Additional file 1: Figure S1.

Electrode sites with positive local field potential (LFP) peaks near the brain surface are not usually associated with strong analogue multiunit activity (AMUA) peaks in the same recording site, suggesting the absence of neurons with action potential firing at these sites. To quantitatively verify this observation we calculated for every site (n = 811 sites in 13 birds) the skewness of its LFP signal, which is negative for negatively peaked signals and positive for positively peaked signals, and the kurtosis of its AMUA signal, which is a measure for its peakedness. Signals with stronger action potential firing have higher kurtosis. A plot of AMUA kurtosis against LFP skewness shows that sites with negatively peaked LFP signals overall have much stronger action potential firing. The difference in kurtosis between sites with negatively peaked LFP (median kurtosis: 32.9; n = 716) and positively peaked LFP (median kurtosis: 2.3; n = 95) is highly significant (Mann–Whitney-U test; u = 11,657, P <0.001). Kurtosis is calculated as excess kurtosis, so that the kurtosis of a normal distribution equals zero.

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Beckers et al. BMC Biology 2014 12:16   doi:10.1186/1741-7007-12-16