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Hi Yale,

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<div>This question has been asked before: <a href="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" originalsrc="https://mailman.science.ru.nl/pipermail/fieldtrip/2021-September/041054.html" originalsrc="https://mailman.science.ru.nl/pipermail/fieldtrip/2021-September/041054.html">https://mailman.science.ru.nl/pipermail/fieldtrip/2021-September/041054.html</a></div>

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<div>The answer is also documented here: <a href="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" originalsrc="https://www.fieldtriptoolbox.org/faq/spectral/phaseslopeindex/" originalsrc="https://www.fieldtriptoolbox.org/faq/spectral/phaseslopeindex/">https://www.fieldtriptoolbox.org/faq/spectral/phaseslopeindex/</a></div>

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<div>Jan-Mathijs</div>

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<blockquote type="cite">

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

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<div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;">Hello<o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;"><o:p> </o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;">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 3<sup>rd</sup><span class="Apple-converted-space"> </span>index is frequency.<o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;"><o:p> </o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;">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.<span class="Apple-converted-space"> </span><o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;"><o:p> </o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;">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?<o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;"><o:p> </o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;">Thank you!<o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 14pt;"><o:p> </o:p></span></div>

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<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 10pt; font-family: Calibri, sans-serif;">-- <o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 10pt; font-family: Calibri, sans-serif;">Yale E. Cohen, Ph.D.<span class="Apple-converted-space"> </span><o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 10pt; font-family: Calibri, sans-serif;">Professor<br>

Associate Dean for Research Infrastructure and Strategy<o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 10pt; font-family: Calibri, sans-serif;">Chair, Bioengineering Graduate Group<br>

Department of Otorhinolaryngology:Head and Neck Surgery<br>

University of Pennsylvania School of Medicine<br>

3400 Spruce - <a href="x-apple-data-detectors://1" title="x-apple-data-detectors://1"><span style="">5 Ravdin</span></a><br>

<a href="x-apple-data-detectors://1" title="x-apple-data-detectors://1"><span style="">Philadelphia, PA 19104</span></a><br>

<br>

Office: G12A Stemmler <o:p></o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 10pt; font-family: Calibri, sans-serif;"><o:p> </o:p></span></div>

<div style="margin: 0in; font-size: 11pt; font-family: Aptos, sans-serif;"><span style="font-size: 10pt; font-family: Calibri, sans-serif;">E-mail: <a href="mailto:ycohen@pennmedicine.upenn.edu" title="mailto:ycohen@pennmedicine.upenn.edu"><span style="">ycohen@pennmedicine.upenn.edu</span></a><br>

Phone: <a href="tel:215%20898%207504" title="tel:215%20898%207504"><span style="">215 898 7504</span></a>    <br>

FAX: <a href="tel:215%20898%209994" title="tel:215%20898%209994"><span style="">215 898 9994</span></a><br>

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