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Oscillatory phase coherence12/28/2023 ![]() While revisiting previously accepted findings may not always be popular, it will likely be essential if neuroscientists want to accurately understand how new memories are formed. The next step therefore is to apply these more robust analysis techniques to data from the human brain. Phase-phase coupling of theta and gamma waves has also been reported in the human hippocampus. Instead, the simulations suggest that what appeared to be statistically significant coupling may in reality be an artifact of the previous analysis. Using stringent control analyses, Scheffer-Teixeira and Tort find no evidence for prominent theta-gamma phase-phase coupling in the hippocampus. The new results suggest that previous reports describing the phenomenon may have relied on inadequate statistical techniques. Using computer simulations and recordings from the rat hippocampus, Scheffer-Teixeira and Tort have now reexamined the evidence for theta-gamma phase-phase coupling. This was thought to help the hippocampus to encode memories. Phase-phase coupling is the idea that gamma and theta waves align themselves, such that gamma waves always begin at the same relative position within a theta wave. Since gamma waves are faster than theta waves, multiple cycles of gamma can occur during a single cycle of theta. Recent work has suggested that gamma waves and theta waves show a phenomenon called phase-phase coupling. Gamma waves are faster, with a frequency of up to 100 Hertz. Theta waves are relatively slow waves, with a frequency between 5 and 10 Hertz. Within the hippocampus, an area of the brain involved in memory, two types of oscillations dominate: theta waves and gamma waves. Moreover, oscillations with different frequencies can co-exist and interact with one another. Recordings reveal that the frequency of these oscillations – the number of cycles of a wave per second, measured in Hertz – can vary between brain regions, and within a single region over time. These arise when large numbers of neurons fire in synchrony. Placing electrodes on the scalp or lowering them into the brain itself reveals rhythmic waves of activity known as oscillations. Neuroscientists have long sought to understand how the brain works by analyzing its electrical activity. 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. We also show that waveform asymmetry and frequency harmonics may generate artifactual n:m phase-locking. 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. However, by analyzing simulated and actual LFPs, here we question the existence of theta-gamma phase-phase coupling in the rat hippocampus. 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. Phase-amplitude coupling between theta and multiple gamma sub-bands is a hallmark of hippocampal activity and believed to take part in information routing.
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