Auditory Gamma Entrainment Leads to Theta Oscillations Asymmetric behavior in both Alzheimer’s Disease and Healthy Participants

Background Brain oscillations known for handling various information across brain. The gamma rhythms observed in high‐level cognitive process (Fries et al., Trends in neurosciences 2018) and theta cycle observed to process sensory information and interact with gamma oscillations (Amemiya et al., Cell Reports 2018). Disruption in synchronization of gamma oscillations have been observed in Alzheimer’s disease patients (Uhlhaas et al., Neuron 2006). Gamma stimulation had been shown to reduce the mentioned dysfunction and reported to have therapeutic results (Martorell et al., Cell 2019; Suk et al. Alzheimer’s and Dementia 2020). This study examines the effect of gamma band activity induced by auditory chirp stimulation as a driver for other frequency sources. The chirp signal is designed to entrain 40Hz which had been shown to have therapeutics effects. Method 33 participants were included in this study including 17 mild AD, 6 MCI and 10 healthy elderlies. A session consists of interleaved periods of forty second stimulation and twenty second silence. The stimulation is a 5kHz tone modulated by a 40Hz chirp at 0.1 duty cycle. EEG data were collected by a 10/20 system. Relative magnitude of the 40Hz oscillations with respect to neighbors’ magnitude during the stimulation was measured as an indicator of the entrained gamma oscillations. Total participants are categorized into entrained and not‐entrained group based on the indicator. Phase locking Value (PLV) is then calculated in theta frequency band. Result 40Hz entrainment have been observed in healthy participants and also some Alzheimer’s disease patients. Based on the power spectral density (PSD) at forty hertz 23 entrained and 10 not‐entrained participants are separated and PLV analysis have been calculated for the two groups. Theta oscillatory asymmetry have been observed in posterior temporal cortex during auditory stimulation. Conclusion Entraining gamma oscillations using auditory stimulation results in cross frequency variation. Phase‐amplitude or phase‐phase coupling measures the simultaneous correlation between specific oscillations. This study suggests that despite the concurrent association of theta‐gamma frequencies, some brain network changes occur in response to gamma oscillation which should be more discussed in related researches.