ABSTRACT
To reveal the difference/connection between two-dimensional and three- dimensional (2D and 3D) chiroptical properties and their relation with 2D/3D symmetry/breaking, we develop an improved coupled dipole theory (ICDT) based on a model system of nanorod (NR) dimer. Our analytical ICDT can overcome the shortcoming of the traditional coupled dipole theory and points out the important role of scattering circular dichroism (SCD) in characterizing 2D chirality. The ICDT, supported by finite-difference time-domain (FDTD) simulation, reveals the physical origin of 2D chiroptical response: the interaction induced asymmetric effective polarizability for two identical NRs in a symmetry broken configuration. By tuning the NR's position/inter-particle distance, we find an optimal structure of maximum SCD due to the competition between geometric symmetry breaking and interaction. In addition, the interplay between 2D in-plane mirror symmetry breaking and three-dimensional (3D) mirror symmetry breaking leads to a symmetry broken system with zero SCD. The relation between chirality and reciprocity has also been addressed.
ABSTRACT
Previous studies indicated that Tai Chi might be an effective way to improve or prevent cognitive impairments in older populations. However, existing research does not provide clear recommendations about the optimal dose of Tai Chi practice, which is the most effective in improving cognitive function in older adults. The purpose of this systematic review and meta-analysis was to investigate the dose-response relationship between Tai Chi and cognition in community-dwelling older adults. A total of 16 studies with 1121 subjects were included in this study. Meta-regression analyses of Tai Chi duration (Tai Chi session duration, Tai Chi practice duration per week, study duration, and Tai Chi practice duration for the entire study) on the study effect size (ES) were performed to examine the dose-response association of Tai Chi and cognition. The results showed that there was a positive effect of Tai Chi on cognitive function, but there were no statistically significant dose duration effects on cognition. The findings suggest that Tai Chi has beneficial effects on cognitive function, but a longer duration was not associated with larger effects. In order to establish evidence-based clinical interventions using Tai Chi, future research should clearly demonstrate intervention protocol, particularly the style and intensity of Tai Chi.
