Figure 2.

(A)Neurophysiological signals acquisition: EEG or MEG recordings are continuous time series representing electric or magnetic field in the scalp. Local field potentials (LFP) is a macroscopic scale recording generated by the electrical mixed transmembrane currents produced by neurons. From the extracellular recordings and using sorting spikes algorithms, spike trains corresponding to simultaneously recorded neurons can be obtained. (B) These recordings have to be pre-processed in order to filter, remove artifacts, or establish time windows. (C) Output from different functional connectivity methods. Spectral coherence (SC) provides coupling per frequency, and it is bounded in the interval (0, 1). Red line shows surrogate level, needed to conclude a statistically significant association. Partial spectral coherence (PSC) is proposed as a tool able to distinguish between direct and indirect (via another system) connections. We show the representation for four systems (note the symmetry). Wavelet coherence, similarly to spectral coherence, provides a measure for the coupling between a pair of sensors but in this case in both time and frequency domains. Representation of the wavelet coherence for a pair of system is a 3D plot (time, frequency, and coherence). Granger-based methods are developed to provide information flow direction. In this case the representation is similar to one from SC or PSC (for four sensors) but now connections from sensor x to sensor y are not the same as connections from sensor y to sensor x (asymmetry). Mutual information and synchronization likelihood provide a measure of coupling between a pair of signals. When signals are filtered in spectral bands of interest we obtain a bar diagram as a possible representation. Finally, we represent, for illustrative purpose, a scheme of the common source (volume conduction) problem. It has to be assumed that when dealing with nearby sensors a high probability of capturing activity of common sources will exist, and therefore spurious strong correlation could arise. Some methods (phase lag index or the imaginary part of coherence) have been proposed to overcome this problem.