[FieldTrip] creating difference wave

[Xianwei Che]

Dear list,

I have some concerns of how to create difference wave between two
conditions. Here is what I want to look at:

I have grand average time-freqency data of two conditions ("GA1","GA2"),
and one behavioural measurement. Now I want to do so some
regression/correlation analysis between the behavioural measurement and the
contrasted time-freqency data (GA1-GA2).

I did some googling and it is suggested to create the difference wave
first, as per here (http://www.fieldtriptoolbox.org/faq/how_can_i_test_an_
interaction_effect_using_cluster-based_permutation_tests).

>From these 4 data structures, you now make 2 difference data structures in
the following way:

   - Copy GA11 to GAdiff11_12 and perform the assignment
   GAdiff11_12.avg=GA11.avg-GA12.avg.
   - Copy GA21 to GAdiff21_22 and perform the assignment
   GAdiff21_22.avg=GA21.avg-GA22.avg.


I got confused about the '.avg' here. Powspctrm is 4-d data in each GA
(subject.channel.frequency.time); so what is and how to calculate the
average (.avg) in each GA structure.

Or is it just a filed in each GA as I cannot find one.

Thanks a lot

*-------------*
*Mr Xianwei Che*
*PhD Candidate*
*Monash Alfred Psychiatry Research Centre (MAPrc)*
*Central Clinical School & the Alfred *
*Monash University*
*Level 4, 607 St Kilda Road, Melbourne **3004, **Australia*
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