Confusing values of normalized PTE (?)

Phase transfer entropy was introduced by Lobier et al., 2014, with this definition. PTE cannot be negative and its magnitude does not have a meaningful upper bound. To address this issue, a way to normalize PTE was introduced by Hillebrand et al., 2016. Where PTE values are normalized by the sum to the total (bidrectional) PTE between two signals.

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With this normalization, dPTE values range from 0 to 1, dPTExy + dPTEyx = 1, and the largest dPTE indicates the preferential flow of the information. This normalization is carried out in Brainstorm within the script PhaseTE_MF.

An additional step is carried on the dPTE values in the Brainstorm inside the script bst_connectivity, where 0.5 is subtracted from dPTE values to center them around 0. However, this centering seems problematic as it can lead to wrong interpretations:

For example, if dPTExy = 0.8 and dPTEyx = 0.2, this indicates a information goes preferentially from x to y. Removing 0.5 on both metrics results in dPTExy = 0.3 and dPTEyx = -0.3, while the numeric difference in the metrics is kept, the meaning does not look as clear. This is most notorious in the extreme case where dPTExy = 1 and dPTEyx = 0.

What is the reason of the zero-centering?
Is it a common practice?

Best,
Raymundo

@MartinC @Sylvain @hossein27en?

Zero-centering of normalized PTE values has been removed from Brainstorm.