Fig. 2.
Illustration of electroencephalogram channels, and coherence measurement. (A) Visual representation of channel locations and the two bipolar frontal channels, F7 and F8, which we used for coherence analysis. Areas in red are purely illustrative for the explanation of coherence. The bipolar frontal channels overlaying these regions may not record signals solely from the underlying cortex. (B) Simulated signals to illustrate interpretation of coherence. Signal “A” and signal “B” appear highly correlated in time, whereas signal “C” appears uncorrelated with both signals A and B. (C–E) Spectrogram for simulated signals in B. The spectrogram plots signal power or energy as a function of time and frequency. Signals A, B, and C produce almost identical spectrograms; however, their coherograms will reflect differences in functional connectivity that may otherwise be overlooked. (F and G) The coherence indicates the correlation coefficient between two signals as a function of frequency (0 for no correlation, with a maximum value of 1 for perfect correlation). The coherogram plots the coherence as function of time, much like the spectrogram. This example shows how the simulated signals have almost identical spectrograms, but very different coherograms, consistent with the degree of correlation evident in the time domain traces shown in B. The coherogram also indicates the frequencies over which two signals are correlated. In this example, signals A and B are correlated at frequencies below approximately 20 Hz.