4.1. Voxel-level integration
REGRESSION, CORTEX
The voxel-level analysis in our study did not reveal any significant correlation between EFs and changes in BOLD signal, which aligns with our initial hypothesis. In contrast, the only previous study that investigated the correlation between EFs and fMRI at the voxel-level reported a significant association between EF distribution patterns in a standard brain and a group-level functional map using a voxel-level approach [The interpretation of voxel-level results in the context of transcranial electrical stimulation poses several challenges. One important consideration is the spatial alignment of the EF and BOLD signal maps. For meaningful correlation or regression analyses, it is crucial to ensure that both maps are in the same space, either subject space or an averaged standard space, with the same resolution [Despite the lack of significant correlations at the voxel level, it is important to consider the potential localized effects of EFs on neural activity. Although the overall voxel-level analysis did not reveal significant associations, a small proportion of voxels exhibited large effect sizes, suggesting that localized regions may respond more prominently to EF-induced effects. This localized influence may be attributed to the intricate interplay between stimulation parameters, neural circuitry, and individual differences in brain structure and function. Understanding the factors contributing to these localized effects can provide valuable insights into the optimization of transcranial electrical stimulation protocols and individualized treatment approaches. In summary, our voxel-level analysis did not identify significant correlations between EFs and BOLD signal changes across the entire cortex. However, the challenges inherent to voxel-wise analysis, including spatial alignment, SNR limitations, and the need for rigorous multiple comparison correction, should be considered when interpreting these findings. The identification of a small proportion of voxels with large effect sizes suggests potential localized effects of EFs on neural activity. Future studies should further investigate these localized effects and explore the factors influencing the variability in EF-induced responses across different brain regions.