dSPM values can be interpreted similarly to Z-scores.
Example: Z-score=1.65 => p-value=0.049; Z=2.35 => p=0.009
https://neuroimage.usc.edu/brainstorm/Tutorials/SourceEstimation#Standardization_of_source_maps
We recommend you only use these thresholds for the interactive exploration of the results. For properly assessing the significance of the minimum norm results, use non-parametric tests across participants.
https://neuroimage.usc.edu/brainstorm/Tutorials/Statistics
https://neuroimage.usc.edu/brainstorm/Tutorials/Workflows