Artefactual origin of biphasic cortical spike-LFP correlation

Abstract

Electrophysiological data acquisition systems introduce various distortions into the signals they record. While such distortions were discussed previously, their effects are often not appreciated. Here I show that the biphasic shape of cortical spike-triggered LFP average (stLFP), reported in multiple studies, is likely an artefact introduced by high-pass filter of the neural data acquisition system when the actual stLFP has a single trough around the zero lag.

Effect of Stimulus Contrast and Visual Attention on Spike-Gamma Phase Relationship in Macaque Primary Visual Cortex

“…However, all three filters had negligible group delay (<0.1 ms) between 500–5,000 Hz, suggesting that these filtering operations did not change the position of the spike appreciably. Similarly, the high-pass filter had a large group delay at very low frequencies, as shown by Okun ( 2017 ), but it was negligible in the gamma range. Therefore, these three filters did not have an appreciable effect on the spike-gamma phase estimation.…”

“…Butterworth filters have a linear relationship between phase delay and frequency, such that the group delay (which roughly translates to how much each frequency component of the signal shifts in time due to the filtering process) is almost constant over a wide frequency range. We removed the low-pass filtering effect by dividing the Fourier Transform of the LFP by the Fourier Transform of the low-pass LFP filter (4th order Butterworth filter with a low-pass cutoff at 500 Hz; constructed in MATLAB using the command “butter”) and subsequently taking the inverse Fourier Transform (Okun, 2017 ). The correction was only done between 0 and 500 Hz because the power of the LFP (as well as the filter) was very less beyond 500 Hz.…”

“…Note that since attention is thought to increase the effective stimulus contrast (Carrasco et al, 2004 ), testing whether gamma phase varies with attentional state is also a test for gamma PC for contrast. We further studied the effect of the online causal filter used to obtain the LFP, which introduces a delay in the LFP and has been shown to influence spike-LFP relationships (Okun, 2017 ), but has not been accounted for in previous studies.…”

Genetically expressed voltage sensor ArcLight for imaging large scale cortical activity in the anesthetized and awake mouse

“…47 However, more recent works have noted the potential influence of filtering properties on relating the LFP to the membrane potential, and thus caution needs to be used in interpreting this relationship. 48 Our comparison between the temporal dynamics of the LFP and the ArcLight response may not account for this complex relationship between the LFP and the membrane potential.…”

“…47 However, this finding should be regarded with caution as the filtering properties have been shown to strongly influence the nature of the LFP and membrane potential relationship. 48 In paired intracellular and LFP recording experiments, LFP explains only a limited amount of the signal variance in the membrane potential. 46 Finally, due to the large hemodynamic noise in the raw single trial and the limitations of the Off-ROI subtraction technique, careful consideration must be taken when examining ArcLight single trials.…”