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The Analysis of the Multisensory Stimulation Effects on Brain Signals in Alzheimer's Patients

Nourmohammadi, Parinaz | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58824 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Aghajan, Hamid
  7. Abstract:
  8. This thesis investigates EEG-based biomarkers for differentiating healthy older adults, individuals with mild cognitive impairment (MCI), and patients with mild Alzheimer’s disease (Mild-AD) during auditory stimulation at 40 Hz. Early and accurate differentiation between MCI and Mild-AD is clinically essential, as therapeutic interventions are considerably more effective at the prodromal stage of cognitive decline. Fifty-one participants were divided into three groups (Healthy, MCI, Mild-AD). Each participant completed twenty-five six-second auditory trials, consisting of three seconds of 40-Hz stimulation followed by three seconds of post-stimulus rest. EEG data were preprocessed through filtering, artifact removal, and independent component analysis (ICA). Three analytical approaches were applied: (1) spectral power and topographic mapping across alpha, theta, and gamma bands; (2) time-resolved phase–amplitude coupling (PAC) between the low-frequency phase (alpha/theta at Fz) and high-frequency amplitude (gamma at Pz) to examine feedback (top-down) versus feedforward interactions; and (3) repetition-dependent analysis comparing early versus late trials. Results revealed that healthy participants exhibited a sharp, transient alpha–gamma PAC peak immediately after stimulus offset, reflecting efficient inhibitory gating and precise timing of sensory reprocessing. In MCI, this response appeared delayed and broader, becoming stronger during later repetitions. In Mild-AD, the alpha–gamma coupling response was markedly attenuated or absent, accompanied by abnormally elevated and sustained gamma power in posterior, indicative of impaired alpha-mediated inhibition. Furthermore, MCI participants showed compensatory enhancement of theta–gamma coupling, suggesting a shift of feedback control toward slower rhythms, whereas this compensation failed in Mild-AD. These findings suggest that the temporal dynamics of PAC encode distinct diagnostic information across sensory (feedforward) and feedback processing phases. Disrupted alpha–gamma coupling likely reflects impaired inhibitory and feedback control mechanisms underlying early cognitive decline. EEG-based PAC measures may thus serve as noninvasive, cost-effective biomarkers for early detection and monitoring of Alzheimer’s disease progression, particularly in differentiating MCI from Mild-AD
  9. Keywords:
  10. Electroencephalography ; Phase-Amplitude Coupling (PAC) ; Mild Alzheimer’s Disease (Mild-AD) ; 40-Hz Auditory ; Mild Cognitive Impairment (MCI) ; Feedback Connectivity ; Inhibitory Gating ; Alpha-Gamma Interaction

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