Abstract
Phase-amplitude coupling between theta and multiple gamma sub-bands is a hallmark of hippocampal activity and believed to take part in information routing. More recently, theta and gamma oscillations were also reported to exhibit phase-phase coupling, or n:m phase-locking, suggesting an important mechanism of neuronal coding that has long received theoretical support. However, by analyzing simulated and actual LFPs, here we question the existence of theta-gamma phase-phase coupling in the rat hippocampus. We show that the quasi-linear phase shifts introduced by filtering lead to spurious coupling levels in both white noise and hippocampal LFPs, which highly depend on epoch length, and that significant coupling may be falsely detected when employing improper surrogate methods. We also show that waveform asymmetry and frequency harmonics may generate artifactual n:m phase-locking. Studies investigating phase-phase coupling should rely on appropriate statistical controls and be aware of confounding factors; otherwise, they could easily fall into analysis pitfalls.
Highlights
Local field potentials (LFPs) exhibit oscillations of different frequencies, which may co-occur and interact with one another (Jensen and Colgin, 2007; Tort et al, 2010; Hyafil et al, 2015)
Crossfrequency phase-amplitude coupling between theta and gamma oscillations has been well described in the hippocampus, whereby the instantaneous amplitude of gamma oscillations depends on the instantaneous phase of theta (Scheffer-Teixeira et al, 2012; Schomburg et al, 2014)
The concentration of the phase difference distribution is used as a metric of n:m phase-locking. This metric is defined as the length of the mean resultant vector (Rn:m) over unitary vectors whose angle is the instantaneous phase difference, and thereby it varies between 0 and 1
Summary
Local field potentials (LFPs) exhibit oscillations of different frequencies, which may co-occur and interact with one another (Jensen and Colgin, 2007; Tort et al, 2010; Hyafil et al, 2015). Cross-frequency phase–phase coupling, or n:m phaselocking, relies on assessing the constancy of the difference between two phase time series (Tass et al, 1998). In this case the original phase time series are accelerated, so that their instantaneous frequencies can match. The instantaneous phase of theta oscillations at 8 Hz needs to be accelerated five times to match in Scheffer-Teixeira and Tort.
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