In order to estimate the relationship between spike timing and the ongoing LFP at different frequencies and over time, spike-field coherograms were computed for each spike train. The spike-field coherogram estimates the coherency C between the spike spectrum and the LFP spectrum for each frequency f: (3)CXY(f)=SXY(f)SXX(f)SYY(f) with (4)SXY(f)=1K∑k=1K xk(f)yk(f) where xk and yk are the LFP spectrum (Equation 2) and the spike spectrum (the Fourier transform of the spike train's autocorrelation function; Jarvis and Mitra, 2001) respectively. Coherency is a complex quantity with magnitude and phase information; the magnitude is known as the coherence, and varies between 0 and 1, where 0 indicates independence and 1 perfect phase locking (Fries et al., 2001). We used the Chronux cohgramcpt function, with the following parameters: window size, 0.5 s; time step, 100 ms; nine tapers; 10 Hz bandwidth, using spike and LFP data taken from the same tetrode. Coherograms were averaged over all time steps to obtain the overall coherence for each spike train.
