EEG harmonic responses to photic stimulation in normal aging and Alzheimerʹs disease: differences in interhemispheric coherence

Objectives: The distinction between electroencephalogram (EEG) driving responses in normal aging and Alzheimerʹs disease (AD) is unclear. This study was conducted to explore the structure of harmonic responses of EEG coherence during photic stimulation (PS) in AD. Methods: EEG interhemispheric coherence was analyzed for each harmonic responses during PS (white flickers at 5, 10 and 15 flashes/sec) in younger subjects (20–25 years), non-demented elderly subjects (50–67 years) and AD patients (43–66 years). Results: The elderly subjects had higher interhemispheric coherence during PS than the younger subjects, with significant differences found in the 6th (30 Hz) and 8th (40 Hz) harmonic responses to 5 Hz PS. The AD patients had significantly smaller coherence than the elderly subjects in the 2nd (10 Hz) and 4th (20 Hz) harmonic responses to 5 Hz PS, and in the fundamental (10 Hz) and 2nd (20 Hz) harmonic responses to 10 Hz PS. These coherence values did not correlate with duration of the disease or with cognitive functioning as evaluated by MMSE score. Conclusions: A failure of stimulation-related brain activation in AD patients was observed in alpha-related frequencies, suggesting that AD is not the end result of normal aging of the brain, at least in terms of the mechanisms which are involved in PS-related functional organization.