[FieldTrip] Negative values of debiased wPLI

[Joram van Driel]

Hi Marta,
It's correct that you can get negative values with dwPLI compared to
classic PLI, it's indeed due to the debiasing procedure which involves a
linear subtraction. What you want to do with it, depends on how you want to
evaluate statistical significance. I wouldn't take the absolute values,
though.
- You could add a fixed value to all dwPLI values to have everything above
zero with a fixed shift, then do a baseline correction such as percent
signal change or dB (which you can't do with negative numbers). This is a
bit crude though, and I actually wonder whether it's "allowed".
- Just use the values as they are, with some being negative, if you want do
condition comparisons, and use permutation testing with condition label
shuffling at the group level to asses statistical difference between
conditions.
- Convert the dwPLI values to a z-score within-subject through permutation
testing, where you shuffle one phase-angle vector of one of the two
channels with respect to the other in e.g. 1000 permutations, thereby
creating a null-distribution under the null-hypothesis of chance-level
dwPLI, to which you can compare you're observed dwPLI. This approach is
blind to the fact that some values are negative. How/what you shuffle
depends on whether you compute dwPLI over trials or over time.

Good luck,
Joram

On Tue, Oct 13, 2015 at 5:59 PM, Rodrigo Montefusco <
rmontefusco at med.uchile.cl> wrote:

> Hello...anybody any guess, explanation? I getting some too...close to zero
> thought, but still negatives.
>
> On Mon, Feb 16, 2015 at 9:09 AM, Marta Bortoletto <
> marta.bortoletto at cognitiveneuroscience.it> wrote:
>
>> Dear Community,
>> I am using the debiased wPLI to estimate connectivity between 70 EEG
>> electrodes. I have about 150 trials for each subject. I noticed that some
>> values of my 70x70 dwPLI matrix are negative. My understanding is that all
>> values should be between 0 and 1, but for some reason I can get negative
>> values from the debiasing process. My question is: Shall I calculate the
>> absolute value of these negative values? Otherwise what shall I do with
>> them?
>>
>> Thank you in advance for your help.
>> Marta
>>
>>
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-- 
Joram van Driel, PhD
Postdoc @ Vrije Universiteit Amsterdam
Cognitive Psychology
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