[FieldTrip] Sign/directionality of phase-slope index

[Aliette Tia]

Dear all,

For our experiment, we recorded EEG activity during presentation of visual
stimuli and we would like to quantify causal influences between brain
regions using the phase-slope index. I was a bit confused about the sign of
PSI and the temporal precedence it indicates.

In the help documentation, I saw that :
"If the phase slope index is positive, then the first
% chan (1st dim) becomes more lagged (or less leading) with higher
frequency,
% indicating that it is causally driven by the second channel (2nd dim)."
https://github.com/fieldtrip/fieldtrip/blob/master/connectivity/ft_connectivity_psi.m

But I read the opposite in previous discussions:
https://mailman.science.ru.nl/pipermail/fieldtrip/2019-May/038879.html
If I understood correctly, this means that when PSI is positive, the first
channel (1st dim) should lead in time the second channel (2nd dim).

Could you let me know which is the correct one?
Many thanks in advance!
Best wishes,

Aliette
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