Femtosecond Optical Kerr Effect in Alzheimer's Brain Tissue.

The optical Kerr effect (OKE) spectroscopy is a powerful technique to study ultrafast dynamics of a material's response to an optical field. In this study, we investigated OKE in Alzheimer's disease (AD) and normal human brain tissues from the hippocampus and Brodmann areas 9 and 17. The Kerr signals from both AD and normal samples exhibited a distinct double-peak profile, corresponding to the electronic and plasma mechanisms. Analysis of plasma relaxation and dielectric response times revealed that conductivity in the AD hippocampus was approximately 46%-61% higher than in normal tissue, due to an increase in structural inhomogeneity, which may arise from tau and amyloid-beta (Aβ) protein accumulation and elevated water content. Overall, the mean conductivity across the three regions was 38% higher in AD compared to normal tissue. These findings demonstrate that OKE can provide valuable information on underlying molecular mechanisms of the brain that occur on ultrafast timescale.