[FieldTrip] Question on ft_connectivity_psi

[Schoffelen, J.M. (Jan Mathijs)]

Hi Yale,

This question has been asked before: https://mailman.science.ru.nl/pipermail/fieldtrip/2021-September/041054.html<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmailman.science.ru.nl%2Fpipermail%2Ffieldtrip%2F2021-September%2F041054.html&data=05%7C02%7Cfieldtrip%40science.ru.nl%7C823bdce6c26a4ca4e01508ddd4ca4421%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C638900687361804907%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=RU%2BB7ruu1XJOKHMoyL2ZvYdIsn2lxLy2OlJj4D%2Fmlzg%3D&reserved=0>

The answer is also documented here: https://www.fieldtriptoolbox.org/faq/spectral/phaseslopeindex/<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.fieldtriptoolbox.org%2Ffaq%2Fspectral%2Fphaseslopeindex%2F&data=05%7C02%7Cfieldtrip%40science.ru.nl%7C823bdce6c26a4ca4e01508ddd4ca4421%7C084578d9400d4a5aa7c7e76ca47af400%7C1%7C0%7C638900687366181488%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=%2FfXTvyxwvz0kvZPlzL5AWQ7XKnGKwrWKYNX3%2BTbiQo4%3D&reserved=0>

Jan-Mathijs


On 17 Jul 2025, at 22:45, Yale Cohen via fieldtrip <fieldtrip at science.ru.nl> wrote:

Hello

If I calculate the PSI value between 2 channels [channels A and B], the output of ft_connectivity_psi is a 3D matrix in which the first 2 indices are the PSI values of AA [always null], AB, BA, and BB [null]. The AB and BA values are the same magnitude but opposite sign indicating causal direction. The 3rd index is frequency.

If I now have more than 2 channels [A1, A2, etc, B1, B2,etc], the output of the matrix is NumChannelsA X NumChannelsB X Frequency.

My question is how do I interpret the sign of the PSI value? Is it always correct to assume that the positive value indicates that Ax is driving Bx?

Thank you!

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