Gamma phase synchronization during olfactory stimulation as a marker for Alzheimer’s disease

Background Altered gamma oscillations have been observed in patients of Alzheimer’s disease (AD). A large number of studies have linked the quality of gamma oscillations in the frontal cortex to the performance of memory retrieval tasks. Synchronization of gamma oscillations has a strong correlation with cognitive functions (Fell et al., Nature Reviews Neuroscience 2011), and deficit in gamma synchronization in the frontal cortex has been reported as a marker for AD (Sedghizadeh et al., PLOS One 2020). In this study, we examine the phase synchronization of gamma oscillations in the EEG data recorded from the frontal cortex during olfactory stimulation as a marker for AD. Method Twenty‐four elderly participants (13 Normal and 11 mild AD patients) participated in this study. The cognitive status of the participants was quantified using the MMSE test. Then, each participant underwent an olfactory stimulation session consisting of 120 trials of 2s stimuli presentation with a random selection of two odors (lemon as the frequent and rose as the rare odor) followed by 8s of rest. EEG data were simultaneously recorded and pre‐processed for artifact and noise removal and trial averaging. To analyze gamma band synchronization, the phase‐locking value (PLV) for lower gamma oscillations (35‐45Hz) was calculated between the Fz and Cz electrodes. Fig. 1 illustrates the measurement and processing flow. Result Our study shows that the PLV is significantly higher for normal participants compared to AD patients (Fig. 2). Also, the phase difference between the Fz and Cz electrodes in the lower gamma band remains unchanged across trials for normal participants while it widely varies between ‐180 and 180 degrees for AD patients (Fig. 3). Conclusion This study shows that there is a significant difference between normal participants and AD patients in terms of the synchronization of gamma oscillations in the frontal cortex in response to olfactory stimulation. These results suggest that the PLV between the Fz and Cz electrodes under olfactory stimulation can be used as a marker for AD diagnosis.