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Dear Aliette,
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<div class="">Well spotted! The good thing about the discrepancy, is that one of the interpretations is correct at least.</div>
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<div class="">Have you thought of trying to run a small simulation, to resolve the issue? In general I’d recommend that: ‘When in doubt, simulate’</div>
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<div class="">So, I wrote a little matlab script, posted below, which should allow you to resolve the issue for yourself:</div>
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clear <span style="color: #aa04f9" class="">all</span>;</div>
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% simulate some data</div>
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fsample = 1000;</div>
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nsample = fsample*30;</div>
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dat = randn(1,nsample+100);</div>
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data.trial{1}(1,:) = dat(1,1:nsample) + 0.1.*randn(1,nsample);</div>
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data.trial{1}(2,:) = dat(1,100+(1:nsample)) + 0.1.*randn(1,nsample);</div>
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data.time{1} = (1:nsample)./fsample;</div>
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<span style="color: #000000" class="">data.label = {</span><span style="color: #aa04f9" class="">'a'</span><span style="color: #000000" class="">;</span><span style="color: #aa04f9" class="">'b'</span><span style="color: #000000" class="">};
</span>% channel 2 is leading channel 1</div>
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figure;plot(data.time{1},data.trial{1}); xlim([0 1]);</div>
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% cut into 2 second snippets</div>
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cfg = [];</div>
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cfg.length = 2;</div>
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data = ft_redefinetrial(cfg, data);</div>
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% spectral decomposition</div>
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cfg = [];</div>
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cfg.method = <span style="color: #aa04f9" class="">'mtmfft'</span>;</div>
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cfg.output = <span style="color: #aa04f9" class="">'fourier'</span>;</div>
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cfg.tapsmofrq = 2;</div>
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cfg.foilim = [0 100];</div>
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freq = ft_freqanalysis(cfg, data);</div>
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% connectivity estimation</div>
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cfg = [];</div>
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cfg.method = <span style="color: #aa04f9" class="">'psi'</span>;</div>
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cfg.bandwidth = 5;</div>
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psi = ft_connectivityanalysis(cfg, freq);</div>
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% visualization</div>
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cfg = [];</div>
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cfg.parameter = <span style="color: #aa04f9" class="">'psispctrm'</span>;</div>
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ft_connectivityplot(cfg, psi);</div>
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<div class=""><font color="#000000" class=""><span style="caret-color: rgb(0, 0, 0);" class="">So, I think that this should help you on the way. Could I ask you, once you have resolved the discrepancy for yourself, to indicate whether the function’s documentation
needs to be changed. Also, the above code might be a good example to document on the fieldtrip website, under Frequently Asked Questions. Feel free and welcome to draft a little piece of documentation that we can upload as a new FAQ!</span></font></div>
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<div class=""><font color="#000000" class=""><span style="caret-color: rgb(0, 0, 0);" class="">Best wishes,</span></font></div>
<div class=""><font color="#000000" class=""><span style="caret-color: rgb(0, 0, 0);" class="">Jan-Mathijs</span></font></div>
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<div class="">On 7 Sep 2021, at 22:04, Aliette Tia via fieldtrip <<a href="mailto:fieldtrip@science.ru.nl" class="">fieldtrip@science.ru.nl</a>> wrote:</div>
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<div dir="ltr" class="">Dear all,<br class="">
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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.
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In the help documentation, I saw that :<br class="">
"If the phase slope index is positive, then the first<br class="">
% chan (1st dim) becomes more lagged (or less leading) with higher frequency,<br class="">
% indicating that it is causally driven by the second channel (2nd dim)."<br class="">
<a href="https://urldefense.com/v3/__https://github.com/fieldtrip/fieldtrip/blob/master/connectivity/ft_connectivity_psi.m__;!!HJOPV4FYYWzcc1jazlU!r7BR-KkIAcwOyc5h-yUay_8wJgVtT38ax3E52E_lOa_jC160_bE7kgq2yqsMgeXlhowhvY3pXeQpA-Q$" class="">https://github.com/fieldtrip/fieldtrip/blob/master/connectivity/ft_connectivity_psi.m</a>
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<div class="">But I read the opposite in previous discussions: </div>
<div class=""><a href="https://mailman.science.ru.nl/pipermail/fieldtrip/2019-May/038879.html" class="">https://mailman.science.ru.nl/pipermail/fieldtrip/2019-May/038879.html</a></div>
<div class="">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).<br class="">
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Could you let me know which is the correct one?<br class="">
Many thanks in advance!<br class="">
Best wishes,<br class="">
<br class="">
Aliette<br class="">
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