of the plot as in <b>Figure 2</b>.

<p>SSVEP responses to stimuli of different sizes; organization of the plot as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112099#pone-0112099-g002" target="_blank"><b>Figure 2</b></a>.</p

SSVEP responses to circle and square stimuli; organization of the plot as in <b>Figure 2</b>.

<p>SSVEP responses to circle and square stimuli; organization of the plot as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112099#pone-0112099-g002" target="_blank"><b>Figure 2</b></a>.</p

Parameters of the stimulus used in Experiment I.

<p>Parameters of the stimulus used in Experiment I.</p